At 2002-04-03 15:14, Hal Daume III wrote:
if I replace
type FM = FiniteMap
with
type FM a b = FiniteMap a b
it works fine. I wasn't aware there was (supposed to be) a difference between these two declarations? Is there?
I don't know about your example, but there is a difference. type FM = FiniteMap ...This defines FM as a type-constructor of kind (* -> * -> *) equivalent to FiniteMap. type FM a b = FiniteMap a b ...This defines FM as a pseudo-type-constructor. It has no kind, but must be specified with two arguments each of kind (*) so as to become a type-constructor (a type) of kind (*). Unfortunately Haskell does not have lambda over types, otherwise the two could be the same. And then one could do things such as type M b a = a -> b; instance Cofunctor (M b) where ... ...which would be very useful, but would probably have unpleasant consequences for type inference... -- Ashley Yakeley, Seattle WA