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 usageLines)
  where usageLines = [
            "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 usageLines)
    where {
      usageLines = [
        "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);
      };
    };
  };
}