simple Haskell loop

Question

I just started learning Haskell, but the absence of loops is infinitely frustrating right now. I figured out how to write loops for functions. My problem, however, is that I want to output some results while iterating the loop. It seems that I have to use debug to perform this simple task.

So right now I would just appreciate an example of how to print out a string 10 times in the main structure.

In other words, I want to do this 10 times:

main = do  
    putStrLn "a string" 

Thanks. I feel this will be very illuminating for my task.

question from:https://stackoverflow.com/questions/16004365/simple-haskell-loop

Answer 1

You could define a recursive function that prints "a string" n times (n being the parameter of the function), like this:

printStringNTimes 0 = return ()
printStringNTimes n =
 do
  putStrLn "a string"
  printStringNTimes (n-1)

main = printStringNTimes 10

A somewhat more general approach would be to define a function that repeats any IO action n times:

repeatNTimes 0 _ = return ()
repeatNTimes n action =
 do
  action
  repeatNTimes (n-1) action

main = repeatNTimes 10 (putStrLn "a string")

The above function already exists in Control.Monad under the name replicateM_.