{-# LANGUAGE FlexibleInstances, UndecidableInstances #-}
{-# LANGUAGE OverlappingInstances #-}

-- ShowList -- a simple example of overlapping instances.
-- See ShowListNO.hs for the same example without overlapping.

module ShowList where

import Data.List

class ShowList a where
    showl :: [a] -> String

-- General instance
instance Show a => ShowList a where
    showl [] = "[]"
    showl xs = "[" ++ concat (intersperse "," (map show xs)) ++ "]"

-- Special instances, for lists of specific types
instance ShowList Bool where
    showl xs = concatMap toBit xs
     where toBit False = "0"
	   toBit True  = "1"

instance ShowList Char where
    showl = id

t1 = showl [10,11::Int]
t2 = showl "abc"
t3 = showl [True,False,True]

{-
*ShowList> t1
"[10,11]"
*ShowList> t2
"abc"
*ShowList> t3
"101"
-}