`Char' is applied to too many type arguments [duplicate]

Question

This question already has answers here:

`String' is applied to too many type arguments (2 answers)

Closed 9 years ago.

I'm trying to write a haskell program that takes in a string and returns a new string that only contains the vowels but I'm getting this error.

`Char' is applied to too many type arguments
In the type signature for `vowels': vowels :: Char a => a -> a

Here's my code:

vowels :: (Char a) => a -> a
vowels str = [ x | x <- str, x `elem` ['A','E','I','U','Y','O']]

Answer 1

Your type signature should be

vowels :: [Char] -> [Char]
vowels str = [ x | x <- str, x `elem` ['A','E','I','U','Y','O']]

or

vowels :: String -> String

In your code, (Char a) => ... indicates that the type a should be an instance of the typeclass Char. However, Char is a data type, not a type class which is why you get the error. Since you don't need any classes to implement this function you can just remove the constraint.

Answer 2

Is this what you're trying to achieve?

vowels :: String -> [Char]
vowels = let
             y =  ['A','a','E','e','I','i','U','u','Y','y','O',o']
         in filter.(\x -> elem x y)