Haskell無法將期望的類型“ Integer”與實際類型“ [Integer]”進行匹配

Question

這是我的代碼，我怎么能說n在2..10之間的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

我建議您使用模式匹配代替警衛：

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

如果要確保不能使用超出特定范圍的值來創建數值，那么在進行排名時，應使用一個smart constructor來驗證此屬性：

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