Confusing message about "type applications should start with a type atom"

[fredcy]

I get the following error message from elm-make (0.16) when compiling the code following that.

-- TYPE MISMATCH ------------------------------------------------------ Main.elm

The type annotation for `update` does not match its definition.

17│ update : Users -> Action -> Users
             ^^^^^^^^^^^^^^^^^^^^^^^^
The type annotation is saying:

    Users -> Action -> Users

But I am inferring that the definition has this type:

    elm-make: type applications should start with a type atom

module Main (..) where

type alias Users =
  { users : List User
  }

type alias User =
  { name : String
  }

type Action
  = X (Result () List)

update : Users -> Action -> Users
update model action model =
  case action of
    X result ->
      case result of
        Ok users ->
          Users (List.map2 (+) model.users users)

[fredcy]

That line with List.map2 is wrong but the error message is little help in determining that.

I have not been able to reproduce the problem with any program smaller than the above.

[jvoigtlaender]

To me, that's not an issue of a confusing message. It's a proper compiler bug. See elm/compiler#1326.

[fredcy]

This case does have an error message very similar to that compiler issue. Should I close this issue and just leave it linked from that one?

[jvoigtlaender]

I think that would make sense, yes.

Additionally, I just now realized that your code is not (only?) wrong in the List.map2 line. The first line of the update definition is bad in its own way:

update model action model =

Note the two parameters with same name. So this is also relevant: elm/compiler#809.

[fredcy]

For the record, it wasn't my code. I was helping someone who posted their uncompilable code as a Gist. We got that working but the original version caused the error message above.