Implementation of a program in which characters of a string repeated certain times in haskell

Question

This is a question from my homework thus tips would be much likely appreciated.

I am learning Haskell this semester and my first assignment requires me to write a function that inputs 2 string (string1 and string2) and returns a string that is composed of (the repeated) characters of first string string1 until a string of same length as string2 has been created.

I am only allowed to use the Prelude function length.

For example: take as string1 "Key" and my name "Ahmed" as string2 the function should return "KeyKe".

Here is what I've got so far:

    makeString :: Int -> [a] -> [a]
    makeString val (x:xs)
        | val > 0 = x : makeString (val-1) xs
        | otherwise = x:xs

Instead of directly giving it two strings i am giving it an integer value (since i can subtitute it for length later on), but this is giving me a runtime-error:

*Main> makeString 8 "ahmed"

"ahmed*** Exception: FirstScript.hs: (21,1)-(23,21) : Non-exhaustive patterns in function makeString

I think it might have something to do my list running out and becoming an empty list(?).

A little help would be much appreciated.

Answer 1

I think this code is enough to solve your problem:

extend :: String -> String -> String
extend src dst = extend' src src (length dst)
    where
        extend' :: String -> String -> Int -> String
        extend' _ _ 0 = []
        extend' [] src size = extend' src src size
        extend' (x:xs) src size  = x : extend' xs src (size - 1)

The extend' function will cycle the first string until is is consumed then will begin to consume it again.

You can also make it using take and cycle like functions:

repeatString :: String -> String
repeatString x = x ++ repeatString x

firstN :: Int -> String -> String
firstN 0 _ = []
firstN n (x:xs) = x : firstN ( n - 1 ) xs

extend :: String -> String -> String
extend src dst = firstN (length dst) (repeatString src)

or a more generic version

repeatString :: [a] -> [a]
repeatString x = x ++ repeatString x

firstN :: (Num n, Eq n ) => n -> [a] -> [a]
firstN 0 _ = []
firstN n (x:xs) = x : firstN ( n - 1 ) xs

extend :: [a] -> [b] -> [a]
extend _ [] = error "Empty target"
extend [] _ = error "Empty source"
extend src dst = firstN (length dst) (repeatString src)

which is capable of taking any type of lists:

>extend [1,2,3,4] "foo bar"
[1,2,3,4,1,2,3]