lib/Hcompta/Amount.hs

   1 {-# LANGUAGE FlexibleContexts #-}
   2 {-# LANGUAGE FlexibleInstances #-}
   3 {-# LANGUAGE TypeFamilies #-}
   4 module Hcompta.Amount where
   5
   6 import Data.Data (Data)
   7 import Data.Eq (Eq(..))
   8 import Data.Function ((.))
   9 import Data.Ord (Ord(..), Ordering)
  10 import Data.Tuple (fst, snd)
  11
  12 import Hcompta.Quantity
  13 import Hcompta.Unit (Unit(..))
  14
  15 -- * Class 'Amount'
  16
  17 class
  18  ( Data a
  19  , Eq   (Amount_Quantity a)
  20  , Zero (Amount_Quantity a)
  21  , Unit (Amount_Unit     a)
  22  ) =>  Amount          a where
  23         type Amount_Quantity a
  24         type Amount_Unit     a
  25         amount_quantity :: a -> Amount_Quantity a
  26         amount_unit     :: a -> Amount_Unit     a
  27 instance -- Type (unit, quantity)
  28  ( Zero quantity
  29  , Eq   quantity
  30  , Unit unit
  31  ) => Amount (unit, quantity) where
  32         type Amount_Quantity (unit, quantity) = quantity
  33         type Amount_Unit     (unit, quantity) = unit
  34         amount_quantity = snd
  35         amount_unit     = fst
  36
  37 amount_sign
  38  :: ( Amount a
  39     , Ord (Amount_Quantity a)
  40     )
  41  => a -> Ordering
  42 amount_sign = quantity_sign . amount_quantity