获取类型中的值列表

Question

在Haskell中，是否可以动态地获取类型中的值列表？一个具体的例子是通过将每一级别和一套牌组合在一起，生成一套完整的扑克牌，以创建一张卡片列表：

data Rank = 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | J | Q | K | A
data Suit = Spades | Hearts | Diamonds | Clubs
data Card = Card Rank Suit

deck :: [Card]
deck = -- get list of ranks and suits to create a deck --

在没有硬编码值的情况下，如何实现这一点？

Answer 1

在这种情况下，最简单的方法是让编译器通过派生的Enum实例生成代码。

data Rank = Two | Three | Four | Five | Six | Seve | Eight | Nine | Ten | J | Q | K | A
    deriving (Show, Enum, Bounded)

data Suit = Spades | Hearts | Diamonds | Clubs
    deriving (Show, Enum, Bounded)

data Card = Card Rank Suit

deck :: [Card]
deck = do
    rank <- enumFrom minBound :: [Rank]
    suit <- enumFrom minBound :: [Suit]
    return $ Card rank suit

或者列表理解版本：

deck :: [Card]
deck = [Card rank suit | rank <- enumFrom minBound :: [Rank], suit <- enumFrom minBound :: [Suit]]