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Check type of if/while/match guard conditions at parse-time#16174

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132ikl:if-condition-typecheck
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Check type of if/while/match guard conditions at parse-time#16174
132ikl wants to merge 2 commits intonushell:mainfrom
132ikl:if-condition-typecheck

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@132ikl 132ikl commented Jul 15, 2025

Description

This PR adds parse-time type checking for the conditions to if, while, and match guards.

Before:

print hi; if "foobar" {}
# => hi
# => Error: nu::shell::cant_convert
# => 
# =>   × Can't convert to boolean.
# =>    ╭─[entry #1:1:14]
# =>  1 │ print hi; if "foobar" {}
# =>    ·              ────┬───
# =>    ·                  ╰── can't convert string to boolean
# =>    ╰────

After (hi doesn't print):

print hi; if "foobar" {}
# => Error: nu::parser::type_mismatch
# => 
# =>   × Type mismatch.
# =>    ╭─[entry #1:1:14]
# =>  1 │ print hi; if "foobar" {}
# =>    ·              ────┬───
# =>    ·                  ╰── expected bool, found string
# =>    ╰────

I have to admit this is not exactly the prettiest solution.if and while are real command in nu-cmd-lang, not parser keywords, so this can't be trivially special cased. Shoving Type into a SyntaxShape is pretty cursed, but there are only a couple other options as far as I can tell:

  1. Do the type checking at compile time: this would probably be the smallest changes lines-of-code wise, but it doesn't really seem right to be mixing up responsibilities like that
  2. Add a new SyntaxShape::Condition or something like that (not to be confused with SyntaxShape::RowCondition), which is basically MathExpression but specifically for booleans.
  3. Give up and wait for the new parser

User-Facing Changes

  • Conditions for if, while, and match guards are now checked at parse-time

Tests + Formatting

N/A

After Submitting

N/A

@github-actions github-actions bot added the A:parser Issues related to parsing label Jul 15, 2025
@132ikl 132ikl mentioned this pull request Jul 15, 2025
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@WindSoilder
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WindSoilder commented Jul 15, 2025

but there are only a couple other options as far as I can tell:

Maybe we can make if and while parser keywords? I remember @sholderbach had mentioned about it

132ikl added a commit that referenced this pull request Jul 29, 2025
@132ikl
Check type of row conditions at parse-time (#16175)
2e4900f 
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# Description
<!--
Thank you for improving Nushell. Please, check our [contributing
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changes.

Description of your pull request goes here. **Provide examples and/or
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As a bonus to #16174, I realized it would be trivial to add a similar
check to where.

Before:
```nushell
1..100 | where 1
# => no output...
```

After:
```nushell
1..100 | where 1
# => Error: nu::parser::type_mismatch
# => 
# =>   × Type mismatch.
# =>    ╭─[entry #3:1:16]
# =>  1 │ 1..100 | where 1
# =>    ·                ┬
# =>    ·                ╰── expected bool, found int
# =>    ╰────
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
* `where` should now error on row condition expressions which are not
booleans
  
# 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 test

# 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
hardfau1t pushed a commit to hardfau1t/nushell that referenced this pull request Aug 25, 2025
@132ikl @hardfau1t
Check type of row conditions at parse-time (nushell#16175)
ab5d270 
<!--
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.
-->

As a bonus to nushell#16174, I realized it would be trivial to add a similar
check to where.

Before:
```nushell
1..100 | where 1
# => no output...
```

After:
```nushell
1..100 | where 1
# => Error: nu::parser::type_mismatch
# => 
# =>   × Type mismatch.
# =>    ╭─[entry nushell#3:1:16]
# =>  1 │ 1..100 | where 1
# =>    ·                ┬
# =>    ·                ╰── expected bool, found int
# =>    ╰────
```

# User-Facing Changes
<!-- List of all changes that impact the user experience here. This
helps us keep track of breaking changes. -->
* `where` should now error on row condition expressions which are not
booleans
  
# 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 test

# 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.
-->
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@cptpiepmatz
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cptpiepmatz commented Mar 14, 2026

This got stale. Open it up again Rose, if you want to. Closing for now.

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@132ikl @WindSoilder @cptpiepmatz