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>

crates/nu-command/src/formats/from/csv.rs

@@ -3,7 +3,9 @@ use super::delimited::{from_delimited_data, trim_from_str};
 use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Value};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct FromCsv;
@@ -15,6 +17,7 @@ impl Command for FromCsv {
 
     fn signature(&self) -> Signature {
         Signature::build("from csv")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
             .named(
                 "separator",
                 SyntaxShape::String,
@@ -54,8 +57,18 @@ impl Command for FromCsv {
         vec![
             Example {
                 description: "Convert comma-separated data to a table",
-                example: "open data.txt | from csv",
-                result: None,
+                example: "\"ColA,ColB\n1,2\" | from csv",
+                result: Some(Value::List {
+                    vals: vec![Value::Record {
+                        cols: vec!["ColA".to_string(), "ColB".to_string()],
+                        vals: vec![
+                            Value::test_int(1),
+                            Value::test_int(2),
+                        ],
+                        span: Span::test_data(),
+                    }],
+                    span: Span::test_data(),
+                })
             },
             Example {
                 description: "Convert comma-separated data to a table, ignoring headers",

crates/nu-command/src/formats/from/eml.rs

@@ -7,7 +7,7 @@ use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::Category;
 use nu_protocol::Config;
 use nu_protocol::{
-    Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Value,
+    Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -22,6 +22,7 @@ impl Command for FromEml {
 
     fn signature(&self) -> Signature {
         Signature::build("from eml")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
             .named(
                 "preview-body",
                 SyntaxShape::Int,
@@ -32,7 +33,7 @@ impl Command for FromEml {
     }
 
     fn usage(&self) -> &str {
-        "Parse text as .eml and create table."
+        "Parse text as .eml and create record."
     }
 
     fn run(
@@ -52,7 +53,7 @@ impl Command for FromEml {
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
-                description: "Convert eml structured data into table",
+                description: "Convert eml structured data into record",
                 example: "'From: test@email.com
 Subject: Welcome
 To: someone@somewhere.com
@@ -89,7 +90,7 @@ Test' | from eml",
                 }),
             },
             Example {
-                description: "Convert eml structured data into table",
+                description: "Convert eml structured data into record",
                 example: "'From: test@email.com
 Subject: Welcome
 To: someone@somewhere.com

crates/nu-command/src/formats/from/ics.rs

@@ -6,7 +6,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, Value,
+    Spanned, Type, Value,
 };
 use std::io::BufReader;
 
@@ -19,7 +19,9 @@ impl Command for FromIcs {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from ics").category(Category::Formats)
+        Signature::build("from ics")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/from/ini.rs

@@ -2,7 +2,8 @@ use indexmap::map::IndexMap;
 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)]
@@ -14,11 +15,13 @@ impl Command for FromIni {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from ini").category(Category::Formats)
+        Signature::build("from ini")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {
-        "Parse text as .ini and create table"
+        "Parse text as .ini and create record"
     }
 
     fn examples(&self) -> Vec<Example> {
@@ -26,7 +29,7 @@ impl Command for FromIni {
             example: "'[foo]
 a=1
 b=2' | from ini",
-            description: "Converts ini formatted string to table",
+            description: "Converts ini formatted string to record",
             result: Some(Value::Record {
                 cols: vec!["foo".to_string()],
                 vals: vec![Value::Record {

crates/nu-command/src/formats/from/json.rs

@@ -2,7 +2,7 @@ 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)]
@@ -19,6 +19,7 @@ impl Command for FromJson {
 
     fn signature(&self) -> nu_protocol::Signature {
         Signature::build("from json")
+            .input_output_types(vec![(Type::String, Type::Any)])
             .switch("objects", "treat each line as a separate value", Some('o'))
             .category(Category::Formats)
     }

crates/nu-command/src/formats/from/nuon.rs

@@ -17,7 +17,9 @@ impl Command for FromNuon {
     }
 
     fn signature(&self) -> nu_protocol::Signature {
-        Signature::build("from nuon").category(Category::Experimental)
+        Signature::build("from nuon")
+            .input_output_types(vec![(Type::String, Type::Any)])
+            .category(Category::Experimental)
     }
 
     fn examples(&self) -> Vec<Example> {

crates/nu-command/src/formats/from/ods.rs

@@ -4,7 +4,7 @@ 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,
 };
 use std::io::Cursor;
 
