In article <20051223143950.GA31551@abridgegame.org>,
David Roundy
On Thu, Dec 22, 2005 at 06:27:41PM -0800, Ashley Yakeley wrote:
David Roundy wrote:
Hello all,
I have a question about how to create the right kind to declare lists to be a class. I have a class Foo
class Foo f where foo :: f a -> Foo
and I want to define that a list of Foos is also a Foo, but can't see how to do it. I imagine something like
instance Foo f => Foo [f] where foo xs = map foo xs
but of course [f] isn't a valid type.
[] and f both have * -> *, and you want to compose them. You can do this like this:
newtype Compose p q a = MkCompose p (q a)
and then
instance Foo f => instance (Compose [] f) where foo (MkCompose fs) = ...
Given:
instance Foo f => Foo (Compose [] f) where foo _ = undefined
ghc gives me the error:
test.hs:24:0: Illegal instance declaration for `Foo (Compose [] f)' (The instance type must be of form (T a b c) where T is not a synonym, and a,b,c are distinct type variables) In the instance declaration for `Foo (Compose ([]) f)'
Oh, you need -fglasgow-exts.
I can, however, define
instance (Functor p, Foo q) => Foo (Compose p q) where foo (MkCompose fs) = MkCompose (fmap foo fs)
but in any case, this doesn't make a list itself a Foo, so you'd still need wrappers, which would defeat the point of putting lists into the class. :(
Your problem is that you want to apply function "foo" to something of type "[f a]". But foo has type "f' a -> Foo", and "[f a]" and "f' a" won't unify. It doesn't matter what else you do with classes and instances, you'll have to change the type of one or the other. -- Ashley Yakeley, Seattle WA