Declare input and output types of commands (#6796)

* Add failing test that list of ints and floats is List<Number>

* Start defining subtype relation

* Make it possible to declare input and output types for commands

- Enforce them in tests

* Declare input and output types of commands

* Add formatted signatures to `help commands` table

* Revert SyntaxShape::Table -> Type::Table change

* Revert unnecessary derive(Hash) on SyntaxShape

Co-authored-by: JT <547158+jntrnr@users.noreply.github.com>

Cargo.lock

@@ -2743,6 +2743,8 @@ dependencies = [
  "num-format",
  "serde",
  "serde_json",
+ "strum 0.24.1",
+ "strum_macros 0.24.3",
  "sys-locale",
  "thiserror",
  "typetag",

crates/nu-command/src/bits/and.rs

@@ -2,7 +2,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits and")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required(
                 "target",
                 SyntaxShape::Int,

crates/nu-command/src/bits/not.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits not")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .switch(
                 "signed",
                 "always treat input number as a signed number",

crates/nu-command/src/bits/or.rs

@@ -2,7 +2,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits or")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required(
                 "target",
                 SyntaxShape::Int,

crates/nu-command/src/bits/rotate_left.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 use num_traits::int::PrimInt;
 use std::fmt::Display;
@@ -18,6 +18,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits rol")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required("bits", SyntaxShape::Int, "number of bits to rotate left")
             .switch(
                 "signed",

crates/nu-command/src/bits/rotate_right.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 use num_traits::int::PrimInt;
 use std::fmt::Display;
@@ -18,6 +18,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits ror")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required("bits", SyntaxShape::Int, "number of bits to rotate right")
             .switch(
                 "signed",

crates/nu-command/src/bits/shift_left.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 use num_traits::CheckedShl;
 use std::fmt::Display;
@@ -18,6 +18,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits shl")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required("bits", SyntaxShape::Int, "number of bits to shift left")
             .switch(
                 "signed",

crates/nu-command/src/bits/shift_right.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 use num_traits::CheckedShr;
 use std::fmt::Display;
@@ -18,6 +18,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits shr")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required("bits", SyntaxShape::Int, "number of bits to shift right")
             .switch(
                 "signed",

crates/nu-command/src/bits/xor.rs

@@ -2,7 +2,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("bits xor")
+            .input_output_types(vec![(Type::Int, Type::Int)])
+            .vectorizes_over_list(true)
             .required(
                 "target",
                 SyntaxShape::Int,

crates/nu-command/src/bytes/add.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 
 struct Arguments {
     added_data: Vec<u8>,
@@ -30,6 +30,8 @@ impl Command for BytesAdd {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes add")
+            .input_output_types(vec![(Type::Binary, Type::Binary)])
+            .vectorizes_over_list(true)
             .required("data", SyntaxShape::Binary, "the binary to add")
             .named(
                 "index",

crates/nu-command/src/bytes/at.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 use std::cmp::Ordering;
 
@@ -115,6 +115,8 @@ impl Command for BytesAt {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes at")
+            .input_output_types(vec![(Type::Binary, Type::Binary)])
+            .vectorizes_over_list(true)
             .required("range", SyntaxShape::Any, "the indexes to get bytes")
             .rest(
                 "rest",

crates/nu-command/src/bytes/build_.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 #[derive(Clone)]
@@ -24,6 +24,7 @@ impl Command for BytesBuild {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("bytes build")
+            .input_output_types(vec![(Type::Nothing, Type::Binary)])
             .rest("rest", SyntaxShape::Any, "list of bytes")
             .category(Category::Bytes)
     }

crates/nu-command/src/bytes/collect.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 #[derive(Clone, Copy)]
@@ -16,6 +16,7 @@ impl Command for BytesCollect {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes collect")
+            .input_output_types(vec![(Type::List(Box::new(Type::Binary)), Type::Binary)])
             .optional(
                 "separator",
                 SyntaxShape::Binary,

crates/nu-command/src/bytes/ends_with.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 
 struct Arguments {
     pattern: Vec<u8>,
@@ -28,6 +28,7 @@ impl Command for BytesEndsWith {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes ends-with")
+            .input_output_types(vec![(Type::Binary, Type::Bool)])
             .required("pattern", SyntaxShape::Binary, "the pattern to match")
             .rest(
                 "rest",

crates/nu-command/src/bytes/index_of.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::{Call, CellPath};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 struct Arguments {
@@ -29,6 +29,10 @@ impl Command for BytesIndexOf {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes index-of")
+            .input_output_types(vec![
+                (Type::Binary, Type::Int),
+                (Type::Binary, Type::List(Box::new(Type::Int))),
+            ])
             .required(
                 "pattern",
                 SyntaxShape::Binary,

crates/nu-command/src/bytes/length.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct BytesLen;
@@ -16,6 +16,8 @@ impl Command for BytesLen {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes length")
+            .input_output_types(vec![(Type::Binary, Type::Int)])
+            .vectorizes_over_list(true)
             .rest(
                 "rest",
                 SyntaxShape::CellPath,

crates/nu-command/src/bytes/remove.rs

@@ -3,7 +3,8 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type,
+    Value,
 };
 
 struct Arguments {
@@ -29,6 +30,7 @@ impl Command for BytesRemove {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes remove")
+            .input_output_types(vec![(Type::Binary, Type::Binary)])
             .required("pattern", SyntaxShape::Binary, "the pattern to find")
             .rest(
                 "rest",

crates/nu-command/src/bytes/replace.rs

@@ -3,7 +3,8 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type,
+    Value,
 };
 
 struct Arguments {
@@ -29,6 +30,7 @@ impl Command for BytesReplace {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes replace")
+            .input_output_types(vec![(Type::Binary, Type::Binary)])
             .required("find", SyntaxShape::Binary, "the pattern to find")
             .required("replace", SyntaxShape::Binary, "the replacement pattern")
             .rest(

crates/nu-command/src/bytes/reverse.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 
@@ -17,6 +17,7 @@ impl Command for BytesReverse {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes reverse")
+            .input_output_types(vec![(Type::Binary, Type::Binary)])
             .rest(
                 "rest",
                 SyntaxShape::CellPath,

crates/nu-command/src/bytes/starts_with.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
-use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value};
 
 struct Arguments {
     pattern: Vec<u8>,
@@ -28,6 +28,7 @@ impl Command for BytesStartsWith {
 
     fn signature(&self) -> Signature {
         Signature::build("bytes starts-with")
+            .input_output_types(vec![(Type::Binary, Type::Bool)])
             .required("pattern", SyntaxShape::Binary, "the pattern to match")
             .rest(
                 "rest",

crates/nu-command/src/charting/hashable_value.rs

@@ -13,7 +13,7 @@ use std::hash::{Hash, Hasher};
 /// ```text
 /// assert_eq!(HashableValue::Bool {val: true, span: Span{start: 0, end: 1}}, HashableValue::Bool {val: true, span: Span{start: 90, end: 1000}})
 /// ```
-#[derive(Eq, Debug)]
+#[derive(Eq, Debug, Ord, PartialOrd)]
 pub enum HashableValue {
     Bool {
         val: bool,

crates/nu-command/src/charting/histogram.rs

@@ -1,10 +1,11 @@
 use super::hashable_value::HashableValue;
+use itertools::Itertools;
 use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape,
-    Value,
+    Type, Value,
 };
 use std::collections::HashMap;
 use std::iter;
@@ -24,6 +25,7 @@ impl Command for Histogram {
 
     fn signature(&self) -> Signature {
         Signature::build("histogram")
+            .input_output_types(vec![(Type::List(Box::new(Type::Any)), Type::Table(vec![])),])
             .optional("column-name", SyntaxShape::String, "column name to calc frequency, no need to provide if input is just a list")
             .optional("frequency-column-name", SyntaxShape::String, "histogram's frequency column, default to be frequency column output")
             .named("percentage-type", SyntaxShape::String, "percentage calculate method, can be 'normalize' or 'relative', in 'normalize', defaults to be 'normalize'", Some('t'))
@@ -36,23 +38,48 @@ impl Command for Histogram {
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
-                description: "Get a histogram for the types of files",
+                description: "Compute a histogram of file types",
                 example: "ls | histogram type",
                 result: None,
             },
             Example {
                 description:
-                    "Get a histogram for the types of files, with frequency column named freq",
+                    "Compute a histogram for the types of files, with frequency column named freq",
                 example: "ls | histogram type freq",
                 result: None,
             },
             Example {
-                description: "Get a histogram for a list of numbers",
+                description: "Compute a histogram for a list of numbers",
-                example: "echo [1 2 3 1 1 1 2 2 1 1] | histogram",
+                example: "echo [1 2 1] | histogram",
-                result: None,
+                result: Some(Value::List {
+                        vals: vec![Value::Record {
+                            cols: vec!["value".to_string(), "count".to_string(), "quantile".to_string(), "percentage".to_string(), "frequency".to_string()],
+                            vals: vec![
+                                Value::test_int(1),
+                                Value::test_int(2),
+                                Value::test_float(0.6666666666666666),
+                                Value::test_string("66.67%"),
+                                Value::test_string("******************************************************************"),
+                            ],
+                            span: Span::test_data(),
+                        },
+                        Value::Record {
+                            cols: vec!["value".to_string(), "count".to_string(), "quantile".to_string(), "percentage".to_string(), "frequency".to_string()],
+                            vals: vec![
+                                Value::test_int(2),
+                                Value::test_int(1),
+                                Value::test_float(0.3333333333333333),
+                                Value::test_string("33.33%"),
+                                Value::test_string("*********************************"),
+                            ],
+                            span: Span::test_data(),
+                        }],
+                        span: Span::test_data(),
+                    }
+                 ),
             },
             Example {
-                description: "Get a histogram for a list of numbers, and percentage is based on the maximum value",
+                description: "Compute a histogram for a list of numbers, and percentage is based on the maximum value",
                 example: "echo [1 2 3 1 1 1 2 2 1 1] | histogram --percentage-type relative",
                 result: None,
             }
@@ -213,7 +240,7 @@ fn histogram_impl(
         freq_column.to_string(),
     ];
     const MAX_FREQ_COUNT: f64 = 100.0;
-    for (val, count) in counter.into_iter() {
+    for (val, count) in counter.into_iter().sorted() {
         let quantile = match calc_method {
             PercentageCalcMethod::Normalize => count as f64 / total_cnt as f64,
             PercentageCalcMethod::Relative => count as f64 / max_cnt as f64,

crates/nu-command/src/conversions/fmt.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -19,7 +19,9 @@ impl Command for Fmt {
     }
 
     fn signature(&self) -> nu_protocol::Signature {
-        Signature::build("fmt").category(Category::Conversions)
+        Signature::build("fmt")
+            .input_output_types(vec![(Type::Number, Type::Record(vec![]))])
+            .category(Category::Conversions)
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/conversions/into/binary.rs

