Hmm. If you have class (Diff (D f)) => Diff f where then if I have f :: Diff f => ... f = e then the constraints available for discharging constraints arising from e are Diff f Diff (D f) Diff (D (D f)) Diff (D (D (D f))) ... That's a lot of constraints. Simon | -----Original Message----- | From: haskell-cafe-bounces@haskell.org [mailto:haskell-cafe-bounces@haskell.org] On | Behalf Of Conor McBride | Sent: 17 December 2009 14:48 | To: Haskell Cafe | Subject: Re: [Haskell-cafe] Restrictions on associated types for classes | | 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 | | _______________________________________________ | Haskell-Cafe mailing list | Haskell-Cafe@haskell.org | http://www.haskell.org/mailman/listinfo/haskell-cafe