Fix output signature of split chars/words (#9739)

# Description
Those two commands did *not* vectorize over the input in the pure sense
as they performed a flat map. Now they return a list for each string
that gets split by them.

```
["foo" "bar"] | split chars
```

## Before 

```
╭───┬───╮
│ 0 │ f │
│ 1 │ o │
│ 2 │ o │
│ 3 │ b │
│ 4 │ a │
│ 5 │ r │
╰───┴───╯
```

## After
```
╭───┬───────────╮
│ 0 │ ╭───┬───╮ │
│   │ │ 0 │ f │ │
│   │ │ 1 │ o │ │
│   │ │ 2 │ o │ │
│   │ ╰───┴───╯ │
│ 1 │ ╭───┬───╮ │
│   │ │ 0 │ b │ │
│   │ │ 1 │ a │ │
│   │ │ 2 │ r │ │
│   │ ╰───┴───╯ │
╰───┴───────────╯
```

crates/nu-command/src/strings/split/chars.rs

@@ -16,14 +16,20 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("split chars")
-            .input_output_types(vec![(Type::String, Type::List(Box::new(Type::String)))])
+            .input_output_types(vec![
+                (Type::String, Type::List(Box::new(Type::String))),
+                (
+                    Type::List(Box::new(Type::String)),
+                    Type::List(Box::new(Type::List(Box::new(Type::String)))),
+                ),
+            ])
+            .allow_variants_without_examples(true)
             .switch("grapheme-clusters", "split on grapheme clusters", Some('g'))
             .switch(
                 "code-points",
                 "split on code points (default; splits combined characters)",
                 Some('c'),
             )
-            .vectorizes_over_list(true)
             .category(Category::Strings)
     }
 
@@ -63,6 +69,26 @@ impl Command for SubCommand {
                     span: Span::test_data(),
                 }),
             },
+            Example {
+                description: "Split multiple strings into lists of characters",
+                example: "['hello', 'world'] | split chars",
+                result: Some(Value::test_list(vec![
+                    Value::test_list(vec![
+                        Value::test_string("h"),
+                        Value::test_string("e"),
+                        Value::test_string("l"),
+                        Value::test_string("l"),
+                        Value::test_string("o"),
+                    ]),
+                    Value::test_list(vec![
+                        Value::test_string("w"),
+                        Value::test_string("o"),
+                        Value::test_string("r"),
+                        Value::test_string("l"),
+                        Value::test_string("d"),
+                    ]),
+                ])),
+            },
         ]
     }
 
@@ -85,17 +111,18 @@ fn split_chars(
     let span = call.head;
 
     let graphemes = grapheme_flags(call)?;
-    input.flat_map(
+    input.map(
         move |x| split_chars_helper(&x, span, graphemes),
         engine_state.ctrlc.clone(),
     )
 }
 
-fn split_chars_helper(v: &Value, name: Span, graphemes: bool) -> Vec<Value> {
+fn split_chars_helper(v: &Value, name: Span, graphemes: bool) -> Value {
     match v.span() {
         Ok(v_span) => {
             if let Ok(s) = v.as_string() {
-                if graphemes {
+                Value::List {
+                    vals: if graphemes {
                         s.graphemes(true)
                             .collect::<Vec<_>>()
                             .into_iter()
@@ -107,20 +134,22 @@ fn split_chars_helper(v: &Value, name: Span, graphemes: bool) -> Vec<Value> {
                             .into_iter()
                             .map(move |x| Value::string(x, v_span))
                             .collect()
+                    },
+                    span: v_span,
                 }
             } else {
-                vec![Value::Error {
+                Value::Error {
                     error: Box::new(ShellError::PipelineMismatch {
                         exp_input_type: "string".into(),
                         dst_span: name,
                         src_span: v_span,
                     }),
-                }]
                 }
             }
-        Err(error) => vec![Value::Error {
+        }
+        Err(error) => Value::Error {
             error: Box::new(error),
-        }],
+        },
     }
 }
 

crates/nu-command/src/strings/split/words.rs

@@ -18,8 +18,14 @@ impl Command for SubCommand {
 
     fn signature(&self) -> Signature {
         Signature::build("split words")
-            .input_output_types(vec![(Type::String, Type::List(Box::new(Type::String)))])
-            .vectorizes_over_list(true)
+            .input_output_types(vec![
+                (Type::String, Type::List(Box::new(Type::String))),
+                (
+                    Type::List(Box::new(Type::String)),
+                    Type::List(Box::new(Type::List(Box::new(Type::String))))
+                ),
+            ])
+            .allow_variants_without_examples(true)
             .category(Category::Strings)
             // .switch(
             //     "ignore-hyphenated",
@@ -133,18 +139,13 @@ fn split_words(
     }
     let graphemes = grapheme_flags(call)?;
 
-    input.flat_map(
+    input.map(
         move |x| split_words_helper(&x, word_length, span, graphemes),
         engine_state.ctrlc.clone(),
     )
 }
 
-fn split_words_helper(
-    v: &Value,
-    word_length: Option<usize>,
-    span: Span,
-    graphemes: bool,
-) -> Vec<Value> {
+fn split_words_helper(v: &Value, word_length: Option<usize>, span: Span, graphemes: bool) -> Value {
     // There are some options here with this regex.
     // [^A-Za-z\'] = do not match uppercase or lowercase letters or apostrophes
     // [^[:alpha:]\'] = do not match any uppercase or lowercase letters or apostrophes
@@ -160,7 +161,7 @@ fn split_words_helper(
                 // let words: Vec<&str> = s.split_whitespace().collect();
 
                 let replaced_string = regex_replace.replace_all(&s, " ").to_string();
-                replaced_string
+                let words = replaced_string
                     .split(' ')
                     .filter_map(|s| {
                         if s.trim() != "" {
@@ -182,20 +183,24 @@ fn split_words_helper(
                             None
                         }
                     })
-                    .collect::<Vec<Value>>()
+                    .collect::<Vec<Value>>();
+                Value::List {
+                    vals: words,
+                    span: v_span,
+                }
             } else {
-                vec![Value::Error {
+                Value::Error {
                     error: Box::new(ShellError::PipelineMismatch {
                         exp_input_type: "string".into(),
                         dst_span: span,
                         src_span: v_span,
                     }),
-                }]
                 }
             }
-        Err(error) => vec![Value::Error {
+        }
+        Err(error) => Value::Error {
             error: Box::new(error),
-        }],
+        },
     }
 }