Question

So I have this code here:

toWords :: String -> [String]
toWords "" = []
toWords (nr1 : rest)
    | nr1 == ' ' = toWords rest
    | otherwise = [nr1] : toWords rest

The "toWords" function should simply remove all spaces and return a list with all the words. But this happens:

*Main> toWords "How are you?"

["H","o","w","a","r","e","y","o","u","?"]

Answer 1

Your type should be String -> [String] or [Char] -> [[Char]].

Your input is a string (a list of chars) your output a list of string (a list of chars of chars).

Your type here means it maps a string to ANY type, this is not true.

Edit: alternatively you can use:

splitBy :: [a] -> a -> [[a]]
splitBy [] sep = []
splitBy (nr1 : rest) if nr1 == sep then splitBy rest sep else nr1 : (splitBy rest sep)

Which is polymorphic and splits a list by any separator. (code not tested though) so splitBy "string of words" ' ' should return [“string”,”of”,”words”].

FINALLY, this was annoying and had an obvious and stupid error of [] in lieu of [[]] the functional version is:

splitBy [] sep = [[]]
splitBy (nr1 : rest) sep = if nr1 == sep 
    then [] : splitBy rest sep 
    else (nr1 : head (splitBy rest sep)) : tail (splitBy rest sep)

Such that: splitBy "List of things" ' ' ===> ["list","of","things"]