Yes, it should be - I have corrected: mmap :: (IArray a e, Num e, IArray a e', Num e') => (e -> e') -> Matrix a e -> Matrix a e' It still cannot be an instance of functor though as: instance Functor (Matrix a) where fmap f = mmap x We cannot constrain the instance correctly... We need "IArray a e", but fmap is (expanding) fmap :: b -> c -> Matrix a b -> Matrix a c But 'b' and 'c' cannot be constrained by the instance... because they do not occur in the class, only the function definition. Keean. Bulat Ziganshin wrote:
Hello Keean,
Tuesday, June 14, 2005, 3:54:08 PM, you wrote:
KS> class Functor m where KS> fmap :: a -> b -> m a -> m b
KS> But the type of "mmap" is
KS> mmap :: e -> e -> Matrix a i e -> Matrix a i e
KS> in other words we map values to the same type, not a different type.
mmap should be
mmap :: (e -> e1) -> Matrix a i e -> Matrix a i e1
for example, we can map matrix of strings to matrix of its lengths