@@ -4,7 +4,7 @@ use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,16 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into binary")
+            .input_output_types(vec![
+                (Type::Binary, Type::Binary),
+                (Type::Int, Type::Binary),
+                (Type::Number, Type::Binary),
+                (Type::String, Type::Binary),
+                (Type::Bool, Type::Binary),
+                (Type::Filesize, Type::Binary),
+                (Type::Date, Type::Binary),
+            ])
+            .allow_variants_without_examples(true) // TODO: supply exhaustive examples
             .rest(
                 "rest",
                 SyntaxShape::CellPath,

crates/nu-command/src/conversions/into/bool.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,13 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into bool")
+            .input_output_types(vec![
+                (Type::Int, Type::Bool),
+                (Type::Number, Type::Bool),
+                (Type::String, Type::Bool),
+                (Type::Bool, Type::Bool),
+                (Type::List(Box::new(Type::Any)), Type::Table(vec![])),
+            ])
             .rest(
                 "rest",
                 SyntaxShape::CellPath,
@@ -89,6 +96,11 @@ impl Command for SubCommand {
                 example: "1 | into bool",
                 result: Some(Value::boolean(true, span)),
             },
+            Example {
+                description: "convert decimal to boolean",
+                example: "0.3 | into bool",
+                result: Some(Value::boolean(true, span)),
+            },
             Example {
                 description: "convert decimal string to boolean",
                 example: "'0.0' | into bool",

crates/nu-command/src/conversions/into/datetime.rs

@@ -7,7 +7,7 @@ use nu_protocol::ast::CellPath;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 struct Arguments {
@@ -64,7 +64,11 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into datetime")
-            .named(
+        .input_output_types(vec![
+            (Type::Int, Type::Date),
+            (Type::String, Type::Date),
+        ])
+        .named(
                 "timezone",
                 SyntaxShape::String,
                 "Specify timezone if the input is a Unix timestamp. Valid options: 'UTC' ('u') or 'LOCAL' ('l')",

crates/nu-command/src/conversions/into/decimal.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,11 +15,16 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("into decimal").rest(
+        Signature::build("into decimal")
-            "rest",
+            .input_output_types(vec![
-            SyntaxShape::CellPath,
+                (Type::String, Type::Number),
-            "optionally convert text into decimal by column paths",
+                (Type::Bool, Type::Number),
-        )
+            ])
+            .rest(
+                "rest",
+                SyntaxShape::CellPath,
+                "optionally convert text into decimal by column paths",
+            )
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/conversions/into/duration.rs

@@ -3,7 +3,7 @@ use nu_parser::parse_duration_bytes;
 use nu_protocol::{
     ast::{Call, CellPath, Expr},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Unit,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Unit,
     Value,
 };
 
@@ -17,6 +17,13 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into duration")
+            .input_output_types(vec![
+                (Type::String, Type::Duration),
+                (Type::Duration, Type::Duration),
+                // TODO: --convert option should be implemented as `format duration`
+                (Type::String, Type::String),
+                (Type::Duration, Type::String),
+            ])
             .named(
                 "convert",
                 SyntaxShape::String,
@@ -121,11 +128,19 @@ impl Command for SubCommand {
                     span,
                 }),
             },
+            Example {
+                description: "Convert duration to duration",
+                example: "420sec | into duration",
+                result: Some(Value::Duration {
+                    val: 7 * 60 * 1000 * 1000 * 1000,
+                    span,
+                }),
+            },
             Example {
                 description: "Convert duration to the requested duration as a string",
-                example: "420sec | into duration --convert min",
+                example: "420sec | into duration --convert ms",
                 result: Some(Value::String {
-                    val: "7 min".to_string(),
+                    val: "420000 ms".to_string(),
                     span,
                 }),
             },

crates/nu-command/src/conversions/into/filesize.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,12 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into filesize")
+            .input_output_types(vec![
+                (Type::Int, Type::Filesize),
+                (Type::Number, Type::Filesize),
+                (Type::String, Type::Filesize),
+                (Type::Filesize, Type::Filesize),
+            ])
             .rest(
                 "rest",
                 SyntaxShape::CellPath,

crates/nu-command/src/conversions/into/int.rs

@@ -3,7 +3,7 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 struct Arguments {
@@ -28,6 +28,16 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into int")
+            .input_output_types(vec![
+                (Type::String, Type::Int),
+                (Type::Number, Type::Int),
+                (Type::Bool, Type::Int),
+                // Unix timestamp in seconds
+                (Type::Date, Type::Int),
+                // TODO: Users should do this by dividing a Filesize by a Filesize explicitly
+                (Type::Filesize, Type::Int),
+            ])
+            .vectorizes_over_list(true)
             .named("radix", SyntaxShape::Number, "radix of integer", Some('r'))
             .switch("little-endian", "use little-endian byte decoding", None)
             .rest(

crates/nu-command/src/conversions/into/string.rs

@@ -4,7 +4,7 @@ use nu_protocol::{
     ast::{Call, CellPath},
     engine::{Command, EngineState, Stack},
     into_code, Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature,
-    Span, SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 use nu_utils::get_system_locale;
 use num_format::ToFormattedString;
@@ -32,6 +32,16 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("into string")
+            .input_output_types(vec![
+                (Type::Binary, Type::String),
+                (Type::Int, Type::String),
+                (Type::Number, Type::String),
+                (Type::String, Type::String),
+                (Type::Bool, Type::String),
+                (Type::Filesize, Type::String),
+                (Type::Date, Type::String),
+            ])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
             // FIXME - need to support column paths
             .rest(
                 "rest",
@@ -135,11 +145,12 @@ impl Command for SubCommand {
                     span: Span::test_data(),
                 }),
             },
-            Example {
+            // TODO: This should work but does not; see https://github.com/nushell/nushell/issues/7032
-                description: "convert date to string",
+            // Example {
-                example: "date now | into string",
+            //     description: "convert date to string",
-                result: None,
+            //     example: "'2020-10-10 10:00:00 +02:00' | into datetime | into string",
-            },
+            //     result: Some(Value::test_string("Sat Oct 10 10:00:00 2020")),
+            // },
             Example {
                 description: "convert filepath to string",
                 example: "ls Cargo.toml | get name | into string",
@@ -147,8 +158,8 @@ impl Command for SubCommand {
             },
             Example {
                 description: "convert filesize to string",
-                example: "ls Cargo.toml | get size | into string",
+                example: "1KiB | into string",
-                result: None,
+                result: Some(Value::test_string("1,024 B")),
             },
         ]
     }

crates/nu-command/src/core_commands/alias.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct Alias;
@@ -16,6 +16,7 @@ impl Command for Alias {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("alias")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("name", SyntaxShape::String, "name of the alias")
             .required(
                 "initial_value",
@@ -53,12 +54,12 @@ impl Command for Alias {
             Example {
                 description: "Alias ll to ls -l",
                 example: "alias ll = ls -l",
-                result: None,
+                result: Some(Value::nothing(Span::test_data())),
             },
             Example {
                 description: "Make an alias that makes a list of all custom commands",
                 example: "alias customs = ($nu.scope.commands | where is_custom | get command)",
-                result: None,
+                result: Some(Value::nothing(Span::test_data())),
             },
         ]
     }

crates/nu-command/src/core_commands/ast.rs

@@ -3,7 +3,8 @@ use nu_parser::parse;
 use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack, StateWorkingSet},
-    Category, Example, PipelineData, ShellError, Signature, Spanned, SyntaxShape,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +21,7 @@ impl Command for Ast {
 
     fn signature(&self) -> Signature {
         Signature::build("ast")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required(
                 "pipeline",
                 SyntaxShape::String,
@@ -50,17 +52,17 @@ impl Command for Ast {
             Example {
                 description: "Print the ast of a string",
                 example: "ast 'hello'",
-                result: None,
+                result: Some(Value::nothing(Span::test_data())),
             },
             Example {
                 description: "Print the ast of a pipeline",
                 example: "ast 'ls | where name =~ README'",
-                result: None,
+                result: Some(Value::nothing(Span::test_data())),
             },
             Example {
                 description: "Print the ast of a pipeline with an error",
                 example: "ast 'for x in 1..10 { echo $x '",
-                result: None,
+                result: Some(Value::nothing(Span::test_data())),
             },
         ]
     }

crates/nu-command/src/core_commands/commandline.rs

@@ -4,7 +4,7 @@ use nu_protocol::engine::ReplOperation;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
 use nu_protocol::IntoPipelineData;
-use nu_protocol::{PipelineData, ShellError, Signature, SyntaxShape, Value};
+use nu_protocol::{PipelineData, ShellError, Signature, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct Commandline;
@@ -16,6 +16,7 @@ impl Command for Commandline {
 
     fn signature(&self) -> Signature {
         Signature::build("commandline")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .switch(
                 "append",
                 "appends the string to the end of the buffer",

crates/nu-command/src/core_commands/debug.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Value};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Type, Value};
 
