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I am attempting to run my program using the command line. I am trying to return my command line arguments:

import System.Environment   
import Data.List  

main :: IO()

main = do  
   args <- getArgs  
   progName <- getProgName  
   putStrLn "The arguments are:"  
   mapM putStrLn args  
   putStrLn "The program name is:"  
   putStrLn progName

I am executing the code by calling the main function with my arguments: 

main argument arguments "more arguements"

However, I am getting a complier error: 

<interactive>:33:6: Not in scope: ‘argument’
<interactive>:33:15: Not in scope: ‘arguments’

Is there an issue with how I am calling my function with my arguments?

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  • 1
    are you are calling this in GHCi? Commented Oct 26, 2015 at 6:19
  • Yes, I'm calling this in GHCi. Commented Oct 26, 2015 at 6:20
  • well Zeta was quicker ;) Commented Oct 26, 2015 at 6:20
  • Geez. That was driving me crazy. Thanks! Commented Oct 26, 2015 at 6:21

1 Answer 1

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10

You have to use :main if you want to simulate command line arguments. main alone only executes your IO () action, but doesn't actually build the arguments. For all what GHCi knows, main doesn't necessarily need to be IO (), it could be Int -> Int -> IO ().

However, if you use :main, GHC will use main in the same way it would get invoked during an runhaskell call, e.g. with interpreting the following parameters as command line arguments.

Alternatively, you can use withArgs from System.Environment:

ghci> withArgs ["argument", "arguments", "more arguments"] main
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1 Comment

That totally makes sense. Thanks for the explanation, Zeta!

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