Proposal: overhaul System.Process

[Don Stewart]

duncan.coutts:
> 
> On Tue, 2008-04-22 at 15:35 -0700, Bryan O'Sullivan wrote:
> > Neil Mitchell wrote:
> > 
> > > I would have thought (ExitCode,String) was more appropriate.
> > 
> > Yes, definitely.
> 
> Yes, I mentioned this to Don previously when he published his popen
> code. I think he agreed.
> 
> Duncan

I'd changed, but not pushed out, process-light:

--
-- | readProcess forks an external process, reads its standard output
-- strictly, blocking until the process terminates, and returns either the output
-- string, or, in the case of non-zero exit status, an error code, and
-- any output.
--
-- Output is returned strictly, so this is not suitable for
-- interactive applications.
--
-- Users of this library should compile with -threaded if they
-- want other Haskell threads to keep running while waiting on
-- the result of readProcess.
--
-- >  > readProcess "date" [] []
-- >  Right "Thu Feb  7 10:03:39 PST 2008\n"
--
-- The argumenst are:
--
-- * The command to run, which must be in the $PATH, or an absolute path 
--  
-- * A list of separate command line arguments to the program
--
-- * A string to pass on the standard input to the program.
--
readProcess :: FilePath                     -- ^ command to run
            -> [String]                     -- ^ any arguments
            -> String                       -- ^ standard input
            -> IO (Either (ExitCode,String) String)  -- ^ either the stdout, or an exitcode and any output

readProcess cmd args input = C.handle (return . handler) $ do
    (inh,outh,errh,pid) <- runInteractiveProcess cmd args Nothing Nothing
    output  <- hGetContents outh
    outMVar <- newEmptyMVar
    forkIO $ (C.evaluate (length output) >> putMVar outMVar ())
    when (not (null input)) $ hPutStr inh input
    takeMVar outMVar
    ex     <- C.catch (waitForProcess pid) (\_e -> return ExitSuccess)
    hClose outh
    hClose inh          -- done with stdin
    hClose errh         -- ignore stderr

    return $ case ex of
        ExitSuccess   -> Right output
        ExitFailure _ -> Left (ex, output)

  where
    handler (C.ExitException e) = Left (e,"")
    handler e                   = Left (ExitFailure 1, show e)