Add only command to std-rfc/iter (#16015)

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# Description
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This PR adds the `only` command to `std-rfc/iter`, which is a command I
wrote a while ago that I've found so useful that I think it could have a
place in the standard library. It acts similarly to `get 0`, but ensures
that the value actually exists, and there aren't additional values. I
find this most useful when chained with `where`, when you want to be
certain that no additional elements are accidentally selected when you
only mean to get a single element.

I'll copy the help page here for additional explanation:

> Get the only element of a list or table, ensuring it exists and there
are no extra elements.
> 
> Similar to `first` with no arguments, but errors if there are no
additional
> items when there should only be one item. This can help avoid issues
when more
> than one row than expected matches some criteria.
> 
> This command is useful when chained with `where` to ensure that only
one row
> meets the given condition.
> 
> If a cell path is provided as an argument, it will be accessed after
the first
> element. For example, `only foo` is roughly equivalent to `get 0.foo`,
with
> the guarantee that there are no additional elements.
> 
> Note that this command currently collects streams.

> Examples:
>  
> Get the only item in a list, ensuring it exists and there's no
additional items
> ```nushell
> [5] | only
> # => 5
> ```
> 
> Get the `name` column of the only row in a table
> ```nushell
> [{name: foo, id: 5}] | only name
> # => foo
> ```
> 
> Get the modification time of the file named foo.txt
> ```nushell
> ls | where name == "foo.txt" | only modified
> ```

Here's some additional examples showing the errors:

![image](https://github.com/user-attachments/assets/d5e6f202-db52-42e4-a2ba-fb7c4f1d530a)


![image](https://github.com/user-attachments/assets/b080da2a-7aff-48a9-a523-55c638fdcce3)

Most of the time I chain this with a simple `where`, but here's a couple
other real world examples of how I've used this:

[With `parse`, which outputs a
table](https://git.ikl.sh/132ikl/dotfiles/src/branch/main/.scripts/manage-nu#L53):
```nushell
let commit = $selection | parse "{start}.g{commit}-{end}" | only commit
```

[Ensuring that only one row in a table has a name that ends with a
certain
suffix](https://git.ikl.sh/132ikl/dotfiles/src/branch/main/.scripts/btconnect):
```nushell
$devices | where ($chosen_name ends-with $it.name) | only
```


Unfortunately to get these nice errors I had to collect the stream (and
I think the errors are more useful for this). This should be to be
mitigated with (something like) #16014.


Putting this in `std/iter` might be pushing it, but it seems *just*
close enough that I can't really justify putting it in a different/new
module.

# User-Facing Changes
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* Adds the `only` command to `std-rfc/iter`, which can be used to ensure
that a table or list only has a single element.

# Tests + Formatting
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Added a few tests for `only` including error cases

# After Submitting
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N/A

---------

Co-authored-by: Bahex <Bahex@users.noreply.github.com>

crates/nu-std/std-rfc/iter/mod.nu

@@ -172,3 +172,51 @@ export def recurse [
     generate $fn [{path: ($.), item: ($in) }]
     | if not $depth_first { flatten } else { }
 }
+
+# Helper for `only` errors
+def only-error [msg: string, meta: record, label: string]: nothing -> error {
+  error make {
+    msg: $msg,
+    label: {
+      text: $label,
+      span: $meta.span,
+    }
+  }
+}
+
+# Get the only element of a list or table, ensuring it exists and there are no extra elements.
+#
+# Similar to `first` with no arguments, but errors if there are additional
+# items when there should only be one item. This can help avoid issues when more
+# than one row than expected matches some criteria.
+#
+# This command is useful when chained with `where` to ensure that only one row
+# meets the given condition.
+#
+# If a cell path is provided as an argument, it will be accessed after the first
+# element. For example, `only foo` is roughly equivalent to `get 0.foo`, with
+# the guarantee that there are no additional elements.
+#
+# Note that this command currently collects streams.
+@search-terms first single
+@category filters
+@example "Get the only item in a list, ensuring it exists and there's no additional items" --result 5 {
+  [5] | only
+}
+@example "Get the `name` column of the only row in a table" --result "foo" {
+  [{name: foo, id: 5}] | only name
+}
+@example "Get the modification time of the file named foo.txt" {
+  ls | where name == "foo.txt" | only modified
+}
+export def only [
+  cell_path?: cell-path # The cell path to access within the only element.
+]: [table -> any, list -> any] {
+  let pipe = {in: $in, meta: (metadata $in)}
+  let path = [0 $cell_path] | cell-path-join
+  match ($pipe.in | length) {
+    0 => (only-error "expected non-empty table/list" $pipe.meta "empty")
+    1 => ($pipe.in | get $path)
+    _ => (only-error "expected only one element in table/list" $pipe.meta "has more than one element")
+  }
+}

crates/nu-std/tests/test_std-rfc_iter.nu

@@ -79,3 +79,45 @@ def recurse-example-closure [] {
 
     assert equal $out $expected
 }
+
+@test
+def only-example-list [] {
+  let out = [5] | only
+  assert equal $out 5
+}
+
+@test
+def only-example-table [] {
+  let out = [{name: foo, id: 5}] | only name
+  assert equal $out foo
+}
+
+@test
+def only-more-than-one-list [] {
+  try {
+    [1 2 3] | only
+    assert false
+  } catch {|err|
+    assert ($err.msg has "expected only one")
+  }
+}
+
+@test
+def only-more-than-one-table [] {
+  try {
+    [[name, id]; [foo bar] [5 6]] | only foo
+    assert false
+  } catch {|err|
+    assert ($err.msg has "expected only one")
+  }
+}
+
+@test
+def only-none [] {
+  try {
+    [] | only
+    assert false
+  } catch {|err|
+    (assert ($err.msg has "non-empty"))
+  }
+}