How to repeat a function in this case? Haskell

Question

I have a function that draws a line of strings and a function that draws a box of strings:

duplicate :: [a] -> Int -> [a] -- Duplicates a string/array
duplicate dup n = concat $ replicate n dup

printLine :: String -> Int -> IO () -- Prints a line of strings
printLine str n = putStrLn $ duplicate str n

printBox :: String -> Int -> Int -> IO () -- Prints a box of strings
printBox str width height = putStrLn $ unlines $ replicate height $ duplicate str width

main :: IO ()
main = printBox "-" 10 10 -- Will print 10x10 box of "-" string

I noticed that I should use printLine in printBox , because printLine is a part of the functionality of printBox .

However, I tried many times and failed miserably. How do I use printLine in printBox to achieve the same effect? Should I somehow repeat it?

Answer 1

You can use replicateM_ from Control.Monad to implement this as follows:

import Control.Monad

main :: IO ()
main = printBox "-" 10 10 -- Will print 10x10 box of "-" string

duplicate :: [a] -> Int -> [a] -- Duplicates a string/array
duplicate dup n = concat $ replicate n dup

printLine :: String -> Int -> IO () -- Prints a line of strings
printLine str n = putStrLn $ duplicate str n

printBox :: String -> Int -> Int -> IO () -- Prints a box of strings
printBox str width height = replicateM_ height (printLine str width)

Effectively the function replicates your monadic action n times and discards the results. Documentation can be found here

Demo