{-
	Copyright (c) 2013, Alex Cole
	
	This Source Code Form is subject to the terms of the Mozilla Public License,
	v. 2.0. If a copy of the MPL was not distributed with this file, You can
	obtain one at http://mozilla.org/MPL/2.0/.
-}

{-# LANGUAGE FunctionalDependencies #-}
{-# LANGUAGE MultiParamTypeClasses  #-}
{-# LANGUAGE EmptyDataDecls         #-}
{- # LANGUAGE KindSignatures         #-}
{-# LANGUAGE TypeFamilies           #-}
{-# LANGUAGE DataKinds              #-}

module Data.Types.Reorder.Base where

import Language.Haskell.TH

import GHC.TypeLits
import Data.Constraint

-- This is simply a placeholder to mark the end of our dynamic data type.
data ReorderableEnd
reorderableEnd :: ReorderableEnd
reorderableEnd = undefined

-- This determines the relative ordering of two types.  Generally the first
-- type given is either a concrete type or a container, and the second type is
-- only a concrete type.  If it isn't, then "TypeOrder_Composition" is used to
-- indicate the special case and recurse.
data TypeOrder_Lower
data TypeOrder_Higher
data TypeOrder_Same
data TypeOrder_Composition
data TypeOrder_End

type family TypeOrder t0 t1
type instance TypeOrder ReorderableEnd ReorderableEnd = TypeOrder_End

-- | Get the order of a new type relative to an existing collection of types,
-- i.e. extract the top type in the collection and compare to that.  The
-- "GetReorderableChild" type doesn't actually need the last "x" type parameter.
type family   GetReorderableChild (container :: * -> * -> *) a x
type instance GetReorderableChild container (container l r) x = r
type instance GetReorderableChild container ReorderableEnd  x = ReorderableEnd

-- | Pass the container so we can pattern match on it at the type level.
type GetTypeOrder container a t = TypeOrder (GetReorderableChild container a t) t

class ReorderableInstance (a :: * -> Constraint) (n :: Nat) (r :: Symbol) | a n -> r