[Haskell-cafe] Type families and kind signatures

Louis Wasserman wasserman.louis at gmail.com
Thu Apr 2 11:59:39 EDT 2009

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)
empty = empty
 update (k1, k2) f = update k1 (update k2 f . fromMaybe empty)
look (k1, k2) = look k1 >=> look k2

The compile fails with
    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

Louis Wasserman
wasserman.louis at gmail.com
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