try to define filter function myself in Haskell and failed-CodePudding

Try to define the filter function as follows

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

I put it into the file named myfilter.hs and loaded to ghci. I got the following error

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>

What the error about and how can I fix it

CodePudding user response：

You should call myFilter with the predicate f and the rest of the list xs, so:

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