 #[derive(Clone)]
 pub struct Debug;
@@ -15,11 +15,17 @@ impl Command for Debug {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("debug").category(Category::Core).switch(
+        Signature::build("debug")
-            "raw",
+            .input_output_types(vec![
-            "Prints the raw value representation",
+                (
-            Some('r'),
+                    Type::List(Box::new(Type::Any)),
-        )
+                    Type::List(Box::new(Type::String)),
+                ),
+                (Type::Table(vec![]), Type::List(Box::new(Type::String))),
+                (Type::Any, Type::String),
+            ])
+            .category(Category::Core)
+            .switch("raw", "Prints the raw value representation", Some('r'))
     }
 
     fn run(
@@ -54,12 +60,20 @@ impl Command for Debug {
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
-                description: "Print the value of a string",
+                description: "Debug print a string",
                 example: "'hello' | debug",
                 result: Some(Value::test_string("hello")),
             },
             Example {
-                description: "Print the value of a table",
+                description: "Debug print a list",
+                example: "['hello'] | debug",
+                result: Some(Value::List {
+                    vals: vec![Value::test_string("hello")],
+                    span: Span::test_data(),
+                }),
+            },
+            Example {
+                description: "Debug print a table",
                 example: "echo [[version patch]; [0.1.0 false] [0.1.1 true] [0.2.0 false]] | debug",
                 result: Some(Value::List {
                     vals: vec![

crates/nu-command/src/core_commands/def.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct Def;
@@ -16,6 +16,7 @@ impl Command for Def {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("def")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .required(

crates/nu-command/src/core_commands/def_env.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct DefEnv;
@@ -16,6 +16,7 @@ impl Command for DefEnv {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("def-env")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .required(

crates/nu-command/src/core_commands/describe.rs

@@ -1,7 +1,7 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,7 +17,9 @@ impl Command for Describe {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("describe").category(Category::Core)
+        Signature::build("describe")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Core)
     }
 
     fn run(

crates/nu-command/src/core_commands/do_.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, ListStream, PipelineData, RawStream, ShellError, Signature, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +20,7 @@ impl Command for Do {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("do")
+            .input_output_types(vec![(Type::Any, Type::Any)])
             .required("block", SyntaxShape::Any, "the block to run")
             .switch(
                 "ignore-errors",
@@ -197,8 +198,13 @@ impl Command for Do {
             },
             Example {
                 description: "Run the block, with a positional parameter",
-                example: r#"do {|x| 100 + $x } 50"#,
+                example: r#"do {|x| 100 + $x } 77"#,
-                result: Some(Value::test_int(150)),
+                result: Some(Value::test_int(177)),
+            },
+            Example {
+                description: "Run the block, with input",
+                example: r#"77 | do {|x| 100 + $in }"#,
+                result: None, // TODO: returns 177
             },
         ]
     }

crates/nu-command/src/core_commands/echo.rs

@@ -2,7 +2,8 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, ListStream, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, ListStream, PipelineData, ShellError, Signature, Span, SyntaxShape, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -19,6 +20,7 @@ impl Command for Echo {
 
     fn signature(&self) -> Signature {
         Signature::build("echo")
+            .input_output_types(vec![(Type::Nothing, Type::Any)])
             .rest("rest", SyntaxShape::Any, "the values to echo")
             .category(Category::Core)
     }

crates/nu-command/src/core_commands/export.rs

@@ -2,7 +2,7 @@ use nu_engine::get_full_help;
 use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
-    Category, Example, IntoPipelineData, PipelineData, Signature, Span, Value,
+    Category, Example, IntoPipelineData, PipelineData, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -14,7 +14,9 @@ impl Command for ExportCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("export").category(Category::Core)
+        Signature::build("export")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .category(Category::Core)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/core_commands/export_alias.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type};
 
 #[derive(Clone)]
 pub struct ExportAlias;
@@ -16,6 +16,7 @@ impl Command for ExportAlias {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("export alias")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("name", SyntaxShape::String, "name of the alias")
             .required(
                 "initial_value",

crates/nu-command/src/core_commands/export_def.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct ExportDef;
@@ -16,6 +16,7 @@ impl Command for ExportDef {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("export def")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .required(

crates/nu-command/src/core_commands/export_def_env.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct ExportDefEnv;
@@ -16,6 +16,7 @@ impl Command for ExportDefEnv {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("export def-env")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .required(

crates/nu-command/src/core_commands/export_extern.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type};
 
 #[derive(Clone)]
 pub struct ExportExtern;
@@ -16,6 +16,7 @@ impl Command for ExportExtern {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("export extern")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .category(Category::Core)

crates/nu-command/src/core_commands/export_use.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct ExportUse;
@@ -16,6 +16,7 @@ impl Command for ExportUse {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("export use")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("pattern", SyntaxShape::ImportPattern, "import pattern")
             .category(Category::Core)
     }

crates/nu-command/src/core_commands/extern_.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type};
 
 #[derive(Clone)]
 pub struct Extern;
@@ -16,6 +16,7 @@ impl Command for Extern {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("extern")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("def_name", SyntaxShape::String, "definition name")
             .required("params", SyntaxShape::Signature, "parameters")
             .category(Category::Core)

crates/nu-command/src/core_commands/for_.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, ListStream, PipelineData, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +20,7 @@ impl Command for For {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("for")
+            .input_output_types(vec![(Type::Nothing, Type::List(Box::new(Type::Any)))])
             .required(
                 "var_name",
                 SyntaxShape::VarWithOptType,

crates/nu-command/src/core_commands/help.rs

@@ -1,4 +1,5 @@
 use fancy_regex::Regex;
+use itertools::Itertools;
 use nu_ansi_term::{
     Color::{Default, Red, White},
     Style,
@@ -9,7 +10,7 @@ use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
     span, Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
-    ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 use std::borrow::Borrow;
 #[derive(Clone)]
@@ -22,6 +23,7 @@ impl Command for Help {
 
     fn signature(&self) -> Signature {
         Signature::build("help")
+            .input_output_types(vec![(Type::Nothing, Type::String)])
             .rest(
                 "rest",
                 SyntaxShape::String,
@@ -162,6 +164,16 @@ fn help(
                     span: head,
                 });
 
+                cols.push("signatures".into());
+                vals.push(Value::String {
+                    val: sig
+                        .input_output_types
+                        .iter()
+                        .map(|(i, o)| format!("{:?} => {:?}", i.to_shape(), o.to_shape()))
+                        .join("\n"),
+                    span: head,
+                });
+
                 cols.push("search_terms".into());
                 vals.push(if search_terms.is_empty() {
                     Value::nothing(head)
@@ -259,6 +271,16 @@ fn help(
                     span: head,
                 });
 
+                cols.push("signatures".into());
+                vals.push(Value::String {
+                    val: sig
+                        .input_output_types
+                        .iter()
+                        .map(|(i, o)| format!("{:?} => {:?}", i.to_shape(), o.to_shape()))
+                        .join("\n"),
+                    span: head,
+                });
+
                 cols.push("search_terms".into());
                 vals.push(if search_terms.is_empty() {
                     Value::nothing(head)

crates/nu-command/src/core_commands/hide.rs

@@ -1,7 +1,7 @@
 use nu_protocol::ast::{Call, Expr, Expression};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -14,6 +14,7 @@ impl Command for Hide {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("hide")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("pattern", SyntaxShape::ImportPattern, "import pattern")
             .category(Category::Core)
     }

crates/nu-command/src/core_commands/hide_env.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     did_you_mean, Category, Example, PipelineData, ShellError, Signature, Span, Spanned,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,7 @@ impl Command for HideEnv {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("hide-env")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .rest(
                 "name",
                 SyntaxShape::String,

crates/nu-command/src/core_commands/if_.rs

@@ -2,7 +2,7 @@ use nu_engine::{eval_block, eval_expression, eval_expression_with_input, CallExt
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, FromValue, PipelineData, ShellError, Signature, SyntaxShape, Value,
+    Category, Example, FromValue, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -19,6 +19,7 @@ impl Command for If {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("if")
+            .input_output_types(vec![(Type::Any, Type::Any)])
             .required("cond", SyntaxShape::Expression, "condition to check")
             .required(
                 "then_block",

crates/nu-command/src/core_commands/ignore.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, Type, Value};
 
 #[derive(Clone)]
 pub struct Ignore;
@@ -15,7 +15,9 @@ impl Command for Ignore {
     }
 
     fn signature(&self) -> nu_protocol::Signature {
-        Signature::build("ignore").category(Category::Core)
+        Signature::build("ignore")
+            .input_output_types(vec![(Type::Any, Type::Nothing)])
+            .category(Category::Core)
     }
 
     fn search_terms(&self) -> Vec<&str> {
@@ -37,7 +39,7 @@ impl Command for Ignore {
         vec![Example {
             description: "Ignore the output of an echo command",
             example: "echo done | ignore",
-            result: None,
+            result: Some(Value::nothing(Span::test_data())),
         }]
     }
 }

crates/nu-command/src/core_commands/let_.rs

@@ -1,7 +1,7 @@
 use nu_engine::eval_expression_with_input;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type};
 
 #[derive(Clone)]
 pub struct Let;
@@ -17,6 +17,8 @@ impl Command for Let {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("let")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
+            .allow_variants_without_examples(true)
             .required("var_name", SyntaxShape::VarWithOptType, "variable name")
             .required(
                 "initial_value",

crates/nu-command/src/core_commands/metadata.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::{Call, Expr, Expression};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, DataSource, Example, IntoPipelineData, PipelineData, PipelineMetadata, Signature,
-    Span, SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +20,8 @@ impl Command for Metadata {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("metadata")
+            .input_output_types(vec![(Type::Nothing, Type::Record(vec![]))])
+            .allow_variants_without_examples(true)
             .optional(
                 "expression",
                 SyntaxShape::Any,

crates/nu-command/src/core_commands/module.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Value};
+use nu_protocol::{Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value};
 
 #[derive(Clone)]
 pub struct Module;
@@ -16,6 +16,7 @@ impl Command for Module {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("module")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("module_name", SyntaxShape::String, "module name")
             .required(
                 "block",

crates/nu-command/src/core_commands/register.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape};
+use nu_protocol::{Category, Example, PipelineData, Signature, SyntaxShape, Type};
 
