{-# LANGUAGE AllowAmbiguousTypes #-}

module Foo where

import Control.Applicative

data Currency

class IsCurrency (a :: Currency) where
  
data Money (currency :: Currency) where
  Money :: IsCurrency currency => Int -> Money currency
  
data ExchangeRate from to

convert :: IsCurrency to => ExchangeRate from to -> Money from -> Money to
convert = undefined

foo :: Maybe (Money to) -> Maybe (Money from) -> ExchangeRate from to -> Maybe (Money to)
foo mMoneyTo mMoneyFrom exchangeRate =
  (fmap (convert exchangeRate) mMoneyFrom) <|> mMoneyTo