@@ -18,6 +18,8 @@ impl Command for FromOds {
 
     fn signature(&self) -> Signature {
         Signature::build("from ods")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
+            .allow_variants_without_examples(true)
             .named(
                 "sheets",
                 SyntaxShape::List(Box::new(SyntaxShape::String)),

crates/nu-command/src/formats/from/ssv.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Spanned,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 
 #[derive(Clone)]
@@ -19,6 +19,7 @@ impl Command for FromSsv {
 
     fn signature(&self) -> Signature {
         Signature::build("from ssv")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
             .switch(
                 "noheaders",
                 "don't treat the first row as column names",

crates/nu-command/src/formats/from/toml.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, Span, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -13,18 +13,20 @@ impl Command for FromToml {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from toml").category(Category::Formats)
+        Signature::build("from toml")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {
-        "Parse text as .toml and create table."
+        "Parse text as .toml and create record."
     }
 
     fn examples(&self) -> Vec<Example> {
         vec![
             Example {
                 example: "'a = 1' | from toml",
-                description: "Converts toml formatted string to table",
+                description: "Converts toml formatted string to record",
                 result: Some(Value::Record {
                     cols: vec!["a".to_string()],
                     vals: vec![Value::Int {
@@ -37,7 +39,7 @@ impl Command for FromToml {
             Example {
                 example: "'a = 1
 b = [1, 2]' | from toml",
-                description: "Converts toml formatted string to table",
+                description: "Converts toml formatted string to record",
                 result: Some(Value::Record {
                     cols: vec!["a".to_string(), "b".to_string()],
                     vals: vec![

crates/nu-command/src/formats/from/tsv.rs

@@ -3,7 +3,9 @@ use super::delimited::{from_delimited_data, trim_from_str};
 use nu_engine::CallExt;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Example, PipelineData, ShellError, Signature, SyntaxShape, Value};
+use nu_protocol::{
+    Category, Example, PipelineData, ShellError, Signature, Span, SyntaxShape, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct FromTsv;
@@ -15,6 +17,7 @@ impl Command for FromTsv {
 
     fn signature(&self) -> Signature {
         Signature::build("from tsv")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
             .switch(
                 "noheaders",
                 "don't treat the first row as column names",
@@ -46,6 +49,21 @@ impl Command for FromTsv {
 
     fn examples(&self) -> Vec<Example> {
         vec![
+            Example {
+                description: "Convert tab-separated data to a table",
+                example: "\"ColA\tColB\n1\t2\" | from tsv",
+                result: Some(Value::List {
+                    vals: vec![Value::Record {
+                        cols: vec!["ColA".to_string(), "ColB".to_string()],
+                        vals: vec![
+                            Value::test_int(1),
+                            Value::test_int(2),
+                        ],
+                        span: Span::test_data(),
+                    }],
+                    span: Span::test_data(),
+                })
+            },
             Example {
                 description: "Create a tsv file with header columns and open it",
                 example: r#"$'c1(char tab)c2(char tab)c3(char nl)1(char tab)2(char tab)3' | save tsv-data | open tsv-data | from tsv"#,

crates/nu-command/src/formats/from/url.rs

@@ -1,6 +1,8 @@
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Config, Example, PipelineData, ShellError, Signature, Span, Value};
+use nu_protocol::{
+    Category, Config, Example, PipelineData, ShellError, Signature, Span, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct FromUrl;
@@ -11,11 +13,13 @@ impl Command for FromUrl {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from url").category(Category::Formats)
+        Signature::build("from url")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {
-        "Parse url-encoded string as a table."
+        "Parse url-encoded string as a record."
     }
 
