Assigning command line arguments to variables

Question

I need to convert my command line argument chars to Ints. I would like to assign each one to a variable and use them in my function. I have the following:

import System.Environment   

getIntArg :: IO Int
getIntArg = fmap (read . head) getArgs

main = do
    n <- getIntArg
    print n

However, it doesn't loop through all the arguments only printing one. Also, would I assign each one to a variable to use?

I'm new to FP.

Answer 1

if you want to map all arguments to Ints you should use

getIntArgs :: IO [Int]
getIntArgs = fmap (map read) $ getArgs

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or a bit shorter:

getIntArgs = map read <$> getArgs

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instead.

This way you can do

main = do
    ns <- getIntArgs
    print ns

and then get the ones you are interested in with (!!) or if you know/expect at least a fixed number of Int - arguments with

(n1:n2:n3:_) <- getIntArgs

but of course you should maybe check the length first - also this will fail if the user decides to input things that cannot get parsed into Ints

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So if you don't want to reinvent the wheel (which might be ok if you want to learn or only need a quick solution) you maybe want to look around and use a existing package - for example parseargs to do this for you