[EMAIL PROTECTED] wrote:
>
> I think I get it. But to be sure:
> The forall means: The type for a may not be the same
> throughout the whole function. It just has to follow
> the pattern specified inside the braces.
Not exactly. In:
> > fun::(forall a.[a]->Int)->[b]->[c]->Int
> > fun f x y = f x + f y
the forall means that the first argument to fun must be polymorphic, as
in:
> > fun length [True, False] [1,2]
It is not enough "to follow the pattern". If, e.g., you have a function
sum::[Int]->Int (which does "follow the pattern", if I understand
correctly what you mean with this), you may NOT say something like:
fun sum [1,2] [3,4]
however sensible this might seem at first look.
The reason is that sum only works on integer lists, whereas the given
type for fun requires that the first argument be a function that works
on lists of any type, such as length.
If sum were allowed as argument to fun, then we would also have to be
allowed to say:
fun sum [True, False] [1,2]
which clearly makes no sense.
That Hugs refuses to report the type of fun has to with the fact that
such (rank-2) types cannot be inferred. GHC generally has better support
for higher-ranked types (no general inference, of course).
Hope that helped,
Janis.
--
Janis Voigtlaender
http://wwwtcs.inf.tu-dresden.de/~voigt/
mailto:[EMAIL PROTECTED]
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