 #[derive(Clone)]
 pub struct Register;
@@ -16,6 +16,7 @@ impl Command for Register {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("register")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required(
                 "plugin",
                 SyntaxShape::Filepath,

crates/nu-command/src/core_commands/use_.rs

@@ -2,7 +2,7 @@ use nu_engine::{eval_block, find_in_dirs_env, redirect_env};
 use nu_protocol::ast::{Call, Expr, Expression};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -19,6 +19,7 @@ impl Command for Use {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("use")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required("pattern", SyntaxShape::ImportPattern, "import pattern")
             .category(Category::Core)
     }

crates/nu-command/src/core_commands/version.rs

@@ -1,6 +1,6 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Example, IntoPipelineData, PipelineData, ShellError, Signature, Value};
+use nu_protocol::{Example, IntoPipelineData, PipelineData, ShellError, Signature, Type, Value};
 
 pub mod shadow {
     include!(concat!(env!("OUT_DIR"), "/shadow.rs"));
@@ -16,6 +16,8 @@ impl Command for Version {
 
     fn signature(&self) -> Signature {
         Signature::build("version")
+            .input_output_types(vec![(Type::Nothing, Type::Record(vec![]))])
+            .allow_variants_without_examples(true)
     }
 
     fn usage(&self) -> &str {
@@ -215,3 +217,13 @@ fn features_enabled() -> Vec<String> {
 
     names
 }
+
+#[cfg(test)]
+mod test {
+    #[test]
+    fn test_examples() {
+        use super::Version;
+        use crate::test_examples;
+        test_examples(Version {})
+    }
+}

crates/nu-command/src/date/format.rs

@@ -4,7 +4,8 @@ use nu_engine::CallExt;
 use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type,
+    Value,
 };
 use nu_utils::locale::get_system_locale_string;
 use std::fmt::{Display, Write};
@@ -21,6 +22,11 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("date format")
+            .input_output_types(vec![
+                (Type::Date, Type::String),
+                (Type::String, Type::String),
+            ])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
             .switch("list", "lists strftime cheatsheet", Some('l'))
             .optional(
                 "format string",
@@ -66,8 +72,18 @@ impl Command for SubCommand {
 
     fn examples(&self) -> Vec<Example> {
         vec![
+            // TODO: This should work but does not; see https://github.com/nushell/nushell/issues/7032
+            // Example {
+            //     description: "Format a given date-time using the default format (RFC 2822).",
+            //     example: r#"'2021-10-22 20:00:12 +01:00' | into datetime | date format"#,
+            //     result: Some(Value::String {
+            //         val: "Fri, 22 Oct 2021 20:00:12 +0100".to_string(),
+            //         span: Span::test_data(),
+            //     }),
+            // },
             Example {
-                description: "Format a given date using the default format (RFC 2822).",
+                description:
+                    "Format a given date-time as a string using the default format (RFC 2822).",
                 example: r#""2021-10-22 20:00:12 +01:00" | date format"#,
                 result: Some(Value::String {
                     val: "Fri, 22 Oct 2021 20:00:12 +0100".to_string(),
@@ -75,13 +91,13 @@ impl Command for SubCommand {
                 }),
             },
             Example {
-                description: "Format a given date using a given format string.",
+                description: "Format the current date-time using a given format string.",
-                example: "date format '%Y-%m-%d'",
+                example: r#"date now | date format "%Y-%m-%d %H:%M:%S""#,
                 result: None,
             },
             Example {
-                description: "Format a given date using a given format string.",
+                description: "Format the current date using a given format string.",
-                example: r#"date format "%Y-%m-%d %H:%M:%S""#,
+                example: r#"date now | date format "%Y-%m-%d %H:%M:%S""#,
                 result: None,
             },
             Example {

crates/nu-command/src/date/humanize.rs

@@ -3,7 +3,7 @@ use chrono::{DateTime, FixedOffset, Local};
 use chrono_humanize::HumanTime;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Value};
+use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, Span, Type, Value};
 #[derive(Clone)]
 pub struct SubCommand;
 
@@ -13,7 +13,13 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("date humanize").category(Category::Date)
+        Signature::build("date humanize")
+            .input_output_types(vec![
+                (Type::Date, Type::String),
+                (Type::String, Type::String),
+            ])
+            .allow_variants_without_examples(true)
+            .category(Category::Date)
     }
 
     fn usage(&self) -> &str {
@@ -45,21 +51,11 @@ impl Command for SubCommand {
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![
+        vec![Example {
-            Example {
+            description: "Print a 'humanized' format for the date, relative to now.",
-                description: "Print a 'humanized' format for the date, relative to now.",
+            example: r#""2021-10-22 20:00:12 +01:00" | date humanize"#,
-                example: "date humanize",
+            result: None,
-                result: Some(Value::String {
+        }]
-                    val: "now".to_string(),
-                    span: Span::test_data(),
-                }),
-            },
-            Example {
-                description: "Print a 'humanized' format for the date, relative to now.",
-                example: r#""2021-10-22 20:00:12 +01:00" | date humanize"#,
-                result: None,
-            },
-        ]
     }
 }
 

crates/nu-command/src/date/list_timezone.rs

@@ -2,7 +2,7 @@ use chrono_tz::TZ_VARIANTS;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoInterruptiblePipelineData, PipelineData, Signature, Span, Value,
+    Category, Example, IntoInterruptiblePipelineData, PipelineData, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -14,7 +14,9 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("date list-timezone").category(Category::Date)
+        Signature::build("date list-timezone")
+            .input_output_types(vec![(Type::Nothing, Type::Table(vec![]))])
+            .category(Category::Date)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/date/now.rs

@@ -1,7 +1,7 @@
 use chrono::Local;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, IntoPipelineData, PipelineData, Signature, Value};
+use nu_protocol::{Category, Example, IntoPipelineData, PipelineData, Signature, Type, Value};
 #[derive(Clone)]
 pub struct SubCommand;
 
@@ -11,7 +11,9 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("date now").category(Category::Date)
+        Signature::build("date now")
+            .input_output_types(vec![(Type::Nothing, Type::Date)])
+            .category(Category::Date)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/date/to_record.rs

@@ -2,6 +2,7 @@ use crate::date::utils::parse_date_from_string;
 use chrono::{DateTime, Datelike, FixedOffset, Local, Timelike};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::Type;
 use nu_protocol::{
     Category, Example, PipelineData, ShellError::DatetimeParseError, Signature, Span, Value,
 };
@@ -15,11 +16,17 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("date to-record").category(Category::Date)
+        Signature::build("date to-record")
+            .input_output_types(vec![
+                (Type::Date, Type::Record(vec![])),
+                (Type::String, Type::Record(vec![])),
+            ])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
+            .category(Category::Date)
     }
 
     fn usage(&self) -> &str {
-        "Convert the date into a structured table."
+        "Convert the date into a record."
     }
 
     fn search_terms(&self) -> Vec<&str> {
@@ -38,46 +45,54 @@ impl Command for SubCommand {
     }
 
     fn examples(&self) -> Vec<Example> {
+        let example_result_1 = || {
+            let span = Span::test_data();
+            let cols = vec![
+                "year".into(),
+                "month".into(),
+                "day".into(),
+                "hour".into(),
+                "minute".into(),
+                "second".into(),
+                "timezone".into(),
+            ];
+            let vals = vec![
+                Value::Int { val: 2020, span },
+                Value::Int { val: 4, span },
+                Value::Int { val: 12, span },
+                Value::Int { val: 22, span },
+                Value::Int { val: 10, span },
+                Value::Int { val: 57, span },
+                Value::String {
+                    val: "+02:00".to_string(),
+                    span,
+                },
+            ];
+            Some(Value::Record { cols, vals, span })
+        };
+
         vec![
             Example {
-                description: "Convert the current date into a structured table.",
+                description: "Convert the current date into a record.",
-                example: "date to-table",
+                example: "date to-record",
                 result: None,
             },
             Example {
-                description: "Convert the current date into a structured table.",
+                description: "Convert the current date into a record.",
                 example: "date now | date to-record",
                 result: None,
             },
             Example {
-                description: "Convert a given date into a structured table.",
+                description: "Convert a date string into a record.",
-                example: " '2020-04-12 22:10:57 +0200' | date to-record",
+                example: "'2020-04-12 22:10:57 +0200' | date to-record",
-                result: {
+                result: example_result_1(),
-                    let span = Span::test_data();
-                    let cols = vec![
-                        "year".into(),
-                        "month".into(),
-                        "day".into(),
-                        "hour".into(),
-                        "minute".into(),
-                        "second".into(),
-                        "timezone".into(),
-                    ];
-                    let vals = vec![
-                        Value::Int { val: 2020, span },
-                        Value::Int { val: 4, span },
-                        Value::Int { val: 12, span },
-                        Value::Int { val: 22, span },
-                        Value::Int { val: 10, span },
-                        Value::Int { val: 57, span },
-                        Value::String {
-                            val: "+02:00".to_string(),
-                            span,
-                        },
-                    ];
-                    Some(Value::Record { cols, vals, span })
-                },
             },
+            // TODO: This should work but does not; see https://github.com/nushell/nushell/issues/7032
+            // Example {
+            //     description: "Convert a date into a record.",
+            //     example: "'2020-04-12 22:10:57 +0200' | into datetime | date to-record",
+            //     result: example_result_1(),
+            // },
         ]
     }
 }

crates/nu-command/src/date/to_table.rs

@@ -2,6 +2,7 @@ use crate::date::utils::parse_date_from_string;
 use chrono::{DateTime, Datelike, FixedOffset, Local, Timelike};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
+use nu_protocol::Type;
 use nu_protocol::{
     Category, Example, PipelineData, ShellError::DatetimeParseError, Signature, Span, Value,
 };
@@ -15,7 +16,13 @@ impl Command for SubCommand {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("date to-table").category(Category::Date)
+        Signature::build("date to-table")
+            .input_output_types(vec![
+                (Type::Date, Type::Table(vec![])),
+                (Type::String, Type::Table(vec![])),
+            ])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
+            .category(Category::Date)
     }
 
     fn usage(&self) -> &str {
@@ -38,49 +45,57 @@ impl Command for SubCommand {
     }
 
     fn examples(&self) -> Vec<Example> {
+        let example_result_1 = || {
+            let span = Span::test_data();
+            let cols = vec![
+                "year".into(),
+                "month".into(),
+                "day".into(),
+                "hour".into(),
+                "minute".into(),
+                "second".into(),
+                "timezone".into(),
+            ];
+            let vals = vec![
+                Value::Int { val: 2020, span },
+                Value::Int { val: 4, span },
+                Value::Int { val: 12, span },
+                Value::Int { val: 22, span },
+                Value::Int { val: 10, span },
+                Value::Int { val: 57, span },
+                Value::String {
+                    val: "+02:00".to_string(),
+                    span,
+                },
+            ];
+            Some(Value::List {
+                vals: vec![Value::Record { cols, vals, span }],
+                span,
+            })
+        };
+
         vec![
             Example {
-                description: "Convert the date into a structured table.",
+                description: "Convert the current date into a table.",
                 example: "date to-table",
                 result: None,
             },
             Example {
-                description: "Convert the date into a structured table.",
+                description: "Convert the date into a table.",
                 example: "date now | date to-table",
                 result: None,
             },
             Example {
-                description: "Convert a given date into a structured table.",
+                description: "Convert a given date into a table.",
-                example: " '2020-04-12 22:10:57 +0200' | date to-table",
+                example: "'2020-04-12 22:10:57 +0200' | date to-table",
-                result: {
+                result: example_result_1(),
-                    let span = Span::test_data();
-                    let cols = vec![
-                        "year".into(),
-                        "month".into(),
-                        "day".into(),
-                        "hour".into(),
-                        "minute".into(),
-                        "second".into(),
-                        "timezone".into(),
-                    ];
-                    let vals = vec![
-                        Value::Int { val: 2020, span },
-                        Value::Int { val: 4, span },
-                        Value::Int { val: 12, span },
-                        Value::Int { val: 22, span },
-                        Value::Int { val: 10, span },
-                        Value::Int { val: 57, span },
-                        Value::String {
-                            val: "+02:00".to_string(),
-                            span,
-                        },
-                    ];
-                    Some(Value::List {
-                        vals: vec![Value::Record { cols, vals, span }],
-                        span,
-                    })
-                },
             },
+            // TODO: This should work but does not; see https://github.com/nushell/nushell/issues/7032
+            // Example {
+            //     description: "Convert a given date into a table.",
+            //     example: "'2020-04-12 22:10:57 +0200' | into datetime | date to-table",
+            //     result: example_result_1(),
+            // },
         ]
     }
 }

