Here's what I would do. There's a MaybeT monad in the transormers library that is pretty good for this sort of stuff. I might restructure it like this:

module Main where

import Control.Monad.Trans.Maybe (runMaybeT, MaybeT)
import Control.Monad.Trans (liftIO)
import System.Environment (getArgs)
import Control.Applicative ((<|>))

margs :: MaybeT IO ()
margs = do
[fname] <- liftIO $ getArgs
fstr <- liftIO $ readFile fname
let nWords = length . words $ fstr
nLines = length . lines $ fstr
nChars = length fstr
liftIO . putStrLn . unwords $ [ show nLines, show nWords, show nChars]

mnoargs :: MaybeT IO ()
mnoargs = liftIO $ print "No args"

main = runMaybeT (margs <|> mnoargs)

This exploits the alternative instance of MaybeT. If the pattern match for arguments fails, then the whole function returns nothing. That causes the alternative to be run instead. Also since MaybeT has an instance for MonadIO, you can do any IO you need by using liftIO.

There is also an EitherT type in the errors package that can return *why* something failed, but I haven't messed with it a ton, so I can't really give a tutorial.

On Sun, Jan 27, 2013 at 12:27 PM, Martin Drautzburg <Martin.Drautzburg@web.de> wrote:

Hello all,

in the code snippet below, is there a way to factor out the second "do"?

import System (getArgs)
main :: IO ()
main = do
args <- getArgs
case args of
[fname] -> do fstr <- readFile fname
let nWords = length . words $ fstr
nLines = length . lines $ fstr
nChars = length fstr
putStrLn . unwords $ [ show nLines
, show nWords
, show nChars
, fname]
_ ->putStrLn "usage: wc fname"

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