Question

I try to create an own list type in haskell but, my implementation contains errors.
What is the proper way to do this nice. Please explain me a bit.
Thank you.

My code:

data List a = EmptyList | ListElement a (List a) 

instance (Show a) => Show (List a) where
        show = showList'

showList' EmptyList = showString "[]"
showList' (ListElement a EmptyList) = show a
showList' (ListElement a b) = show a ++ show " " ++ showList' b

Error code:

[1 of 1] Compiling Main             ( tipusok.hs, interpreted )

tipusok.hs:12:39:
    Couldn't match expected type `Prelude.String -> Prelude.String'
                with actual type `[Char]'
    Expected type: ShowS
      Actual type: Prelude.String
    In the return type of a call of `show'
    In the expression: show a
Failed, modules loaded: none.

Answer 1

showList' EmptyList = showString "[]"

The type of showString is String -> ShowS. ShowS is a type synonym for String -> String, so the result of showString "[]" is the function that prepends the string "[]" to its argument. Since you gave no type signature, that equation determines what type is inferred for the function, but the other equations don’t match that type.

You probably wanted simply

showList' EmptyList = "[]"