crates/nu-command/src/date/to_timezone.rs

@@ -5,7 +5,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 use chrono::{FixedOffset, TimeZone};
@@ -20,6 +20,8 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("date to-timezone")
+            .input_output_types(vec![(Type::Date, Type::Date), (Type::String, Type::Date)])
+            .allow_variants_without_examples(true) // https://github.com/nushell/nushell/issues/7032
             .required("time zone", SyntaxShape::String, "time zone description")
             .category(Category::Date)
     }
@@ -62,6 +64,16 @@ impl Command for SubCommand {
     }
 
     fn examples(&self) -> Vec<Example> {
+        let example_result_1 = || {
+            let dt = FixedOffset::east(5 * 3600)
+                .ymd(2020, 10, 10)
+                .and_hms(13, 00, 00);
+            Some(Value::Date {
+                val: dt,
+                span: Span::test_data(),
+            })
+        };
+
         vec![
             Example {
                 description: "Get the current date in UTC+05:00",
@@ -81,19 +93,14 @@ impl Command for SubCommand {
             Example {
                 description: "Get the current date in Hawaii",
                 example: r#""2020-10-10 10:00:00 +02:00" | date to-timezone "+0500""#,
-                // result: None
+                result: example_result_1(),
-                // The following should be the result of the test, but it fails. Cannot figure it out why.
-                result: {
-                    let dt = FixedOffset::east(5 * 3600)
-                        .ymd(2020, 10, 10)
-                        .and_hms(13, 00, 00);
-
-                    Some(Value::Date {
-                        val: dt,
-                        span: Span::test_data(),
-                    })
-                },
             },
+            // TODO: This should work but does not; see https://github.com/nushell/nushell/issues/7032
+            // Example {
+            //     description: "Get the current date in Hawaii, from a datetime object",
+            //     example: r#""2020-10-10 10:00:00 +02:00" | into datetime | date to-timezone "+0500""#,
+            //     result: example_result_1(),
+            // },
         ]
     }
 }

crates/nu-command/src/env/export_env.rs

@@ -2,7 +2,7 @@ use nu_engine::{eval_block, redirect_env, CallExt};
 use nu_protocol::{
     ast::Call,
     engine::{CaptureBlock, Command, EngineState, Stack},
-    Category, Example, PipelineData, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,7 @@ impl Command for ExportEnv {
 
     fn signature(&self) -> Signature {
         Signature::build("export-env")
+            .input_output_types(vec![(Type::Nothing, Type::Nothing)])
             .required(
                 "block",
                 SyntaxShape::Block(Some(vec![])),
@@ -53,11 +54,20 @@ impl Command for ExportEnv {
     }
 
     fn examples(&self) -> Vec<Example> {
-        vec![Example {
+        vec![
-            description: "Set an environment",
+            Example {
-            example: r#"export-env { let-env SPAM = 'eggs' }; $env.SPAM"#,
+                description: "Set an environment variable",
-            result: Some(Value::string("eggs", Span::test_data())),
+                example: r#"export-env { let-env SPAM = 'eggs' }"#,
-        }]
+                result: Some(Value::Nothing {
+                    span: Span::test_data(),
+                }),
+            },
+            Example {
+                description: "Set an environment variable and examine its value",
+                example: r#"export-env { let-env SPAM = 'eggs' }; $env.SPAM"#,
+                result: Some(Value::string("eggs", Span::test_data())),
+            },
+        ]
     }
 }
 

crates/nu-command/src/env/with_env.rs

@@ -4,7 +4,7 @@ use nu_engine::{eval_block, CallExt};
 use nu_protocol::{
     ast::Call,
     engine::{CaptureBlock, Command, EngineState, Stack},
-    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,7 @@ impl Command for WithEnv {
 
     fn signature(&self) -> Signature {
         Signature::build("with-env")
+            .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "variable",
                 SyntaxShape::Any,

crates/nu-command/src/example_test.rs

@@ -1,75 +1,206 @@
 #[cfg(test)]
-use nu_engine::eval_block;
+use nu_protocol::engine::Command;
-#[cfg(test)]
-use nu_parser::parse;
-#[cfg(test)]
-use nu_protocol::{
-    engine::{Command, EngineState, Stack, StateWorkingSet},
-    PipelineData, Span, Value,
-};
-
-#[cfg(test)]
-use crate::To;
-
-#[cfg(test)]
-use super::{
-    Ansi, Date, From, If, Into, LetEnv, Math, Path, Random, Split, SplitColumn, SplitRow, Str,
-    StrJoin, StrLength, StrReplace, Url, Wrap,
-};
 
 #[cfg(test)]
 pub fn test_examples(cmd: impl Command + 'static) {
-    use crate::BuildString;
+    test_examples::test_examples(cmd);
+}
 
-    let examples = cmd.examples();
+#[cfg(test)]
-    let mut engine_state = Box::new(EngineState::new());
+mod test_examples {
-
+    use super::super::{
-    let delta = {
+        Ansi, BuildString, Date, Echo, From, If, Into, LetEnv, Math, Path, Random, Split,
-        // Base functions that are needed for testing
+        SplitColumn, SplitRow, Str, StrJoin, StrLength, StrReplace, Url, Wrap,
-        // Try to keep this working set small to keep tests running as fast as possible
-        let mut working_set = StateWorkingSet::new(&*engine_state);
-        working_set.add_decl(Box::new(Str));
-        working_set.add_decl(Box::new(StrJoin));
-        working_set.add_decl(Box::new(StrLength));
-        working_set.add_decl(Box::new(StrReplace));
-        working_set.add_decl(Box::new(BuildString));
-        working_set.add_decl(Box::new(From));
-        working_set.add_decl(Box::new(If));
-        working_set.add_decl(Box::new(To));
-        working_set.add_decl(Box::new(Into));
-        working_set.add_decl(Box::new(Random));
-        working_set.add_decl(Box::new(Split));
-        working_set.add_decl(Box::new(SplitColumn));
-        working_set.add_decl(Box::new(SplitRow));
-        working_set.add_decl(Box::new(Math));
-        working_set.add_decl(Box::new(Path));
-        working_set.add_decl(Box::new(Date));
-        working_set.add_decl(Box::new(Url));
-        working_set.add_decl(Box::new(Ansi));
-        working_set.add_decl(Box::new(Wrap));
-        working_set.add_decl(Box::new(LetEnv));
-
-        use super::Echo;
-        working_set.add_decl(Box::new(Echo));
-        // Adding the command that is being tested to the working set
-        working_set.add_decl(Box::new(cmd));
-
-        working_set.render()
     };
+    use crate::To;
+    use itertools::Itertools;
+    use nu_engine;
+    use nu_parser;
+    use nu_protocol::{
+        ast::Block,
+        engine::{Command, EngineState, Stack, StateDelta, StateWorkingSet},
+        Example, PipelineData, Signature, Span, Type, Value,
+    };
+    use std::{collections::HashSet, path::PathBuf};
 
-    let cwd = std::env::current_dir().expect("Could not get current working directory.");
+    pub fn test_examples(cmd: impl Command + 'static) {
+        let examples = cmd.examples();
+        let signature = cmd.signature();
+        let mut engine_state = make_engine_state(cmd.clone_box());
 
-    engine_state
+        let cwd = std::env::current_dir().expect("Could not get current working directory.");
-        .merge_delta(delta)
-        .expect("Error merging delta");
 
-    for example in examples {
+        let mut witnessed_type_transformations = HashSet::<(Type, Type)>::new();
-        // Skip tests that don't have results to compare to
+
-        if example.result.is_none() {
+        for example in examples {
-            continue;
+            if example.result.is_none() {
+                continue;
+            }
+            witnessed_type_transformations.extend(
+                check_example_input_and_output_types_match_command_signature(
+                    &example,
+                    &cwd,
+                    &mut make_engine_state(cmd.clone_box()),
+                    &signature.input_output_types,
+                    signature.operates_on_cell_paths(),
+                    signature.vectorizes_over_list,
+                ),
+            );
+            check_example_evaluates_to_expected_output(&example, &cwd, &mut engine_state);
         }
-        let start = std::time::Instant::now();
 
