This is not a bug report but a suggestion concerning a Hugs -98
feature: universal quantifiers. BTW I refer to the September 1999
Version.
First of all, I just discovered that you can now make universal
quantification explicit. That's great!
> data List a = Nil | Cons a (List a)
> mapList :: forall a a'.(a -> a') -> (List a -> List a')
> mapList mapa Nil = Nil
> mapList mapa (Cons v vs) = Cons (mapa v) (mapList mapa vs)
However, slight variations cause syntax errors.
> mapList :: (forall a a'.(a -> a') -> (List a -> List a'))
> mapList :: forall a.forall a'.(a -> a') -> (List a -> List a')
ERROR: Syntax error in type signature (unexpected keyword "forall")
ERROR: Syntax error in type signature (unexpected `;', possibly due to bad layout)
Furthermore, universal quantifiers can only appear on the top level
(ignoring rank-2 type sigantures for the moment). It is not possible to
write `t -> forall a.u' even though the type is equivalent to `forall
a.t -> u' provided `a' does not appear free in `t'. So I must write
> data Twice f a = Twice (f (f a))
> mapTwice :: forall f f' a a'.(forall b b'.(b -> b') -> (f b ->
>f' b'))
> -> (a -> a') -> (Twice f a ->
>Twice f' a')
> mapTwice mapf mapa (Twice v) = Twice (mapf (mapf mapa) v)
instead of
> mapTwice :: forall f f'.(forall b b'.(b -> b') -> (f b -> f'
>b'))
> -> (forall a a'.(a -> a') -> (Twice f a
>-> Twice f' a'))
which is more logical (for what I have in mind).
I think that these problems can be solved if the type language
is extended by a production for quantified types `forall <tvars>.<type>'.
Then in a second pass it is checked whether the type signature has a
rank less or equal two. This extension would also permit quantified types
in type signatures.
What do you think?
Ralf
