{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE UndecidableInstances #-}
{-# LANGUAGE IncoherentInstances #-}
module Control.Interchangeable
  (
    Interchangeable (..)
  )
where

import qualified Data.Set
import qualified Data.Map

class Interchangeable a b where
  -- | Interchange a with b
  aToB :: a -> b
  -- | Interchange b with a
  bToA :: b -> a
  -- | Append a to b
  appendAToB :: a -> b -> b
  appendAToB a b = modifyBWithA b (const a)
  -- | Append b to a
  appendBToA :: b -> a -> a
  appendBToA b a = modifyAWithB a (const b)
  -- | Modify a with an operation to b
  modifyAWithB :: a -> (b -> b) -> a
  modifyAWithB a f = appendBToA (f (aToB a)) a
  -- | Modify a with an operation to b
  modifyBWithA :: b -> (a -> a) -> b
  modifyBWithA b f = appendAToB (f (bToA b)) b

instance (Interchangeable a b) => Interchangeable b a where
  aToB = bToA
  bToA = aToB
  appendAToB = appendBToA
  appendBToA = appendAToB
  
instance (Ord a) => Interchangeable [a] (Data.Set.Set a) where
  aToB = Data.Set.fromList
  bToA = Data.Set.toList
  appendAToB a = Data.Set.union (Data.Set.fromList a)
  appendBToA b a = a ++ Data.Set.toList b

instance (Ord k) => Interchangeable [(k,a)] (Data.Map.Map k a) where
  aToB = Data.Map.fromList
  bToA = Data.Map.toList
  appendAToB a = Data.Map.union (Data.Map.fromList a)
  appendBToA b a = a ++ Data.Map.toList b