{-# 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

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?