https://github.com/lue-bird/elm-review-single-use-type-vars-end-with-underscore

elm-review: type variables only used once end with -_
https://github.com/lue-bird/elm-review-single-use-type-vars-end-with-underscore

elm elm-review type-variable

Last synced: about 1 month ago
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elm-review: type variables only used once end with -_

• Host: GitHub
• URL: https://github.com/lue-bird/elm-review-single-use-type-vars-end-with-underscore
• Owner: lue-bird
• License: mit
• Created: 2021-07-04T05:13:28.000Z (over 3 years ago)
• Default Branch: master
• Last Pushed: 2022-11-14T16:47:03.000Z (about 2 years ago)
• Last Synced: 2024-12-09T18:02:08.005Z (about 2 months ago)
• Topics: elm, elm-review, type-variable
• Language: Elm
• Homepage: https://package.elm-lang.org/packages/lue-bird/elm-review-single-use-type-vars-end-with-underscore/latest/
• Size: 137 KB
• Stars: 0
• Watchers: 1
• Forks: 0
• Open Issues: 0
• Metadata Files:
  • Readme: README.md
  • Changelog: changes.md
  • Contributing: contributing.md
  • License: LICENSE

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README

        
# [elm-review-single-use-type-vars-end-with-underscore](https://package.elm-lang.org/packages/lue-bird/elm-review-single-use-type-vars-end-with-underscore/latest/)

[`elm-review`](https://package.elm-lang.org/packages/jfmengels/elm-review/latest/) rule

to make sure that type variables on module scope declarations

which are only used once

are marked with the suffix -\_.

## why would you want this?

-\_ at the end of a type variable is a good indication that it is used only in this one place.

Some types have a lot of type variables, most of them only used once.

If you see a -\_

  - you know not to focus on these

    - -\_ in the result: can be inferred as anything

    - -\_ in an argument: anything allowed

  

  - through the review rule you can make sure that this type variable isn't used anywhere else

    → 2 type variables can't accidentally be the same.

### example

Which one is easier to understand?

```elm

at :

    Nat (ArgIn indexMin minLengthMinus1 indexIfN)

    -> LinearDirection

    -> Arr (In (Nat1Plus minLengthMinus1) maxLength) element

    -> element

```

```elm

at :

    Nat (ArgIn indexMin_ minLengthMinus1 indexIfN_)

    -> LinearDirection

    -> Arr (In (Nat1Plus minLengthMinus1) maxLength_) element

    -> element

```

(from [typesafe-array: Arr.at](https://package.elm-lang.org/packages/lue-bird/elm-typesafe-array/latest/Arr#at))

Once you're used to this, it feels similar to

```elm

at :

    Nat (ArgIn _ minLengthMinus1 _)

    -> LinearDirection

    -> Arr (In (Nat1Plus minLengthMinus1) _) element

    -> element

```

## Provided rules

🔧[`OnlyAllSingleUseTypeVarsEndWith_`](OnlyAllSingleUseTypeVarsEndWith_)

reports in types of module scope declarations

  - single-use type variables that don't have a -\_ suffix

  - multi-use type variables that have a -\_ suffix

## Configuration

```elm

module ReviewConfig exposing (config)

import OnlyAllSingleUseTypeVarsEndWith_

import Review.Rule exposing (Rule)

config : List Rule

config =

    [ OnlyAllSingleUseTypeVarsEndWith_.rule

    ]

```

## Why you might not want this

  - You want to keep the conventional way of naming type variables for consistency?

  - You already use the -\_ suffix in possibly multi-use type variables?

  - You dislike having possibly multi-use -\_ suffixed type variables in your let declarations?

Ultimately, the solution to fix all three

would be an editor extension that de-emphasizes (less contrast, ...) single-use type variables.