Improve type mismatch error: this need to be `a` but it is `a` about roc

[eckertliam]

@Anton-4 are you working on a fix for this? I would like to give it a shot.

[Anton-4]

No, I've now assigned you to this issue :)
Thanks for your help @eckertliam!

[Anton-4]

Due to the multiple type mismatches this example is pretty difficult so if we can get a simpler case to investigate that would be great :)

[jwoudenberg]

This might be simpler example:

interface Foo
    exposes [bar]
    imports []

foo : a, (a -> a) -> a
foo = \x, f -> bar x f

bar : a, (a -> a) -> a
bar = \x, f ->
    baz : a
    baz = f x

    foo x f

It fails with just the single error:

── TYPE MISMATCH in Foo.roc ────────────────────────────────────────────────────

Something is off with the body of the baz definition:

10│      baz : a
11│      baz = f x
               ^^^

This f call produces:

    a

But the type annotation on baz says it should be:

    a

────────────────────────────────────────────────────────────────────────────────

1 error and 1 warning found in 26 ms

The error goes away after any of the following changes:

• Remove implementation of baz
• Remove type annotation of baz
• Remove type annotation of bar
• Remove implementation of foo

[Anton-4]

Awesome, thanks @jwoudenberg :)

[ayazhafiz]

I don't think either of these should actually be type errors. Unless I'm misunderstanding something I think the programs should compile fine. The issue is that a in the type annotation is not used in the inner annotations' a.

[lukewilliamboswell]

I wonder if we could add a suggestion? maybe it would be possible to detect this somehow and provide a suggestion like "the types are different even though they are have the same name"

Or perhaps there is a way to prevent this kind of issue by making it invalid to use the same type variable in an inner annotation?