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 ""
)
