haskell Differences in two words [closed]

Question

I have 2 words with the same length, for example: "mother" and "father". My program should:

a) if the letter in the x position in the first word is the same as in the x position in the second word: the letter should stay on its place

b) if the letter in the x position in the first word isn't the same as in the x position in the second word: the letter should be replaced with sign "-"

function mother father
--result "--ther"

So far I made this:

function :: (Eq a) => [a] -> [a] -> [a]
function [] [] = []
function [x] [] = [x]
function [] [x] = [x]
function (x:xs) (y:ys) = if x == y
                           then x : function xs ys
                       else
                            "-" : function xs ys

It gives error

Couldn't match type `a' with `[Char]

I have no clue how make last line works. Do you have any idea what should I do here?

Answer 1

You use "-", hence that means that the result is a list of Strings. You probably want to use '-', then it is a list of Chars, so a String:

function :: String -> String -> String
function x [] = x
function [] x = x
function (x:xs) (y:ys)
    | x == y = x : function xs ys
    | otherwise = '-' : function xs ys