props for Hugs over System FC

[AntC]

heh heh. I've got to record this for posterity. I've never known an example before.

Some code that compiles in Hugs and works fine; but GHC (8.6.4) can't typecheck so rejects.

It's an example in the 2011 'System F with Type Equality Coercions', section 2.3 discussing FunDeps; and used to justify the extra power of type inference in Type Families as opposed to FunDeps. Full details discussed here: https://gitlab.haskell.org/ghc/ghc/issues/16430#note_189393

    class F a b | a -> b

    instance F Int Bool

    class D a where { op :: F a b => a -> b }

    instance D Int where { op _ = True }

True that doesn't compile as given. Hugs says: 'Inferred type is not general enough'. 

GHC says 'Couldn't match expected type `b' with actual type `Bool''/ '`b' is a rigid type variable'. (So essentially the same failure of typechecking.)

With a little help for the type inference, this compiles in Hugs.     
    class F a b | a -> b
    instance F Int Bool

    class D a where { op :: (F a b) => a -> b } 
    instance (TypeCast Bool b') => D Int where { op _ = typeCast True }

With `TypeCast` defined as for HList.
That dangling type variable `b'` in the constraint is weird.
But GHC still rejects it; and rejects a version with a `(~)` constraint instead of the `TypeCast`.

AntC

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