How do I merge signals of nested tagged types? - elm

Question

As part of the Elm app I’m building, I want to keep signals of environment changes (like resizing the window) from data changes (rendering a filterable list of models to the browser). I thought I would model these as different extensible types:
type WindowUpdate = Resize (Int, Int)
type DataUpdate = TagFilter Model.Tag
type Update update data = WindowUpdate update data
| DataUpdate update data
| NoOp
updates : Signal.Mailbox (Update update data)
updates = Signal.mailbox NoOp
appModel : Signal Model
appModel =
let
applicationUpdates = Signal.mergeMany
[ updates.signal
]
in
Signal.foldp update Model.defaultModel applicationUpdates
windowUpdate : WindowUpdate -> Model -> Model
windowUpdate update model =
let resizeWidth = \windowModel newWidth -> { windowModel | width = newWidth }
in
case update of
Resize (w, _) -> { model | window = (resizeWidth model.window w) }
update : Update -> Model -> Model
update u model =
case u of
WindowUpdate wu data -> windowUpdate (wu data) model
DataUpdate du data -> model
otherwise -> model
Unfortunately I can’t get my update function to work correctly. I get the following compiler errors:
— TYPE MISMATCH —————————————————————— ./app/Updates.elm
The 3rd argument to function `foldp` is causing a mismatch.
36│ Signal.foldp update Model.defaultModel applicationUpdates
^^^^^^^^^^^^^^^^^^
Function `foldp` is expecting the 3rd argument to be:
Signal (Update a)
But it is:
Signal Update
Hint: I always figure out the type of arguments from left to right. If an
argument is acceptable when I check it, I assume it is "correct" in subsequent checks. So the problem may actually be in how previous arguments interact with the 3rd.
What am I doing wrong?

Answer 1

You forgot the type parameters of Update in the signature of update, it should be (code untested):
update : Update update data -> Model -> Model