loop in haskell to repeat a block of code [duplicate]

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.

Solution 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_.

Solution 2:

Well Haskell's IO is a bit tricky when you're just starting out since it's based on monads.

Your problem though has a simple solution:

main = replicateM_ 10 $ putStrLn "a string"

This is using the combinator replicateM_ from Control.Monad

It has lots of useful functions for composing and executing monadic actions.

Solution 3:

I am also a beginner of Haskell, and I have a solution that is less elegant and yet is pragmatically useful.

main = do 
    putStr result
    where
        string = "a string"
        result = concat [string ++ "\n" | i <- [1,2..10]]

So here, we have defined a list, the elements of which are the strings that you want to print out followed by a new line character.