+        check_all_signature_input_output_types_entries_have_examples(
+            signature,
+            witnessed_type_transformations,
+        );
+    }
+
+    fn make_engine_state(cmd: Box<dyn Command>) -> Box<EngineState> {
+        let mut engine_state = Box::new(EngineState::new());
+
+        let delta = {
+            // Base functions that are needed for testing
+            // Try to keep this working set small to keep tests running as fast as possible
+            let mut working_set = StateWorkingSet::new(&*engine_state);
+            working_set.add_decl(Box::new(Str));
+            working_set.add_decl(Box::new(StrJoin));
+            working_set.add_decl(Box::new(StrLength));
+            working_set.add_decl(Box::new(StrReplace));
+            working_set.add_decl(Box::new(BuildString));
+            working_set.add_decl(Box::new(From));
+            working_set.add_decl(Box::new(If));
+            working_set.add_decl(Box::new(To));
+            working_set.add_decl(Box::new(Into));
+            working_set.add_decl(Box::new(Random));
+            working_set.add_decl(Box::new(Split));
+            working_set.add_decl(Box::new(SplitColumn));
+            working_set.add_decl(Box::new(SplitRow));
+            working_set.add_decl(Box::new(Math));
+            working_set.add_decl(Box::new(Path));
+            working_set.add_decl(Box::new(Date));
+            working_set.add_decl(Box::new(Url));
+            working_set.add_decl(Box::new(Ansi));
+            working_set.add_decl(Box::new(Wrap));
+            working_set.add_decl(Box::new(LetEnv));
+            working_set.add_decl(Box::new(Echo));
+            // Adding the command that is being tested to the working set
+            working_set.add_decl(cmd);
+
+            working_set.render()
+        };
+
+        engine_state
+            .merge_delta(delta)
+            .expect("Error merging delta");
+        engine_state
+    }
+
+    fn check_example_input_and_output_types_match_command_signature<'a>(
+        example: &Example,
+        cwd: &PathBuf,
+        engine_state: &mut Box<EngineState>,
+        signature_input_output_types: &Vec<(Type, Type)>,
+        signature_operates_on_cell_paths: bool,
+        signature_vectorizes_over_list: bool,
+    ) -> HashSet<(Type, Type)> {
+        let mut witnessed_type_transformations = HashSet::<(Type, Type)>::new();
+
+        // Skip tests that don't have results to compare to
+        if let Some(example_output) = example.result.as_ref() {
+            if let Some(example_input_type) =
+                eval_pipeline_without_terminal_expression(example.example, &cwd, engine_state)
+            {
+                let example_input_type = example_input_type.get_type();
+                let example_output_type = example_output.get_type();
+
+                let example_matches_signature =
+                    signature_input_output_types
+                        .iter()
+                        .any(|(sig_in_type, sig_out_type)| {
+                            example_input_type.is_subtype(sig_in_type)
+                                && example_output_type.is_subtype(sig_out_type)
+                                && {
+                                    witnessed_type_transformations
+                                        .insert((sig_in_type.clone(), sig_out_type.clone()));
+                                    true
+                                }
+                        });
+
+                // The example type checks as vectorization over an input list if both:
+                // 1. The command is declared to vectorize over list input.
+                // 2. There exists an entry t -> u in the type map such that the
+                //    example_input_type is a subtype of list<t> and the
+                //    example_output_type is a subtype of list<u>.
+                let example_matches_signature_via_vectorization_over_list =
+                    signature_vectorizes_over_list
+                        && match &example_input_type {
+                            Type::List(ex_in_type) => {
+                                match signature_input_output_types.iter().find_map(
+                                    |(sig_in_type, sig_out_type)| {
+                                        if ex_in_type.is_subtype(sig_in_type) {
+                                            Some((sig_in_type, sig_out_type))
+                                        } else {
+                                            None
+                                        }
+                                    },
+                                ) {
+                                    Some((sig_in_type, sig_out_type)) => match &example_output_type
+                                    {
+                                        Type::List(ex_out_type)
+                                            if ex_out_type.is_subtype(sig_out_type) =>
+                                        {
+                                            witnessed_type_transformations.insert((
+                                                sig_in_type.clone(),
+                                                sig_out_type.clone(),
+                                            ));
+                                            true
+                                        }
+                                        _ => false,
+                                    },
+                                    None => false,
+                                }
+                            }
+                            _ => false,
+                        };
+
+                // The example type checks as a cell path operation if both:
+                // 1. The command is declared to operate on cell paths.
+                // 2. The example_input_type is list or record or table, and the example
+                //    output shape is the same as the input shape.
+                let example_matches_signature_via_cell_path_operation =
+                    signature_operates_on_cell_paths
+                        && example_input_type.accepts_cell_paths()
+                        // TODO: This is too permissive; it should make use of the signature.input_output_types at least.
+                        && example_output_type.to_shape() == example_input_type.to_shape();
+
+                if !(example_matches_signature
+                    || example_matches_signature_via_vectorization_over_list
+                    || example_matches_signature_via_cell_path_operation)
+                {
+                    assert!(
+                false,
+                "The example `{}` demonstrates a transformation of type {:?} -> {:?}. \
+                However, this does not match the declared signature: {:?}.{} \
+                For this command, `vectorizes_over_list` is {} and `operates_on_cell_paths()` is {}.",
+                example.example,
+                example_input_type,
+                example_output_type,
+                signature_input_output_types,
+                if signature_input_output_types.is_empty() { " (Did you forget to declare the input and output types for the command?)" } else { "" },
+                signature_vectorizes_over_list,
+                signature_operates_on_cell_paths
+            );
+                };
+            };
+        }
+        witnessed_type_transformations
+    }
+
+    fn check_example_evaluates_to_expected_output(
+        example: &Example,
+        cwd: &PathBuf,
+        engine_state: &mut Box<EngineState>,
+    ) {
         let mut stack = Stack::new();
 
         // Set up PWD
@@ -85,30 +216,82 @@ pub fn test_examples(cmd: impl Command + 'static) {
             .merge_env(&mut stack, &cwd)
             .expect("Error merging environment");
 
-        let (block, delta) = {
+        let empty_input = PipelineData::new(Span::test_data());
-            let mut working_set = StateWorkingSet::new(&*engine_state);
+        let result = eval(example.example, empty_input, &cwd, engine_state);
-            let (output, err) = parse(
+
-                &mut working_set,
+        // Note. Value implements PartialEq for Bool, Int, Float, String and Block
-                None,
+        // If the command you are testing requires to compare another case, then
-                example.example.as_bytes(),
+        // you need to define its equality in the Value struct
-                false,
+        if let Some(expected) = example.result.as_ref() {
-                &[],
+            assert_eq!(
+                &result, expected,
+                "The example result differs from the expected value",
+            )
+        }
+    }
+
+    fn check_all_signature_input_output_types_entries_have_examples(
+        signature: Signature,
+        witnessed_type_transformations: HashSet<(Type, Type)>,
+    ) {
+        let declared_type_transformations =
+            HashSet::from_iter(signature.input_output_types.into_iter());
+        assert!(
+            witnessed_type_transformations.is_subset(&declared_type_transformations),
+            "This should not be possible (bug in test): the type transformations \
+            collected in the course of matching examples to the signature type map \
+            contain type transformations not present in the signature type map."
+        );
+
+        if !signature.allow_variants_without_examples {
+            assert_eq!(
+                witnessed_type_transformations,
+                declared_type_transformations,
+                "There are entries in the signature type map which do not correspond to any example: \
+                {:?}",
+                declared_type_transformations
+                    .difference(&witnessed_type_transformations)
+                    .map(|(s1, s2)| format!("{} -> {}", s1, s2))
+                    .join(", ")
             );
+        }
+    }
 
-            if let Some(err) = err {
+    fn eval(
-                panic!("test parse error in `{}`: {:?}", example.example, err)
+        contents: &str,
-            }
+        input: PipelineData,
+        cwd: &PathBuf,
+        engine_state: &mut Box<EngineState>,
+    ) -> Value {
+        let (block, delta) = parse(contents, engine_state);
+        eval_block(block, input, cwd, engine_state, delta)
+    }
 
-            (output, working_set.render())
+    fn parse(contents: &str, engine_state: &Box<EngineState>) -> (Block, StateDelta) {
-        };
+        let mut working_set = StateWorkingSet::new(&*engine_state);
+        let (output, err) =
+            nu_parser::parse(&mut working_set, None, contents.as_bytes(), false, &[]);
 
+        if let Some(err) = err {
+            panic!("test parse error in `{}`: {:?}", contents, err)
+        }
+
+        (output, working_set.render())
+    }
+
+    fn eval_block(
+        block: Block,
+        input: PipelineData,
+        cwd: &PathBuf,
+        engine_state: &mut Box<EngineState>,
+        delta: StateDelta,
+    ) -> Value {
         engine_state
             .merge_delta(delta)
             .expect("Error merging delta");
 
         let mut stack = Stack::new();
 
-        // Set up PWD
         stack.add_env_var(
             "PWD".to_string(),
             Value::String {
@@ -117,33 +300,34 @@ pub fn test_examples(cmd: impl Command + 'static) {
             },
         );
 
-        match eval_block(
+        match nu_engine::eval_block(&engine_state, &mut stack, &block, input, true, true) {
-            &engine_state,
+            Err(err) => panic!("test eval error in `{}`: {:?}", "TODO", err),
-            &mut stack,
+            Ok(result) => result.into_value(Span::test_data()),
-            &block,
+        }
-            PipelineData::new(Span::test_data()),
+    }
-            true,
-            true,
-        ) {
-            Err(err) => panic!("test eval error in `{}`: {:?}", example.example, err),
-            Ok(result) => {
-                let result = result.into_value(Span::test_data());
-                println!("input: {}", example.example);
-                println!("result: {:?}", result);
-                println!("done: {:?}", start.elapsed());
 
-                // Note. Value implements PartialEq for Bool, Int, Float, String and Block
+    fn eval_pipeline_without_terminal_expression(
-                // If the command you are testing requires to compare another case, then
+        src: &str,
-                // you need to define its equality in the Value struct
+        cwd: &PathBuf,
-                if let Some(expected) = example.result {
+        engine_state: &mut Box<EngineState>,
-                    if result != expected {
+    ) -> Option<Value> {
-                        panic!(
+        let (mut block, delta) = parse(src, engine_state);
-                            "the example result is different to expected value: {:?} != {:?}",
+        match block.pipelines.len() {
-                            result, expected
+            1 => {
-                        )
+                let n_expressions = block.pipelines[0].expressions.len();
-                    }
+                block.pipelines[0].expressions.truncate(&n_expressions - 1);
+
+                if !block.pipelines[0].expressions.is_empty() {
+                    let empty_input = PipelineData::new(Span::test_data());
+                    Some(eval_block(block, empty_input, &cwd, engine_state, delta))
+                } else {
+                    Some(Value::nothing(Span::test_data()))
                 }
             }
+            _ => {
+                // E.g. multiple semicolon-separated statements
+                None
+            }
         }
     }
 }

crates/nu-command/src/filters/all.rs

@@ -2,7 +2,8 @@ use nu_engine::{eval_block, CallExt};
 use nu_protocol::{
     ast::Call,
     engine::{CaptureBlock, Command, EngineState, Stack},
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +16,10 @@ impl Command for All {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
+            .input_output_types(vec![
+                (Type::List(Box::new(Type::Any)), Type::Bool),
+                (Type::Table(vec![]), Type::Bool),
+            ])
             .required(
                 "predicate",
                 SyntaxShape::RowCondition,

