elm-review-single-use-type-vars-end-with-underscore

The elm-review rule 🔧OnlyAllSingleUseTypeVarsEndWith_ enforces that type variables which are only used once in annotations of module scope declaration are marked with the suffix -_ and other type variables are not.

why would you want this?

You can use -_ at the end of a type variable to indicate 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

The review rule makes sure that this type variable isn't used anywhere else so that 2 type variables can't accidentally be the same.

httpRequestSend :
    { url : String
    , method : String
    , onResponse : Bytes -> msg
    , headers :
          List
              { name : String
              , value : String
              }
    , body : Maybe Bytes
    }
    -> Task error_ msg

We can see at a glance that error_ isn't related to an argument, it's free to take on any type whereas msg is something an argument has to provide.

Likewise, if we see -_ in an argument, we also know it's not used in any other part of the type.

nodeCount :
    lockingTag
    -> TaggedTree label_ lockingTag
    -> Int

configure

module ReviewConfig exposing (config)

import OnlyAllSingleUseTypeVarsEndWith_
import Review.Rule

config : List Review.Rule.Rule
config =
    [ OnlyAllSingleUseTypeVarsEndWith_.rule
    ]

why you might not want this

• The conventional way of naming type variables is used consistently in pretty much all existing elm code and is already familiar to elm folks
• you already use -/_ for separate reasons in some type variables
• since let declaration annotations may need to refer to types that are single-use in the module-scope declaration annotation, this review rule does not consider let type variables at all which might be confusing

Ultimately, the solution to fix all three would be an editor extension that de-emphasizes (less contrast, ...) single-use type variables.