部分函数组合的语法

Question

module Luhn (isValid) where

import qualified Data.Char as C

isAsciiAlpha :: Char -> Bool
isAsciiAlpha = C.isAsciiLower || C.isAsciiUpper

isValid :: String -> Bool
isValid n
  | any ((isAsciiAlpha || C.isSpace) . not) n = False
  | otherwise = ys > 1 && sum xxs `mod` 10 == 0
  where
    xs = reverse [c | c <- n, isAsciiAlpha c]
    ys = length xs
    zs = zip xs (cycle [1, 2])
    xxs = [convert x y | (x, y) <- zs]

convert :: Char -> Int -> Int
convert c mul =
  do
    let n = C.digitToInt c
    case () of
      _
        | mul == 2 && (n > 4) -> n * mul - 9
        | otherwise -> n * mul

我正在为这条线苦苦挣扎： any ((isAsciiAlpha || C.isSpace) . not) n = False 。 我想要的很明显； 查找是否有任何字符不是 ASCII 字母或空格。

尽管尝试了各种语法，但我在这一行不断收到编译错误，比如

• Couldn't match expected type ‘Bool’
                  with actual type ‘Char -> Bool’

Answer 1

你不能在两个函数上使用(||) :: Bool -> Bool -> Bool ：参数应该都是Bool 。 您可以做的是构造一个函数，将字符c映射到isAsciiAlpha c || C.isSpace c isAsciiAlpha c || C.isSpace c ，所以\c -> isAsciiAlpha c || C.isSpace c \c -> isAsciiAlpha c || C.isSpace c ，或者你可以使用liftA2 :: Applicative f => (a -> b -> c) -> fa -> fb -> fc with liftA2 (||) isAsciiAlpha C.isSpace 。 not :: Bool -> Bool也应该应用于函数的结果，所以：

import Control.Applicative(liftA2)

isAsciiAlpha :: Char -> Bool
isAsciiAlpha = liftA2 (||) C.isAsciiLower C.isAsciiUpper

isValid :: String -> Bool
isValid n
  | any (not . liftA2 (||) isAsciiAlpha C.isSpace) n = False
  | otherwise = ys > 1 && sum xxs `mod` 10 == 0
  where -- …

您还可以使用(<||>) :: Applicative a => a Bool -> a Bool -> a Bool或其短路版本(||^) :: Monad m => m Bool -> m Bool -> m Bool protolude包的布尔值。