Tighten how input streams handle nothing, and related (#2847)

crates/nu-cli/src/commands/do_.rs

@@ -2,9 +2,7 @@ use crate::commands::classified::block::run_block;
 use crate::commands::WholeStreamCommand;
 use crate::prelude::*;
 use nu_errors::ShellError;
-use nu_protocol::{
-    hir::CapturedBlock, hir::ExternalRedirection, ReturnSuccess, Signature, SyntaxShape, Value,
-};
+use nu_protocol::{hir::CapturedBlock, hir::ExternalRedirection, Signature, SyntaxShape, Value};
 
 pub struct Do;
 
@@ -48,7 +46,7 @@ impl WholeStreamCommand for Do {
             Example {
                 description: "Run the block and ignore errors",
                 example: r#"do -i { thisisnotarealcommand }"#,
-                result: Some(vec![Value::nothing()]),
+                result: Some(vec![]),
             },
         ]
     }
@@ -98,7 +96,7 @@ async fn do_(raw_args: CommandArgs) -> Result<OutputStream, ShellError> {
                 context.clear_errors();
                 Ok(futures::stream::iter(output).to_output_stream())
             }
-            Err(_) => Ok(OutputStream::one(ReturnSuccess::value(Value::nothing()))),
+            Err(_) => Ok(OutputStream::empty()),
         }
     } else {
         result.map(|x| x.to_output_stream())

crates/nu-cli/src/commands/math/eval.rs

@@ -50,37 +50,48 @@ pub async fn eval(args: CommandArgs) -> Result<OutputStream, ShellError> {
     let name = args.call_info.name_tag.span;
     let (SubCommandArgs { expression }, input) = args.process().await?;
 
-    Ok(input
-        .map(move |x| {
-            if let Some(Tagged {
-                tag,
-                item: expression,
-            }) = &expression
-            {
-                UntaggedValue::string(expression).into_value(tag)
-            } else {
-                x
-            }
-        })
-        .map(move |input| {
-            if let Ok(string) = input.as_string() {
-                match parse(&string, &input.tag) {
-                    Ok(value) => ReturnSuccess::value(value),
-                    Err(err) => Err(ShellError::labeled_error(
-                        "Math evaluation error",
-                        err,
-                        &input.tag.span,
-                    )),
+    if let Some(string) = expression {
+        match parse(&string, &string.tag) {
+            Ok(value) => Ok(OutputStream::one(ReturnSuccess::value(value))),
+            Err(err) => Err(ShellError::labeled_error(
+                "Math evaluation error",
+                err,
+                &string.tag.span,
+            )),
+        }
+    } else {
+        Ok(input
+            .map(move |x| {
+                if let Some(Tagged {
+                    tag,
+                    item: expression,
+                }) = &expression
+                {
+                    UntaggedValue::string(expression).into_value(tag)
+                } else {
+                    x
                 }
-            } else {
-                Err(ShellError::labeled_error(
-                    "Expected a string from pipeline",
-                    "requires string input",
-                    name,
-                ))
-            }
-        })
-        .to_output_stream())
+            })
+            .map(move |input| {
+                if let Ok(string) = input.as_string() {
+                    match parse(&string, &input.tag) {
+                        Ok(value) => ReturnSuccess::value(value),
+                        Err(err) => Err(ShellError::labeled_error(
+                            "Math evaluation error",
+                            err,
+                            &input.tag.span,
+                        )),
+                    }
+                } else {
+                    Err(ShellError::labeled_error(
+                        "Expected a string from pipeline",
+                        "requires string input",
+                        name,
+                    ))
+                }
+            })
+            .to_output_stream())
+    }
 }
 
 pub fn parse<T: Into<Tag>>(math_expression: &str, tag: T) -> Result<Value, String> {

crates/nu-cli/src/commands/math/sum.rs

@@ -73,7 +73,11 @@ pub fn summation(values: &[Value], name: &Tag) -> Result<Value, ShellError> {
     let sum = reducer_for(Reduce::Summation);
 
     let first = values.get(0).ok_or_else(|| {
-        ShellError::unexpected("Cannot perform aggregate math operation on empty data")
+        ShellError::labeled_error(
+            "Cannot perform aggregate math operation on empty data",
+            "expected input",
+            name.span,
+        )
     })?;
 
     match first {

crates/nu-cli/src/commands/reduce.rs

@@ -6,7 +6,7 @@ use crate::{CommandArgs, Example, OutputStream};
 use futures::stream::once;
 use nu_errors::ShellError;
 use nu_parser::ParserScope;
-use nu_protocol::{hir::CapturedBlock, Primitive, Signature, SyntaxShape, UntaggedValue, Value};
+use nu_protocol::{hir::CapturedBlock, Signature, SyntaxShape, UntaggedValue, Value};
 use nu_source::Tagged;
 
