KindedTypeParameters.hs

KindedTypeParameters.hs

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE KindSignatures #-}

module Lib where

data Owner = Merchant | Customer

newtype MoneyAmount (a :: Owner)
  = MoneyAmount Rational

data BalanceCurrency
  = BX
  | BY
  | BZ

data SettlmentCurrency
  = SX
  | SY

--
-- How I can add constraint to type `b`?
-- I want it to be only `BalanceCurrency` or `SettlementCurrency`.
-- But not any other type.
--
data Money (a :: Owner) b
  = Money
      { amt :: MoneyAmount a,
        cc :: b
      }

{-# LANGUAGE DataKinds #-}
{-# LANGUAGE GADTs #-}
{-# LANGUAGE KindSignatures #-}

module Lib where

data Owner = Merchant | Customer

newtype MoneyAmount (a :: Owner)
  = MoneyAmount Rational

data Balance
  = B_EUR
  | B_USD
  | B_BTC
  | B_LTC

data Settlement
  = S_BTC
  | S_LTC

class Currency a

instance Currency Balance

instance Currency Settlement

data Money (a :: Owner) b where
  Money :: Currency b => {amt :: MoneyAmount a, cc :: b} -> Money a b

x0 :: Money Merchant Balance
x0 = Money {amt = MoneyAmount 1, cc = B_EUR}

x1 :: Money Merchant Settlement
x1 = Money {amt = MoneyAmount 1, cc = S_BTC}

x2 :: Money Customer Balance
x2 = Money {amt = MoneyAmount 1, cc = B_EUR}
-- this will not compile
--x3 :: Money Customer String
--x3 = Money {amt = MoneyAmount 1, cc = "B_EUR"}