haskell - 无法将预期类型 ‘Int -> [Char]’ 与实际类型 ‘Int’ 匹配 可能的原因 : ‘mod’ is applied to too many arguments

Question

我的代码:

weekDay :: Int -> Int -> Int -> Int -> [Char]
weekDay day month year = mod (day + x + (31 * m0) `div` 12) 7
          where y0 = year - ((14 - month) `div` 12)
                 x = y0 + y0 `div` 4 - y0 `div `100 + y0 `div` 400
                 m0 = month + 12 * ((14 - month) `div` 12) - 2
                 day month year = case day month year of
                           0 -> Sonntag
                           1 -> Montag
                           2 -> Dienstag
                           3 -> Mittwoch
                           4 -> Donnerstag
                           5 -> Freitag
                           6 -> Samstag
                           otherwise -> error

错误:

 error:
    • Couldn't match expected type ‘Int -> [Char]’
                  with actual type ‘Int’
    • Possible cause: ‘mod’ is applied to too many arguments
      In the expression: mod (day + x + (31 * m0) `div` 12) 7
error:
 • Non type-variable argument in the constraint: Eq ([Char] -> a)
      (Use FlexibleContexts to permit this)
    • When checking the inferred type
        day :: forall a t1 t2.
               (Eq ([Char] -> a), Num ([Char] -> a)) =>
               t1 -> t2 -> [Char] -> a

Answer 1

您可以简化代码以确定错误的原因。例如

weekDay :: Int -> Int -> Int -> Int -> [Char]
weekDay day month year = mod 4 7

会导致同样的问题

weekDay :: Int -> Int -> Int -> Int -> [Char]
weekDay day month year = 10

和

weekDay :: Int -> [Char]
weekDay = 10

我发现这种技术非常通用，可用于解决各种棘手的问题。