Hi all On 17 Dec 2009, at 14:22, Tom Schrijvers wrote:
class MyClass k where type AssociatedType k :: *
Is there a way of requiring AssociatedType be of class Eq, say?
Have you tried:
{-# LANGUAGE TypeFamilies #-} {-# LANGUAGE FlexibleContexts #-}
class Eq (AssociatedType k) => MyClass k where type AssociatedType k :: *
I just got very excited about this. I'm supposed to be setting a test, but this is far more interesting. I tried this
{-# LANGUAGE TypeFamilies, FlexibleContexts, EmptyDataDecls, TypeOperators #-}
module DDD where
class (Diff (D f)) => Diff f where type D f plug :: D f x -> x -> f x
newtype K a x = K a deriving Show
data Void magic :: Void -> a magic x = x `seq` error "haha"
instance Diff (K a) where type D (K a) = K Void plug (K c) x = magic c
newtype I x = I x deriving Show
instance Diff I where type D I = K () plug (K ()) x = I x
data (f :+: g) x = L (f x) | R (g x) deriving Show
instance (Diff f, Diff g) => Diff (f :+: g) where type D (f :+: g) = D f :+: D g plug (L f') x = L (plug f' x) plug (R g') x = R (plug g' x)
data (f :*: g) x = f x :& g x deriving Show
instance (Diff f, Diff g) => Diff (f :*: g) where type D (f :*: g) = (D f :*: g) :+: (f :*: D g) plug (L (f' :& g)) x = plug f' x :& g plug (R (f :& g')) x = f :& plug g' x
But I got this message [1 of 1] Compiling DDD ( DDD.lhs, interpreted ) DDD.lhs:5:2: Cycle in class declarations (via superclasses): DDD.lhs:(5,2)-(7,28): class (Diff (D f)) => Diff f where { type family D f; } Failed, modules loaded: none. and now I have to go back to setting my class test. Sorry for spam Conor