haskell - 不在范围内 : data constructor?

Question

我是haskell的新手。我遇到了这个问题:

Assignment12.hs:5:12:不在范围内:数据构造函数“Suit”

Assignment12.hs:5:18:不在范围内:数据构造函数“Rank”

Assignment12.hs:6:11:不在范围内:数据构造函数“Rank”

Assignment12.hs:7:11:不在范围内:数据构造函数“Rank”

Assignment12.hs:8:11:不在范围内:数据构造函数“否则”
失败，加载模块:无。

这是我的代码:

data Suit = Clubs | Diamonds | Hearts | Spades deriving (Show, Eq)
data Rank = Jack | Queen | King | Ace | Num Int deriving (Show, Eq)
type Card = (Suit, Rank)
cardValue :: Card -> Int
cardValue (Suit, Rank)
     | Rank == Ace = 11
     | Rank == Ace = 11
     | Rank == Jack = 10
     | Otherwise = Num

我真的很感谢你的帮助。谢谢

Answer 1

在 haskell 中，变量名必须以小写字母开头。任何大写字母都会被编译器解释为 Data Constructor这就是您收到该错误的原因。

当您定义 cardRank变量 (suit, rank)必须以小写字母开头，您的代码才能编译。

这应该工作

data Suit = Clubs | Diamonds | Hearts | Spades deriving (Show, Eq)
data Rank = Jack | Queen | King | Ace | Num Int deriving (Show, Eq)
type Card = (Suit, Rank)

cardValue :: Card -> Int
cardValue (suit, rank)
     | rank == Ace = 11
     | rank == Ace = 11
     | rank == Jack = 10
cardValue (suit, Num x) = x