simple Haskell loop

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.

5 Answers

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_.

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.

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.

I think the most imperative looking form of doing a for loop is:

for list action = mapM_ action list main :: IO Int main = do for [0..10] (\ i -> do print(i^2) ) return 0 

This actually looks pretty much like C code to me.

Doing something like to this allows you to loop a specific function, making it more reusable (instead of writing it out for each new thing you want to loop).

loop :: Int -> (IO()) -> IO() loop 0 _ = return () loop n f = do f loop (n - 1) f 

Examples:

main = do loop 5 (do putStr "hello " putStrLn "there") 

main = do loop 3 (do loop 4 (putStrLn "Hi") putStrLn "" )