[Hugs-users] props for Hugs over System FC

[Anthony Clayden]

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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