 pub struct Reduce;
@@ -93,22 +93,25 @@ async fn process_row(
 }
 
 async fn reduce(raw_args: CommandArgs) -> Result<OutputStream, ShellError> {
+    let span = raw_args.call_info.name_tag.span;
     let context = Arc::new(EvaluationContext::from_raw(&raw_args));
     let (reduce_args, mut input): (ReduceArgs, _) = raw_args.process().await?;
     let block = Arc::new(reduce_args.block);
-    let (ioffset, start) = match reduce_args.fold {
-        None => {
-            let first = input
-                .next()
-                .await
-                .expect("empty stream expected to contain Primitive::Nothing");
-            if let UntaggedValue::Primitive(Primitive::Nothing) = first.value {
-                return Err(ShellError::missing_value(None, "empty input"));
-            }
+    let (ioffset, start) = if !input.is_empty() {
+        match reduce_args.fold {
+            None => {
+                let first = input.next().await.expect("non-empty stream");
 
-            (1, first)
+                (1, first)
+            }
+            Some(acc) => (0, acc),
         }
-        Some(acc) => (0, acc),
+    } else {
+        return Err(ShellError::labeled_error(
+            "Expected input",
+            "needs input",
+            span,
+        ));
     };
 
     if reduce_args.numbered.item {

crates/nu-cli/src/commands/str_/collect.rs

@@ -49,12 +49,39 @@ pub async fn collect(args: CommandArgs) -> Result<OutputStream, ShellError> {
 
     let strings: Vec<Result<String, ShellError>> =
         input.map(|value| value.as_string()).collect().await;
-    let strings: Vec<String> = strings.into_iter().collect::<Result<_, _>>()?;
-    let output = strings.join(&separator);
+    let strings: Result<Vec<_>, _> = strings.into_iter().collect::<Result<_, _>>();
 
-    Ok(OutputStream::one(ReturnSuccess::value(
-        UntaggedValue::string(output).into_value(tag),
-    )))
+    match strings {
+        Ok(strings) => {
+            let output = strings.join(&separator);
+
+            Ok(OutputStream::one(ReturnSuccess::value(
+                UntaggedValue::string(output).into_value(tag),
+            )))
+        }
+        Err(err) => match err.error {
+            nu_errors::ProximateShellError::TypeError { actual, .. } => {
+                if let Some(item) = actual.item {
+                    Err(ShellError::labeled_error_with_secondary(
+                        "could not convert to string",
+                        format!("tried to convert '{}' in input to a string", item),
+                        tag.span,
+                        format!("'{}' value originated here", item),
+                        actual.span,
+                    ))
+                } else {
+                    Err(ShellError::labeled_error_with_secondary(
+                        "could not convert to string",
+                        "failed to convert input to strings",
+                        tag.span,
+                        "non-string found here",
+                        actual.span,
+                    ))
+                }
+            }
+            _ => Err(err),
+        },
+    }
 }
 
 #[cfg(test)]

crates/nu-cli/tests/commands/math/sum.rs

@@ -33,13 +33,6 @@ fn all() {
     })
 }
 
-#[test]
-fn outputs_zero_with_no_input() {
-    let actual = nu!(cwd: ".", "math sum");
-
-    assert_eq!(actual.out, "0");
-}
-
 #[test]
 #[allow(clippy::unreadable_literal)]
 #[allow(clippy::float_cmp)]

crates/nu-cli/tests/commands/reduce.rs

@@ -103,5 +103,5 @@ fn error_reduce_empty() {
         )
     );
 
-    assert!(actual.err.contains("empty input"));
+    assert!(actual.err.contains("needs input"));
 }

crates/nu-stream/src/input.rs

@@ -1,5 +1,5 @@
 use crate::prelude::*;
-use futures::stream::{iter, once};
+use futures::stream::iter;
 use nu_errors::ShellError;
 use nu_protocol::{Primitive, Type, UntaggedValue, Value};
 use nu_source::{PrettyDebug, Tag, Tagged, TaggedItem};
@@ -14,7 +14,7 @@ pub struct InputStream {
 impl InputStream {
     pub fn empty() -> InputStream {
         InputStream {
-            values: once(async { UntaggedValue::nothing().into_untagged_value() }).boxed(),
+            values: futures::stream::empty().boxed(),
             empty: true,
         }
     }