尝试在 Haskell 中自己定义过滤器函数但失败了

Question

尝试定义过滤函数如下

myFilter :: (a -> Bool) -> [a] -> [a]
myFilter f [] = []
myFilter f (x:xs) 
  | f x = x : myFilter xs 
  | otherwise = myFilter xs

我把它放到了名为myfilter.hs的文件中并加载到ghci。我收到以下错误

myfilter.hs:5:26: error:
    • Couldn't match expected type: a1 -> Bool
                  with actual type: [a]
    • In the first argument of ‘myFilter’, namely ‘xs’
      In the expression: myFilter xs
      In an equation for ‘myFilter’:
          myFilter f (x : xs)
            | f x = x : myFilter xs
            | otherwise = myFilter xs
    • Relevant bindings include
        xs :: [a] (bound at myfilter.hs:3:15)
        x :: a (bound at myfilter.hs:3:13)
        f :: a -> Bool (bound at myfilter.hs:3:10)
        myFilter :: (a -> Bool) -> [a] -> [a] (bound at myfilter.hs:2:1)
  |
5 |   | otherwise = myFilter xs
  |                          ^^
Failed, no modules loaded.
ghci>

错误是什么以及如何解决

Answer 1

您应该使用谓词 f 和列表的其余部分 xs 调用 myFilter，因此：

myFilter :: (a -> Bool) -> [a] -> [a]
myFilter f [] = []
myFilter f (x:xs) 
  | f x = x : myFilter <strong>f</strong> xs -- ← call with f as first parameter
  | otherwise = myFilter <strong>f</strong> xs

尝试在 Haskell 中自己定义过滤器函数但失败了

try to define filter function myself in Haskell and failed

haskell

filter

higher-order-functions