{-# LANGUAGE FlexibleContexts #-}
{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE TypeFamilies #-}
module Hcompta.Amount where

import Data.Data (Data)
import Data.Eq (Eq(..))
import Data.Function ((.))
import Data.Ord (Ord(..), Ordering)
import Data.Tuple (fst, snd)

import Hcompta.Quantity
import Hcompta.Unit (Unit(..))

-- * Class 'Amount'

class
 ( Data a
 , Eq   (Amount_Quantity a)
 , Zero (Amount_Quantity a)
 , Unit (Amount_Unit     a)
 ) =>  Amount          a where
	type Amount_Quantity a
	type Amount_Unit     a
	amount_quantity :: a -> Amount_Quantity a
	amount_unit     :: a -> Amount_Unit     a
instance -- Type (unit, quantity)
 ( Zero quantity
 , Eq   quantity
 , Unit unit
 ) => Amount (unit, quantity) where
	type Amount_Quantity (unit, quantity) = quantity
	type Amount_Unit     (unit, quantity) = unit
	amount_quantity = snd
	amount_unit     = fst

amount_sign
 :: ( Amount a
    , Ord (Amount_Quantity a)
    )
 => a -> Ordering
amount_sign = quantity_sign . amount_quantity