2009/4/2 Louis Wasserman
The following module does not compile, and I can't figure out why: {-# LANGUAGE TypeFamilies #-} {-# LANGUAGE KindSignatures #-}
module Foo where
import Control.Monad import Data.Maybe
class Key k where type Map k :: * -> * empty :: Map k v look :: k -> Map k v -> Maybe v update :: k -> (Maybe v -> Maybe v) -> Map k v -> Map k v
instance (Key k1, Key k2) => Key (k1, k2) where type Map (k1, k2) v = Map k1 (Map k2 v)
The arity of the instance has to be *exactly* the same as is declared. So the v is one too many parameters. That does make your life a little more difficult (but points to an abstraction you may not have seen :-). I would resolve this as: type Map (k1,k2) = Map k1 `O` Map k2 Where O is functor composition from TypeCompose on hackage.
empty = empty update (k1, k2) f = update k1 (update k2 f . fromMaybe empty) look (k1, k2) = look k1 >=> look k2
The compile fails with Foo.hs:16:1: Number of parameters must match family declaration; expected 1 In the type synonym instance declaration for `Map' In the instance declaration for `Key (k1, k2)'
Is this a bug with type synonym families? Is there something silly I'm missing?
Louis Wasserman wasserman.louis@gmail.com
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