10-datatypes.hs

-- data Name = Constructor1 <args> | Constructor2 <args>

data Calculation
  = Add Int Int
  | Sub Int Int

calc :: Calculation -> Int
calc (Add x y) = x + y
calc (Sub x y) = x - y

-- * Main> calc 1 1

-- <interactive>:76:1: error:
--     • Couldn't match expected type ‘t0 -> t’ with actual type ‘Int’
--     • The function ‘calc’ is applied to two arguments,
--       but its type ‘Calculation -> Int’ has only one
--       In the expression: calc 1 1
--       In an equation for ‘it’: it = calc 1 1
--     • Relevant bindings include it :: t (bound at <interactive>:76:1)