crash when unannotated definition has type variable in sub-annotation

[marick]

Here is an example:

fanOut model  =
  let
    step : (a, Int)
    step =
        ( model , 3 )
  in
    5

Deleting the let type annotation, or adding one to the function causes it to work. (In the latter case, it works whether or not the let has a type annotation.

Error text:

elm-make IVFlat/Generic/TEA.elm
[                                                  ] - 0 / 1elm-make: It looks like something went wrong with the type inference algorithm.

Unable to generalize a type variable. It is not unranked.

Elm 0.18, Mac OSX 10.11.6

For reference, the above code was binary-chopped down from this:

import Tuple

type alias Return msg model = (model, Cmd msg)
type alias Unbatched msg model = (model, List (Cmd msg))

type alias FieldUpdater msg field = msg -> field -> Return msg field

type alias Lens field model = ( model -> field, field -> model -> model )
  
-- fanOut : msg -> model -> FieldUpdater msg field -> List (Lens field model)
--       -> Return msg model
fanOut msg originalModel update accessors =
  let
    step : Lens field model -> Unbatched msg model -> Unbatched msg model
    step (getter, setter) (changingModel, cmdList) =
      let 
        (newPart, newCmd) = update msg (getter originalModel)
      in
        ( setter newPart changingModel
        , newCmd :: cmdList
        )
  in  
    accessors
      |> List.foldl step (originalModel, [])
      |> Tuple.mapSecond Cmd.batch

[process-bot]

Thanks for the issue! Make sure it satisfies this checklist. My human colleagues will appreciate it!

Here is what to expect next, and if anyone wants to comment, keep these things in mind.

[evancz]

Interesting! I've never seen someone do annotations with type variables inside, but not outside.

Anyway, added to #1613