demo.hs

-- Does anyone understand what this code does?
-- What is a and b?
-- We add two amounts
-- We know that an "amount" is an Int

type TransferId1 = Int

type Amount1 = Int

add1 :: Amount1 -> Amount1 -> Amount1
add1 a b = a + b

-- Is this an improvement?
-- Our function name is shorter
-- And it's clear that we add together amounts

-- but we can

transferId1 = 1 :: TransferId1
amount1 = 42 :: Amount1

-- add1 amount1 amount1 
-- -> 84 OK!
-- add1 transferId1 amount1
-- -> 43 should ERROR!

newtype TransferId2 = TransferId2 Int
  deriving (Show)

newtype Amount2 = Amount2 Int
  deriving (Show, Num)

add2 :: Amount2 -> Amount2 -> Amount2
add2 (Amount2 a) (Amount2 b) = Amount2 $ a + b

transferId2 = TransferId2 1
amount2 = Amount2 42

-- add2 amount2 amount2
-- -> Amount2 84 OK!
-- add2 transferId2 amount2
-- -> ERROR, nice!

data Currency = SEK | NOK
  deriving (Eq, Show)

-- We want to represent

amountSek2 = Amount2 499
amountNok2 = Amount2 299

-- add2 amountSek2 amountNok2
-- -> 798 should ERROR!

data Amount3 = Amount3
  { amount :: Amount2
  , currency :: Currency
  } deriving (Show)

add3 :: Amount3 -> Amount3 -> Maybe Amount3
add3 (Amount3 a c1) (Amount3 b c2) 
  | c1 == c2  = Just $ Amount3 (a + b) c1
  | otherwise = Nothing

amountSek3 = Amount3 499 SEK
amountNok3 = Amount3 499 NOK

-- add3
-- add3 amountSek3 amountNok3
-- -> Nothing OK!

-- So we get Nothing here, can we do better than this?
-- What do you think?

data SEK4 = SEK4
  deriving (Show)

data NOK4 = NOK4
  deriving (Show)

data Amount4 a = Amount4
  { amount :: Amount2
  , currency :: a
  } deriving (Show)

add4 :: Amount4 a -> Amount4 a -> Amount4 a
add4 (Amount4 a c) (Amount4 b _) = Amount4 a c

amountSek4 = Amount4 499 SEK4
amountNok4 = Amount4 499 NOK4

-- add4
-- add4 amountSek4 amountNok4
-- -> ERROR, nice!