crates/nu-command/src/filters/any.rs

@@ -2,7 +2,8 @@ use nu_engine::{eval_block, CallExt};
 use nu_protocol::{
     ast::Call,
     engine::{CaptureBlock, Command, EngineState, Stack},
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +16,10 @@ impl Command for Any {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
+            .input_output_types(vec![
+                (Type::List(Box::new(Type::Any)), Type::Bool),
+                (Type::Table(vec![]), Type::Bool),
+            ])
             .required(
                 "predicate",
                 SyntaxShape::RowCondition,

crates/nu-command/src/filters/append.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,10 @@ impl Command for Append {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("append")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required("row", SyntaxShape::Any, "the row, list, or table to append")
             .category(Category::Filters)
     }

crates/nu-command/src/filters/collect.rs

@@ -2,7 +2,7 @@ use nu_engine::{eval_block, redirect_env, CallExt};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, Signature, SyntaxShape, Value,
+    Category, Example, IntoPipelineData, PipelineData, Signature, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,7 @@ impl Command for Collect {
 
     fn signature(&self) -> Signature {
         Signature::build("collect")
+            .input_output_types(vec![(Type::List(Box::new(Type::Any)), Type::Any)])
             .required(
                 "block",
                 SyntaxShape::Block(Some(vec![SyntaxShape::Any])),

crates/nu-command/src/filters/columns.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
-    Signature, Span, Value,
+    Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,7 +15,12 @@ impl Command for Columns {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build(self.name()).category(Category::Filters)
+        Signature::build(self.name())
+            .input_output_types(vec![(
+                Type::Table(vec![]),
+                Type::List(Box::new(Type::String)),
+            )])
+            .category(Category::Filters)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/filters/compact.rs

@@ -1,7 +1,7 @@
 use nu_engine::CallExt;
 use nu_protocol::{
     ast::Call, engine::Command, engine::EngineState, engine::Stack, Category, Example,
-    PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -14,6 +14,19 @@ impl Command for Compact {
 
     fn signature(&self) -> Signature {
         Signature::build("compact")
+            .input_output_types(vec![
+                (
+                    Type::List(Box::new(Type::Any)),
+                    Type::List(Box::new(Type::Any)),
+                ),
+                (Type::Table(vec![]), Type::Table(vec![])),
+                (
+                    // TODO: Should table be a subtype of List<Any>? If so then this
+                    // entry would be unnecessary.
+                    Type::Table(vec![]),
+                    Type::List(Box::new(Type::Any)),
+                ),
+            ])
             .rest(
                 "columns",
                 SyntaxShape::Any,

crates/nu-command/src/filters/default.rs

@@ -1,7 +1,9 @@
 use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, Signature, Span, Spanned, SyntaxShape, Value};
+use nu_protocol::{
+    Category, Example, PipelineData, Signature, Span, Spanned, SyntaxShape, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct Default;
@@ -13,6 +15,9 @@ impl Command for Default {
 
     fn signature(&self) -> Signature {
         Signature::build("default")
+            // TODO: Give more specific type signature?
+            // TODO: Declare usage of cell paths in signature? (It seems to behave as if it uses cell paths)
+            .input_output_types(vec![(Type::Any, Type::Any)])
             .required(
                 "default value",
                 SyntaxShape::Any,

crates/nu-command/src/filters/drop/column.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::{Call, CellPath};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, FromValue, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
-    ShellError, Signature, Span, SyntaxShape, Value,
+    ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,7 @@ impl Command for DropColumn {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
+            .input_output_types(vec![(Type::Table(vec![]), Type::Table(vec![]))])
             .optional(
                 "columns",
                 SyntaxShape::Int,
@@ -25,7 +26,7 @@ impl Command for DropColumn {
     }
 
     fn usage(&self) -> &str {
-        "Remove the last number of columns. If you want to remove columns by name, try 'reject'."
+        "Remove N columns at the right-hand end of the input table. To remove columns by name, use 'reject'."
     }
 
     fn search_terms(&self) -> Vec<&str> {

crates/nu-command/src/filters/drop/drop_.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,16 +17,19 @@ impl Command for Drop {
 
     fn signature(&self) -> Signature {
         Signature::build("drop")
-            .optional(
+            .input_output_types(vec![
-                "rows",
+                (Type::Table(vec![]), Type::Table(vec![])),
-                SyntaxShape::Int,
+                (
-                "starting from the back, the number of rows to remove",
+                    Type::List(Box::new(Type::Any)),
-            )
+                    Type::List(Box::new(Type::Any)),
+                ),
+            ])
+            .optional("rows", SyntaxShape::Int, "The number of items to remove")
             .category(Category::Filters)
     }
 
     fn usage(&self) -> &str {
-        "Remove the last several rows of the input. Counterpart of 'skip'. Opposite of 'last'."
+        "Remove items/rows from the end of the input list/table. Counterpart of 'skip'. Opposite of 'last'."
     }
 
     fn search_terms(&self) -> Vec<&str> {
@@ -37,7 +40,7 @@ impl Command for Drop {
         vec![
             Example {
                 example: "[0,1,2,3] | drop",
-                description: "Remove the last item of a list/table",
+                description: "Remove the last item of a list",
                 result: Some(Value::List {
                     vals: vec![Value::test_int(0), Value::test_int(1), Value::test_int(2)],
                     span: Span::test_data(),
@@ -45,7 +48,7 @@ impl Command for Drop {
             },
             Example {
                 example: "[0,1,2,3] | drop 0",
-                description: "Remove zero item of a list/table",
+                description: "Remove zero item of a list",
                 result: Some(Value::List {
                     vals: vec![
                         Value::test_int(0),
@@ -58,12 +61,24 @@ impl Command for Drop {
             },
             Example {
                 example: "[0,1,2,3] | drop 2",
-                description: "Remove the last two items of a list/table",
+                description: "Remove the last two items of a list",
                 result: Some(Value::List {
                     vals: vec![Value::test_int(0), Value::test_int(1)],
                     span: Span::test_data(),
                 }),
             },
+            Example {
+                description: "Remove the last row in a table",
+                example: "[[a, b]; [1, 2] [3, 4]] | drop 1",
+                result: Some(Value::List {
+                    vals: vec![Value::Record {
+                        cols: vec!["a".to_string(), "b".to_string()],
+                        vals: vec![Value::test_int(1), Value::test_int(2)],
+                        span: Span::test_data(),
+                    }],
+                    span: Span::test_data(),
+                }),
+            },
         ]
     }
 

crates/nu-command/src/filters/drop/nth.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::{Call, RangeInclusion};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, FromValue, IntoInterruptiblePipelineData, PipelineData, PipelineIterator,
-    Range, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Range, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,10 @@ impl Command for DropNth {
 
     fn signature(&self) -> Signature {
         Signature::build("drop nth")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "row number or row range",
                 // FIXME: we can make this accept either Int or Range when we can compose SyntaxShapes

crates/nu-command/src/filters/each.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Signature,
-    Span, SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -35,6 +35,10 @@ with 'transpose' first."#
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("each")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "block",
                 SyntaxShape::Block(Some(vec![SyntaxShape::Any])),

crates/nu-command/src/filters/each_while.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Signature,
-    Span, SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -24,6 +24,10 @@ impl Command for EachWhile {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build(self.name())
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "block",
                 SyntaxShape::Block(Some(vec![SyntaxShape::Any])),

crates/nu-command/src/filters/empty.rs

@@ -2,7 +2,7 @@ use nu_engine::CallExt;
 use nu_protocol::ast::{Call, CellPath};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, Signature, Span, SyntaxShape, Value,
+    Category, Example, IntoPipelineData, PipelineData, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +15,7 @@ impl Command for Empty {
 
     fn signature(&self) -> Signature {
         Signature::build("is-empty")
+            .input_output_types(vec![(Type::Any, Type::Bool)])
             .rest(
                 "rest",
                 SyntaxShape::CellPath,

crates/nu-command/src/filters/every.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,10 @@ impl Command for Every {
 
     fn signature(&self) -> Signature {
         Signature::build("every")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "stride",
                 SyntaxShape::Int,

crates/nu-command/src/filters/find.rs

@@ -9,7 +9,7 @@ use nu_protocol::{
     ast::Call,
     engine::{CaptureBlock, Command, EngineState, Stack},
     Category, Config, Example, IntoInterruptiblePipelineData, ListStream, PipelineData, ShellError,
-    Signature, Span, SyntaxShape, Value,
+    Signature, Span, SyntaxShape, Type, Value,
 };
 use nu_utils::get_ls_colors;
 
@@ -23,6 +23,20 @@ impl Command for Find {
 
     fn signature(&self) -> Signature {
         Signature::build(self.name())
+            .input_output_types(vec![
+                (
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter it would be List<T> -> List<T>.
+                    Type::List(Box::new(Type::Any)),
+                    Type::List(Box::new(Type::Any)),
+                ),
+                (Type::String, Type::String),
+                (
+                    // For find -p
+                    Type::Table(vec![]),
+                    Type::Table(vec![]),
+                ),
+            ])
             .named(
                 "predicate",
                 SyntaxShape::Block(Some(vec![SyntaxShape::Any])),

crates/nu-command/src/filters/first.rs

@@ -16,6 +16,24 @@ impl Command for First {
 
     fn signature(&self) -> Signature {
         Signature::build("first")
+            .input_output_types(vec![
+                (
+                    // TODO: This variant duplicates the functionality of
+                    // `take`. See #6611, #6611, #6893
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter style it would be List<T> ->
+                    // List<T>.
+                    Type::List(Box::new(Type::Any)),
+                    Type::List(Box::new(Type::Any)),
+                ),
+                (
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter it would be List<T> -> T.
+                    Type::List(Box::new(Type::Any)),
+                    Type::Any,
+                ),
+                (Type::Binary, Type::Binary),
+            ])
             .optional(
                 "rows",
                 SyntaxShape::Int,

crates/nu-command/src/filters/flatten.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::{Call, CellPath, PathMember};
 
