问构造一个类型

EN

module SumProduct where


  data GuessWhat = Chickenbutt deriving (Eq, Show)

  data Id a = MkId a deriving (Eq, Show)

  data Product a b = Product a b deriving (Eq, Show)

  data Sum a b = First a
                | Second b
                deriving (Eq, Show)

  data RecordProduct a b = RecordProduct { pfirst :: a , psecond :: b }
                          deriving (Eq, Show)


  newtype NumCow = NumCow Int deriving (Eq, Show)

  newtype NumPig = NumPig Int deriving (Eq, Show)

  data Farmhouse = Farmhouse NumCow NumPig deriving (Eq, Show)
  type Farmhouse' = Product NumCow NumPig


  newtype NumSheep = NumSheep Int deriving (Eq, Show)

  data BigFarmhouse = BigFarmhouse NumCow NumPig NumSheep deriving (Eq, Show) 
  type BigFarmhouse' = Product NumCow (Product NumPig NumSheep)

  type Name = String
  type Age = Int
  type LovesMud = Bool


  type PoundsOfWool = Int
  data CowInfo = CowInfo Name Age
                 deriving (Eq, Show)

  data PigInfo = PigInfo Name Age LovesMud deriving (Eq, Show)

  data SheepInfo = SheepInfo Name Age PoundsOfWool deriving (Eq, Show)


  data Animal = Cow CowInfo 
               | Pig PigInfo
               | Sheep SheepInfo
               deriving (Eq, Show)


  type Animal' = Sum CowInfo (Sum PigInfo SheepInfo)(Sum PigInfo SheepInfo)

Prelude> let bess = First (CowInfo "Bess" 4) :: Animal'
Prelude> let elmer' = Second (SheepInfo "Elmer" 5 5)