Hello,
On Sun, Jan 13, 2013 at 12:05 PM, Conal Elliott <co...@conal.net> wrote: > > > so there is really no way for GHC to figure out what is the intended value >> for `a`. >> > > Indeed. Though I wonder: does the type-checker really need to find a > binding for `a` in this case, i.e., given the equation `(forall a. F a) == > (forall a'. F a')`? > Wouldn't that make `F` a constant type family, and so one could just skip the `a` parameter? Anyway, if there was a way to assert something like that about a type-function, than there would be no problem. When something like that happens (i.e., GHC figures out that it does not know how to instantiate a type variable, but it is sure that the actual instantiation does not matter), GHC instantiates the variable a special type called `Any`, which has a very polymorphic kind. By the way, Simon recently reworked the ambiguity checker in GHC, and now HEAD correctly rejects `foo` as being ambiguous (the new ambiguity check uses exactly what's in your example: a value `f :: S` is ambiguous, if `g :: S; g = f` results in an error). -Iavor
_______________________________________________ Glasgow-haskell-users mailing list Glasgow-haskell-users@haskell.org http://www.haskell.org/mailman/listinfo/glasgow-haskell-users
