In Haskell how can you multiply a string?

Question

I'm trying to write a function that takes a String and an Int and returns that string "int" times. That is:

duplicate :: String -> Int -> String

If I were to write duplicate "Hello" 3 the output should be "HelloHelloHello".

Answer 1

Easily:

duplicate :: String -> Int -> String
duplicate string n = concat $ replicate n string

The $ is a function of type (a -> b) -> a -> b. The language allows the functions with non-alpha-numeric names to be used in infix form (as operators). I.e., the body of the function above is absolutely identical to the following expression:

($) concat (replicate n string)

What $ does is just allows you to get rid of braces. Meaning that the above expressions are just an alternative to the following expression:

concat (replicate n string)

Answer 2

A String is just a synonym for a list of Char, and the list type is a Monad. Therefore

duplicate :: Int -> String -> String
duplicate n str = [1..n] >>= const str

Or, if you wanted to get all point-free

duplicate = (. const) . (>>=) . enumFromTo 1

---

Edit

As suggested in the comments

duplicate n str = [1..n] >> str

or

duplicate = (>>) . enumFromTo 1

Answer 3

You can use replicate and concat as follows:

duplicate :: [a] -> Int -> [a]
duplicate = flip $ (concat .) . replicate

-- or as larsmans suggested:

duplicate :: [a] -> Int -> [a]
duplicate = (concat .) . flip replicate

Then use it as duplicate "Hello" 3.