Higher level interface to System.Process

Ian Lynagh igloo at earth.li
Sun Dec 17 19:43:58 EST 2006

Hi Don,

On Sat, Dec 16, 2006 at 07:03:18PM +1100, Donald Bruce Stewart wrote:
> Ok, I really want to push forwards the effort to add a nice popen to
> base.  Here's my first effort at a minimal clean interface, that we
> might be proud to demonstrate in a tutorial ;)

That would be great!

Some comments:

>     readProcess cmd args input = C.handle (return . handler) $ do
>         (inh,outh,errh,pid) <- runInteractiveProcess cmd args Nothing Nothing
>         -- fork off a thread to start consuming the output
>         output  <- hGetContents outh
>         outMVar <- newEmptyMVar
>         forkIO $ C.evaluate (length output) >> putMVar outMVar ()

outMVar isn't needed, is it? Just putting the "hClose outh" in place of
the putMVar should work AFAICS. The only difference is that it wouldn't
guarantee that the handle will be closed by the time the main functions
is finished, but maybe with the finite number of file handles we have
that is worth spending an MVar for.

>         -- now write and flush any input
>         when (not (null input)) $ hPutStr inh input

          unless (null input) $ hPutStr inh input

>         hClose inh          -- done with stdin
>         hClose errh         -- ignore stderr

This will cause the command to fail if it tries to write to stderr (and
checks the result of the write). Doing the same thing with stderr that
you do with stdout would work.

>         -- wait on the output
>         takeMVar outMVar
>         hClose outh
>         -- wait on the process
>         ex <- C.catch (waitForProcess pid) (\_ -> return ExitSuccess)

Catching all exceptions and treating them as success doesn't seem right?
Why not just let them propagate?

>         return $ case ex of
>             ExitSuccess   -> Right output
>             ExitFailure _ -> Left ex
>       where
>         handler (C.ExitException e) = Left e
>         handler e                   = Left (ExitFailure 1)

Again, why not let the exceptions propagate? If you do want to catch
everything and return it as a value then I think you should allow the
possibility to return an Exception.


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