Type checker compiler crash

[folkertdev]

I'm working with this snippet

points : List a -> List ((a, List a))
points elems = 
    case elems of
        [] -> 
            []
        (x::xs) -> 
            (x, xs) :: (List.map (\(b, bs) -> (b, x::bs)) xs)

Compiling this code result in a compiler crash:

-- TYPE MISMATCH ---------------------------------------------- DecisionTree.elm

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

142│ points : List a -> List ((a, List a))
              ^^^^^^^^^^^^^^^^^^^^^^^^^^^^
The type annotation is saying:

    List a -> List ( a, List a )

But I am inferring that the definition has this type:

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

checked with elm-make v0.16.0 and elm-lang.org/try

(as an aside: elm-lang.org/try gives no value when the compiler crashes. This may be a conscious decision (maybe security concerns), but I'd rather see what went wrong on the backend)

[mgold]

You need to replace List ((a, List a)) with List (a, List a).

@jvoigtlaender and I had a discussion on whether more than the minimal number of parens should be allowed by the compiler, but we deleted it to keep the issue clean.

[folkertdev]

@mgold

Removing the parenthesis does not solve the problem:

points : List a -> List (a, List a)

This still crashes (on elm-lang.org/try). I added the parens because I thought the type checker maybe confused parenthesis with the tuple type.

[mgold]

Huh, weird! This is something really strange then.

[jvoigtlaender]

Possibly similar to or even related to #1231

You are causing the type-checker to try to create (and then pretty-print) an infinite type there. Note how when you map over xs, you try to draw a pair from it, even though xs is only known to be a List a.

[folkertdev]

right, that seems to be it.

 (x, xs) :: (List.map (\(b, bs) -> (b, x::bs)) (points xs))

is how this should be written, and that compiles.

[evancz]

Closing as duplicate of #1231