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,13 @@ impl Command for Flatten {
 
     fn signature(&self) -> Signature {
         Signature::build("flatten")
+            .input_output_types(vec![
+                (
+                    Type::List(Box::new(Type::Any)),
+                    Type::List(Box::new(Type::Any)),
+                ),
+                (Type::Record(vec![]), Type::Table(vec![])),
+            ])
             .rest(
                 "rest",
                 SyntaxShape::String,

crates/nu-command/src/filters/get.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::{Call, CellPath};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Signature,
-    SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +20,15 @@ impl Command for Get {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("get")
+            .input_output_types(vec![
+                (
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter it would be List<T> -> T.
+                    Type::List(Box::new(Type::Any)),
+                    Type::Any,
+                ),
+                (Type::Table(vec![]), Type::Any),
+            ])
             .required(
                 "cell_path",
                 SyntaxShape::CellPath,
@@ -92,6 +101,24 @@ impl Command for Get {
     }
     fn examples(&self) -> Vec<Example> {
         vec![
+            Example {
+                description: "Get an item from a list",
+                example: "[0 1 2] | get 1",
+                result: Some(Value::test_int(1)),
+            },
+            Example {
+                description: "Get a column from a table",
+                example: "[{A: A0}] | get A",
+                result: Some(Value::List {
+                    vals: vec![Value::test_string("A0")],
+                    span: Span::test_data(),
+                }),
+            },
+            Example {
+                description: "Get a cell from a table",
+                example: "[{A: A0}] | get 0.A",
+                result: Some(Value::test_string("A0")),
+            },
             Example {
                 description: "Extract the name of files as a list",
                 example: "ls | get name",
@@ -125,3 +152,15 @@ impl Command for Get {
         ]
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn test_examples() {
+        use crate::test_examples;
+
+        test_examples(Get)
+    }
+}

crates/nu-command/src/filters/group.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, Signature, Span, Spanned,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,14 @@ impl Command for Group {
 
     fn signature(&self) -> Signature {
         Signature::build("group")
+            // TODO: It accepts Table also, but currently there is no Table
+            // example. Perhaps Table should be a subtype of List, in which case
+            // the current signature would suffice even when a Table example
+            // exists.
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::List(Box::new(Type::Any)))),
+            )])
             .required("group_size", SyntaxShape::Int, "the size of each group")
             .category(Category::Filters)
     }

crates/nu-command/src/filters/group_by.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 use indexmap::IndexMap;
@@ -17,15 +17,20 @@ impl Command for GroupBy {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("group-by").optional(
+        Signature::build("group-by")
-            "grouper",
+            // TODO: It accepts Table also, but currently there is no Table
-            SyntaxShape::Any,
+            // example. Perhaps Table should be a subtype of List, in which case
-            "the grouper value to use",
+            // the current signature would suffice even when a Table example
-        )
+            // exists.
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::Record(vec![]),
+            )])
+            .optional("grouper", SyntaxShape::Any, "the grouper value to use")
     }
 
     fn usage(&self) -> &str {
-        "Split a table into groups based on one column's values, and return a record with those groups."
+        "Splits a list or table into groups, and returns a record containing those groups."
     }
 
     fn run(
@@ -47,7 +52,7 @@ impl Command for GroupBy {
             },
             Example {
                 description: "You can also group by raw values by leaving out the argument",
-                example: "echo ['1' '3' '1' '3' '2' '1' '1'] | group-by",
+                example: "['1' '3' '1' '3' '2' '1' '1'] | group-by",
                 result: Some(Value::Record {
                     cols: vec!["1".to_string(), "3".to_string(), "2".to_string()],
                     vals: vec![

crates/nu-command/src/filters/headers.rs

@@ -1,7 +1,8 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Value,
+    Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -13,7 +14,16 @@ impl Command for Headers {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build(self.name()).category(Category::Filters)
+        Signature::build(self.name())
+            .input_output_types(vec![
+                (Type::Table(vec![]), Type::Table(vec![])),
+                (
+                    // Tables with missing values are List<Any>
+                    Type::List(Box::new(Type::Any)),
+                    Type::Table(vec![]),
+                ),
+            ])
+            .category(Category::Filters)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/filters/insert.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::{Call, CellPath, PathMember};
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, FromValue, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
-    ShellError, Signature, Span, SyntaxShape, Value,
+    ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,14 @@ impl Command for Insert {
 
     fn signature(&self) -> Signature {
         Signature::build("insert")
+            .input_output_types(vec![
+                (Type::Record(vec![]), Type::Record(vec![])),
+                // TODO: It accepts table input also (in which case it repeats
+                // the value across all table rows) but currently there is no
+                // example of the table variant so it cannot be in the
+                // signature.
+                // (Type::Table(vec![]), Type::Table(vec![])),
+            ])
             .required(
                 "field",
                 SyntaxShape::CellPath,
@@ -49,8 +57,8 @@ impl Command for Insert {
 
     fn examples(&self) -> Vec<Example> {
         vec![Example {
-            description: "Insert a new value",
+            description: "Insert a new entry into a record",
-            example: "echo {'name': 'nu', 'stars': 5} | insert alias 'Nushell'",
+            example: "{'name': 'nu', 'stars': 5} | insert alias 'Nushell'",
             result: Some(Value::Record {
                 cols: vec!["name".into(), "stars".into(), "alias".into()],
                 vals: vec![

crates/nu-command/src/filters/last.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
-    Signature, Span, SyntaxShape, Value,
+    Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,23 @@ impl Command for Last {
 
     fn signature(&self) -> Signature {
         Signature::build("last")
+            .input_output_types(vec![
+                (
+                    // TODO: This variant duplicates the functionality of
+                    // `take`. See #6611, #6611, #6893
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter style it would be List<T> ->
+                    // List<T>.
+                    Type::List(Box::new(Type::Any)),
+                    Type::List(Box::new(Type::Any)),
+                ),
+                (
+                    // TODO: This is too permissive; if we could express this
+                    // using a type parameter it would be List<T> -> T.
+                    Type::List(Box::new(Type::Any)),
+                    Type::Any,
+                ),
+            ])
             .optional(
                 "rows",
                 SyntaxShape::Int,

crates/nu-command/src/filters/length.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
-    Signature, Span, Value,
+    Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -20,6 +20,10 @@ impl Command for Length {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("length")
+            .input_output_types(vec![
+                (Type::List(Box::new(Type::Any)), Type::Int),
+                (Type::Table(vec![]), Type::Int),
+            ])
             .switch("column", "Show the number of columns in a table", Some('c'))
             .category(Category::Filters)
     }

crates/nu-command/src/filters/lines.rs

@@ -2,7 +2,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, RawStream, ShellError,
-    Signature, Span, Value,
+    Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -19,6 +19,7 @@ impl Command for Lines {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("lines")
+            .input_output_types(vec![(Type::String, Type::List(Box::new(Type::String)))])
             .switch("skip-empty", "skip empty lines", Some('s'))
             .category(Category::Filters)
     }

crates/nu-command/src/filters/merge.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, FromValue, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData,
-    ShellError, Signature, Span, SyntaxShape, Value,
+    ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -29,6 +29,10 @@ repeating this process with row 1, and so on."#
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("merge")
+            .input_output_types(vec![
+                (Type::Record(vec![]), Type::Record(vec![])),
+                (Type::Table(vec![]), Type::Table(vec![])),
+            ])
             .required(
                 "block",
                 // Both this and `update` should have a shape more like <record> | <table> | <block> than just <any>. -Leon 2022-10-27
@@ -64,10 +68,11 @@ repeating this process with row 1, and so on."#
             Example {
                 example: "{a: 1, b: 2} | merge {c: 3}",
                 description: "Merge two records",
-                result: Some(Value::test_record(
+                result: Some(Value::Record {
-                    vec!["a", "b", "c"],
+                    cols: vec!["a".to_string(), "b".to_string(), "c".to_string()],
-                    vec![Value::test_int(1), Value::test_int(2), Value::test_int(3)],
+                    vals: vec![Value::test_int(1), Value::test_int(2), Value::test_int(3)],
-                )),
+                    span: Span::test_data(),
+                }),
             },
             Example {
                 example: "{a: 1, b: 3} | merge { { b: 2 } | merge { c: 4 } }",
@@ -77,6 +82,17 @@ repeating this process with row 1, and so on."#
                     vec![Value::test_int(1), Value::test_int(2), Value::test_int(4)],
                 )),
             },
+            Example {
+                example: "[{columnA: A0 columnB: B0}] | merge [{columnA: 'A0*'}]",
+                description: "Merge two tables, overwriting overlapping columns",
+                result: Some(Value::List {
+                    vals: vec![Value::test_record(
+                        vec!["columnA", "columnB"],
+                        vec![Value::test_string("A0*"), Value::test_string("B0")],
+                    )],
+                    span: Span::test_data(),
+                }),
+            },
         ]
     }
 

crates/nu-command/src/filters/move_.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, ShellError,
-    Signature, Span, Spanned, SyntaxShape, Value,
+    Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone, Debug)]
@@ -26,6 +26,10 @@ impl Command for Move {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("move")
+            .input_output_types(vec![
+                (Type::Table(vec![]), Type::Table(vec![])),
+                (Type::Record(vec![]), Type::Record(vec![])),
+            ])
             .rest("columns", SyntaxShape::String, "the columns to move")
             .named(
                 "after",

crates/nu-command/src/filters/par_each.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, IntoPipelineData, PipelineData, Signature,
-    Span, SyntaxShape, Value,
+    Span, SyntaxShape, Type, Value,
 };
 use rayon::prelude::*;
 
@@ -23,6 +23,10 @@ impl Command for ParEach {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("par-each")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "block",
                 SyntaxShape::Block(Some(vec![SyntaxShape::Any])),

crates/nu-command/src/filters/prepend.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,6 +16,10 @@ impl Command for Prepend {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("prepend")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "row",
                 SyntaxShape::Any,

crates/nu-command/src/filters/range.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::{Call, RangeInclusion};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoInterruptiblePipelineData, PipelineData, ShellError, Signature, Span,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,10 @@ impl Command for Range {
 
     fn signature(&self) -> Signature {
         Signature::build("range")
+            .input_output_types(vec![(
+                Type::List(Box::new(Type::Any)),
+                Type::List(Box::new(Type::Any)),
+            )])
             .required(
                 "rows",
                 SyntaxShape::Range,

crates/nu-command/src/filters/reduce.rs

@@ -5,7 +5,7 @@ use nu_engine::{eval_block, CallExt};
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{CaptureBlock, Command, EngineState, Stack};
 use nu_protocol::{
-    Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape, Value,
+    Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -18,6 +18,7 @@ impl Command for Reduce {
 
     fn signature(&self) -> Signature {
         Signature::build("reduce")
+            .input_output_types(vec![(Type::List(Box::new(Type::Any)), Type::Any)])
             .named(
                 "fold",
                 SyntaxShape::Any,