     fn run(
@@ -33,7 +37,7 @@ impl Command for FromUrl {
     fn examples(&self) -> Vec<Example> {
         vec![Example {
             example: "'bread=baguette&cheese=comt%C3%A9&meat=ham&fat=butter' | from url",
-            description: "Convert url encoded string into a table",
+            description: "Convert url encoded string into a record",
             result: Some(Value::Record {
                 cols: vec![
                     "bread".to_string(),

crates/nu-command/src/formats/from/vcf.rs

@@ -5,7 +5,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, Value,
+    Spanned, Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,7 +17,9 @@ impl Command for FromVcf {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from vcf").category(Category::Formats)
+        Signature::build("from vcf")
+            .input_output_types(vec![(Type::String, Type::Table(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/from/xlsx.rs

@@ -4,7 +4,7 @@ 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,
 };
 use std::io::Cursor;
 
@@ -18,6 +18,8 @@ impl Command for FromXlsx {
 
     fn signature(&self) -> Signature {
         Signature::build("from xlsx")
+            .input_output_types(vec![(Type::Binary, Type::Table(vec![]))])
+            .allow_variants_without_examples(true)
             .named(
                 "sheets",
                 SyntaxShape::List(Box::new(SyntaxShape::String)),

crates/nu-command/src/formats/from/xml.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, Value,
+    Spanned, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,11 +15,13 @@ impl Command for FromXml {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from xml").category(Category::Formats)
+        Signature::build("from xml")
+            .input_output_types(vec![(Type::String, Type::Record(vec![]))])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {
-        "Parse text as .xml and create table."
+        "Parse text as .xml and create record."
     }
 
     fn run(
@@ -40,7 +42,7 @@ impl Command for FromXml {
 <note>
   <remember>Event</remember>
 </note>' | from xml"#,
-            description: "Converts xml formatted string to table",
+            description: "Converts xml formatted string to record",
             result: Some(Value::Record {
                 cols: vec!["note".to_string()],
                 vals: vec![Value::Record {

crates/nu-command/src/formats/from/yaml.rs

@@ -3,7 +3,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, Value,
+    Spanned, Type, Value,
 };
 use serde::de::Deserialize;
 use std::collections::HashMap;
@@ -17,7 +17,9 @@ impl Command for FromYaml {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("from yaml").category(Category::Formats)
+        Signature::build("from yaml")
+            .input_output_types(vec![(Type::String, Type::Any)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/to/csv.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, PipelineData, ShellError, Signature, Span, Spanned, SyntaxShape,
-    Value,
+    Type, Value,
 };
 
 #[derive(Clone)]
@@ -17,6 +17,7 @@ impl Command for ToCsv {
 
     fn signature(&self) -> Signature {
         Signature::build("to csv")
+            .input_output_types(vec![(Type::Any, Type::String)])
             .named(
                 "separator",
                 SyntaxShape::String,

crates/nu-command/src/formats/to/html.rs

@@ -5,7 +5,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Spanned,
-    SyntaxShape, Value,
+    SyntaxShape, Type, Value,
 };
 use rust_embed::RustEmbed;
 use serde::{Deserialize, Serialize};
@@ -90,6 +90,7 @@ impl Command for ToHtml {
 
     fn signature(&self) -> Signature {
         Signature::build("to html")
+            .input_output_types(vec![(Type::Any, Type::String)])
             .switch("html-color", "change ansi colors to html colors", Some('c'))
             .switch("no-color", "remove all ansi colors in output", Some('n'))
             .switch(

crates/nu-command/src/formats/to/json.rs

@@ -2,7 +2,8 @@ use nu_engine::CallExt;
 use nu_protocol::ast::{Call, PathMember};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, SyntaxShape, Type,
+    Value,
 };
 
