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 C a b | a -> b    instance C Int Bool    f :: (C Int b,
TypeCast b Bool) => b -> Bool    f x  = typeCast x

With `TypeCast` defined as for HList.

But GHC still rejects it; and rejects a version with a `(~)`
constraint instead of the `TypeCast`.


AntC
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