Jared Warren wrote:
Consider:
class Thing t where thing :: t
instance Thing Int where thing = 0
instance Thing Char where thing = 'a'
Can someone please explain why
fst (1,thing)
...gets an 'ambiguous type variable' error, but
fst (1,undefined)
...doesn't?
Perhaps because "thing" has an ambiguous type (it's either Int of Char), but "undefined" doesn't, (it's for all a.a). Remember that type inference in the Haskell type system does not assume knowledge of the semantics of functions. In order to deduce the type of an application of "fst", you need to determine the type of its argument - you can't discard one of ithe tuple components just because you know you will lose it when you apply a projection function. Type checking is done before any evaulation, compile-time or run-time. You might argue that it would be a good idea to apply some program transformations early to avoid this kind of unnecessary ambiguity, but I have doubts about its general usefullness.
And is there anything I can change to make the former work as well? Even modifying fst doesn't work:
fst' :: Thing b => (a,b) -> a fst' (~a,~b) = a
You should not expect it to work. The problem is with the type ambiguity, not with the semantics of fst. --brian -- Brian Boutel Wellington New Zealand Note the NOSPAM