 #[derive(Clone)]
@@ -15,6 +16,7 @@ impl Command for ToJson {
 
     fn signature(&self) -> Signature {
         Signature::build("to json")
+            .input_output_types(vec![(Type::Any, Type::String)])
             .switch("raw", "remove all of the whitespace", Some('r'))
             .named(
                 "indent",

crates/nu-command/src/formats/to/md.rs

@@ -3,7 +3,8 @@ use indexmap::map::IndexMap;
 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)]
@@ -16,6 +17,7 @@ impl Command for ToMd {
 
     fn signature(&self) -> Signature {
         Signature::build("to md")
+            .input_output_types(vec![(Type::Any, Type::String)])
             .switch(
                 "pretty",
                 "Formats the Markdown table to vertically align items",

crates/nu-command/src/formats/to/nuon.rs

@@ -4,7 +4,7 @@ use nu_parser::escape_quote_string;
 use nu_protocol::ast::{Call, RangeInclusion};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -16,7 +16,9 @@ impl Command for ToNuon {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to nuon").category(Category::Experimental)
+        Signature::build("to nuon")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Experimental)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/to/text.rs

@@ -3,7 +3,7 @@ use nu_protocol::{
     ast::Call,
     engine::{Command, EngineState, Stack},
     format_duration, format_filesize_from_conf, Category, Config, Example, IntoPipelineData,
-    PipelineData, ShellError, Signature, Value,
+    PipelineData, ShellError, Signature, Type, Value,
 };
 
 #[derive(Clone)]
@@ -15,7 +15,9 @@ impl Command for ToText {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to text").category(Category::Formats)
+        Signature::build("to text")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/to/toml.rs

@@ -13,7 +13,9 @@ impl Command for ToToml {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to toml").category(Category::Formats)
+        Signature::build("to toml")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/to/tsv.rs

@@ -1,7 +1,9 @@
 use crate::formats::to::delimited::to_delimited_data;
 use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
-use nu_protocol::{Category, Config, Example, PipelineData, ShellError, Signature, Span, Value};
+use nu_protocol::{
+    Category, Config, Example, PipelineData, ShellError, Signature, Span, Type, Value,
+};
 
 #[derive(Clone)]
 pub struct ToTsv;
@@ -13,6 +15,7 @@ impl Command for ToTsv {
 
     fn signature(&self) -> Signature {
         Signature::build("to tsv")
+            .input_output_types(vec![(Type::Any, Type::String)])
             .switch(
                 "noheaders",
                 "do not output the column names as the first row",

crates/nu-command/src/formats/to/url.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, Span, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -13,7 +13,9 @@ impl Command for ToUrl {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to url").category(Category::Formats)
+        Signature::build("to url")
+            .input_output_types(vec![(Type::Table(vec![]), Type::String)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {

crates/nu-command/src/formats/to/xml.rs

@@ -4,7 +4,7 @@ use nu_protocol::ast::Call;
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
     Category, Config, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span,
-    Spanned, SyntaxShape, Value,
+    Spanned, SyntaxShape, Type, Value,
 };
 use quick_xml::events::{BytesEnd, BytesStart, BytesText, Event};
 use std::collections::HashSet;
@@ -21,6 +21,7 @@ impl Command for ToXml {
 
     fn signature(&self) -> Signature {
         Signature::build("to xml")
+            .input_output_types(vec![(Type::Record(vec![]), Type::String)])
             .named(
                 "pretty",
                 SyntaxShape::Int,

crates/nu-command/src/formats/to/yaml.rs

@@ -1,7 +1,7 @@
 use nu_protocol::ast::{Call, PathMember};
 use nu_protocol::engine::{Command, EngineState, Stack};
 use nu_protocol::{
-    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Value,
+    Category, Example, IntoPipelineData, PipelineData, ShellError, Signature, Span, Type, Value,
 };
 
 #[derive(Clone)]
@@ -13,7 +13,9 @@ impl Command for ToYaml {
     }
 
     fn signature(&self) -> Signature {
-        Signature::build("to yaml").category(Category::Formats)
+        Signature::build("to yaml")
+            .input_output_types(vec![(Type::Any, Type::String)])
+            .category(Category::Formats)
     }
 
     fn usage(&self) -> &str {