elm/compiler

Identity type alias causes problems

Heimdell opened this issue · 2 comments

Heimdell commented

Quick Summary:

I entered an identity type alias, but elm refuses to unify LHS and RHS of the type alias later.

SSCCE

module Both.Msg exposing (..)

type alias Msg a = a

make : a -> Msg a
make = identity

=>

Something is off with the body of the `make` definition:

6| make = identity
          ^^^^^^^^
This `identity` value is a:

    a -> a

But the type annotation on `make` says it should be:

    a -> Msg a
  • Elm: 0.19.1
  • Browser: irrelevant
  • Operating System: irrelevant

Additional Details

Msg a and a, for all a, by definition, have to be the same type. Yet, Elm refuses to acknowledge that anything a is also Msg a.

Funny thing, the code

module Both.Msg exposing (..)

type alias Msg a = a

make : Msg a -> a
make = identity

compiles just fine.

github-actions commented

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Heimdell commented

Duplicate of #2252

  • 👍2