Haskell无法将期望的类型“ Integer”与实际类型“ [Integer]”进行匹配

Question

这是我的代码，我怎么能说n在2..10之间的int

   data Rank = Numeric Integer | Jack | Queen | King | Ace
        deriving (Eq, Show)

valueRank :: Rank ->Integer
valueRank rank

|rank ==Jack = 10
|rank ==King = 10
|rank ==Queen = 10
|rank ==Ace = 10
|rank == Numeric n = n
  where n =[x|x<-[2..10]]

Answer 1

我建议您使用模式匹配代替警卫：

valueRank :: Rank -> Integer
valueRank Jack = 10
valueRank King = 10
valueRank Queen = 10
valueRank Ace = 10
valueRank (Numeric n) = n

如果要确保不能使用超出特定范围的值来创建数值，那么在进行排名时，应使用一个smart constructor来验证此属性：

makeRank n
  | 1 <= n <= 13 = ...
  | otherwise = error ...