simple Haskell loop

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_.

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.

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.

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