i think what i will do is to instantiate all table types individually: | instance Show c => Show (SimpleTable c) where | showsPrec p t = showParen (p > 10) $ showString "FastTable " . | shows (toLists t)
I was going to propose this solution, as well as define newtype SlowType a = SlowType [[a]] for the ordinary table. That would avoid the conflict with Show [a] instance. It is also good idea to differentiate [[a]] intended to be a table from just any list of lists. (Presumably the table has rows of the same size). Enclosed is a bit spiffed up variation of that idea. {-# LANGUAGE TypeFamilies, FlexibleInstances, FlexibleContexts, UndecidableInstances #-} class Table t where data TName t :: * type TCell t :: * toLists :: TName t -> [[TCell t]] fromLists :: [[TCell t]] -> TName t instance Table [[a]] where newtype TName [[a]] = SlowTable [[a]] type TCell [[a]] = a toLists (SlowTable x) = x fromLists = SlowTable data FastMapRep a -- = ... instance Table (FastMapRep a) where newtype TName (FastMapRep a) = FastTable [[a]] type TCell (FastMapRep a) = a toLists = undefined fromLists = undefined instance Table Int where newtype TName Int = FastBoolTable Int type TCell Int = Bool toLists = undefined fromLists = undefined instance (Table t, Show (TCell t)) => Show (TName t) where showsPrec p t = showParen (p > 10) $ showString "fromLists " . shows (toLists t) t1 :: TName [[Int]] t1 = fromLists [[1..10],[2..20]] -- fromLists [[1,2,3,4,5,6,7,8,9,10], -- [2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20]]