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<!-- if this PR closes one or more issues, you can automatically link the PR with them by using one of the [*linking keywords*](https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue#linking-a-pull-request-to-an-issue-using-a-keyword), e.g. - this PR should close #xxxx - fixes #xxxx you can also mention related issues, PRs or discussions! --> # Description <!-- Thank you for improving Nushell. Please, check our [contributing guide](../CONTRIBUTING.md) and talk to the core team before making major changes. Description of your pull request goes here. **Provide examples and/or screenshots** if your changes affect the user experience. --> 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:   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 <!-- List of all changes that impact the user experience here. This helps us keep track of breaking changes. --> * 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 <!-- Don't forget to add tests that cover your changes. Make sure you've run and fixed any issues with these commands: - `cargo fmt --all -- --check` to check standard code formatting (`cargo fmt --all` applies these changes) - `cargo clippy --workspace -- -D warnings -D clippy::unwrap_used` to check that you're using the standard code style - `cargo test --workspace` to check that all tests pass (on Windows make sure to [enable developer mode](https://learn.microsoft.com/en-us/windows/apps/get-started/developer-mode-features-and-debugging)) - `cargo run -- -c "use toolkit.nu; toolkit test stdlib"` to run the tests for the standard library > **Note** > from `nushell` you can also use the `toolkit` as follows > ```bash > use toolkit.nu # or use an `env_change` hook to activate it automatically > toolkit check pr > ``` --> Added a few tests for `only` including error cases # After Submitting <!-- If your PR had any user-facing changes, update [the documentation](https://github.com/nushell/nushell.github.io) after the PR is merged, if necessary. This will help us keep the docs up to date. --> N/A --------- Co-authored-by: Bahex <Bahex@users.noreply.github.com>
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 sometablehelpers.
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