Haskell：无法将预期类型“IO t0”与实际类型“[Char]”匹配

Question

myTakeWhile :: (a-> Bool ) -> [a] -> [a]
myTakeWhile _ [] = []
myTakeWhile pred (head:tail) = if pred head
                           then head:myTakeWhile pred tail
                           else []

main =  myTakeWhile (/= ' ') "This is practice."

gives me this error:

Couldn't match expected type ‘IO t0’ with actual type ‘[Char]’
    • In the expression: main
      When checking the type of the IO action ‘main’

Answer 1

A main always has type IO something , not a String which is the result type myTakeWhile (/= ' ') "This is practice." .

You can work with print:: Show a => a -> IO () to print it as a string literal , or putStrLn:: String -> IO () to print the content of the string, so:

main :: IO ()
main =   (myTakeWhile (/= ' ') "This is practice.")