Main.hs

module Main where

import System.Environment 
import System.Exit   

type Filename = String   

data Command = Foo Filename | Bar   

parseCommand :: [String] -> Maybe Command   
parseCommand ["--foo", file] = Just (Foo file)   
parseCommand ["--bar"] = Just Bar   
parseCommand _ = Nothing   

printUsage :: IO ()   
printUsage = putStrLn (  
    unlines ([
      "Usage: ",
      "myProgram [--foo file | --bar]"
    ])
  ) 

runCommand :: Command -> IO ()   
runCommand (Foo file) =  
  putStrLn ("Running Foo on: " ++ file)   
runCommand Bar =  
  putStrLn "Running Bar"   

main :: IO ()   
main = do    
  args <- getArgs   
  let mbCmd = parseCommand args  
  case mbCmd of    
    Just cmd -> runCommand cmd   
    Nothing -> do    
      printUsage   
      exitWith (ExitFailure 84)   

Main2.hs

module Main where {
  import System.Environment;
  import System.Exit;

  type Filename = String;

  data Command = Foo Filename | Bar;

  parseCommand :: [String] -> Maybe Command;
  parseCommand ["--foo", file] = Just (Foo file);
  parseCommand ["--bar"] = Just Bar;
  parseCommand _ = Nothing;

  printUsage :: IO ();
  printUsage = putStrLn (
    unlines ([
      "Usage: ",
      "myProgram [--foo file | --bar]"
    ])
  );

  runCommand :: Command -> IO ();
  runCommand (Foo file) =
    putStrLn ("Running Foo on: " ++ file);
  runCommand Bar =
    putStrLn "Running Bar";

  main :: IO ();
  main = do {
    args <- getArgs;
    let {
      mbCmd = parseCommand args;
    };
    case mbCmd of {
      Just cmd -> runCommand cmd;
      Nothing -> do {
        printUsage;
        exitWith (ExitFailure 84);
      };
    };
  };
}