deriving over renamed types
Wed, 3 Apr 2002 15:36:01 -0800
At 2002-04-03 15:14, Hal Daume III wrote:
>if I replace
>> type FM = FiniteMap
>> 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
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