Dispatching to correct function with command line arguments in Haskell

Question

I\'m writing a little command-line program in Haskell. I need it to dispatch to the correct encryption function based on the command line arguments. I\'ve gotten that far, but t

Answer 1

One way is to wrap your functions inside functions that do further command line processing. e.g.

dispatch::[(String, [String]->IO ())]
dispatch = [("EEncipher", takesSingleArg ExptCiphers.exptEncipherString)
            ("EDecipher", takesSingleArg ExptCiphers.exptDecipherString)
            ("CEncipher", takesTwoArgs CaesarCiphers.caesarEncipherString)
            ("CDecipher", takesTwoArgs CaesarCiphers.caesarDecipherString)
            ("CBruteForce", takesSingleArg CaesarCiphers.bruteForceCaesar)]

-- a couple of wrapper functions:

takesSingleArg :: (String -> IO ()) -> [String] -> IO ()
takesSingleArg act [arg] = act arg
takesSingleArg _   _     = showUsageMessage

takesTwoArgs :: (String -> String -> IO ()) -> [String] -> IO ()
takesTwoArgs act [arg1, arg2] = act arg1 arg2
takesTwoArgs _   _            = showUsageMessage

-- put it all together

main = do
    (command:args) <- getArgs
    case lookup command dispatch of
         Just act -> act args
         Nothing  -> showUsageMessage

You can extend this by having variants of the wrapper functions perform error checking, convert (some of) their arguments into Ints / custom datatypes / etc as necessary.

As dbaupp notes, the way we pattern match on getArgs above isn't safe. A better way is

run :: [String] -> IO ()
run [] = showUsageMessage
run (command : args)
   = case lookup command dispatch of
          Just act -> act args
          Nothing  -> showUsageMessage

main = run =<< getArgs