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nushell/crates/nu-std/std-rfc
132ikl 0b202d55f0 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>
2025-06-23 16:29:58 -05:00
..
2025-02-10 09:06:09 -05:00

std-rfc

Overview and Requirements

This module includes potential candidate commands (and other definitions) for inclusion in the Standard Library (std) that is built in to Nushell. As a general guideline, candidates should:

  • Be general purpose features that will be useful to a number of users
  • Include doc comments for definitions and parameters that can be used with help <command>
  • Include tests
  • Since doc comments are fairly limited, additional documentation can be included in a GitHub discussion. This documentation can then be moved to the main website when the feature is promoted to std. See this example for some table helpers.

Showcase and Discussion

While primary feedback should take place in the PR, we have also established a Drawing Board Discord channel which can be used for several purposes:

  • Ideation before a PR is submitted
  • Raise awareness of the feature
  • Short-term questions and discussion

Note: The Drawing Board is not just for std-rfc. Please tag your topic with std-library if it is about a Standard Library idea.

Promotion Evaluation

In general, new std-rfc features will be evaluated after they have been trialed for a sufficient period, which may vary depending on the feature. After that period, the feature may be:

  • Promoted to std
  • Removed from std-rfc
  • Or changes may be requested and then reevaluated later