Re: [Haskell-cafe] Intercept stdin in Haskell

Hello, That last version was still off. I think we are going with John's idea, which is slightly more concretely in: http://lpaste.net/98017 I expect there are still issues concerning how much input to request, so that getLine, getChar and getContents all behave as they do in the console. -- Adam On Sun, Jan 5, 2014 at 7:45 PM, Andrew Gibiansky wrote:

You are a saviour!

We'd actually already tried going down this path with takeMVars and putMVars, but somehow it didn't quite work. Let's see if this works in practice in IHaskell!

Working program below:

import Control.Concurrent import Control.Monad import GHC.IO.Handle import GHC.IO.Handle.Types import System.IO import System.Posix.IO

main = do -- Create a pipe using System.Posix and turn it into handles. (readEnd, writeEnd) <- createPipe newStdin <- fdToHandle readEnd stdinInput <- fdToHandle writeEnd

-- Store old stdin and swap in new stdin. oldStdin <- hDuplicate stdin hDuplicateTo newStdin stdin

-- In a separate thread, wait for the read. forkIO $ forever $ do let FileHandle _ mvar = stdin threadDelay $ 200 * 1000 empty <- isEmptyMVar mvar when empty $ do putStrLn "Empty!" hPutStrLn stdinInput "foo" hFlush stdinInput

putStrLn "Waiting." threadDelay $ 3 * 1000 * 1000 putStrLn "Reading." getChar >>= print

On Sun, Jan 5, 2014 at 7:11 PM, John Lato wrote:

...

Here's a stupid idea:

A Handle contains an MVar Handle__, and when a thread calls hGetLine stdin, it will take that MVar, attempt to read from the buffered device, and then block until there's data available to be read from the device. You could check if the MVar is empty, and if so, assume that something is trying to read from stdin and write your input into the device.

Horrible, unsound hack, I'm sure, but it's all I've got...

On Sun, Jan 5, 2014 at 3:14 PM, Andrew Gibiansky wrote:

...

Looks like the excitement was a bit premature. The types work, and in Haskell that often means the program works... but looks like hDupTo relies on the `dup2` of the IODevice class, and attempts to cast one IODevice to another IODevice. Since I'm trying to replace stdin (with IODevice type Fd) with my own IODevice, the cast fails and raises an exception. Practically ClassCastException.... yeesh.

On Sun, Jan 5, 2014 at 5:19 PM, Andrew Gibiansky wrote:

...

I think we found a way! (With a *ton* of help from @aavogt - might actually be more correct to say he found the way :) )

You can use `hDupTo` to change what a Handle points to. You can use `mkFileHandle` in GHC.IO.Internal to create a new file handle. You can implement your own IODevice and BufferedIO datatype to give to `mkFileHandle` instead of using `Fd`. Then, when your "device" is being read from, you just implement `newBuffer` and `readBuffer` to do whatever you need them to.

Results pending.

-- Andrew

On Sun, Jan 5, 2014 at 4:14 PM, Donn Cave wrote:

...

I bet a quarter you can't do it. You'd need access to the process state - whether it's blocking for I/O and whether one of the units in the input set is 0 ("stdin".) Even if you could get that? you'd have to poll for it, which would be hideous.

That's the UNIX I/O model. I've always found it a little annoying, because I could do this with the VMS `mailbox' device, analogous to UNIX pipes - in various ways a more sophisticated interprocess communication system than UNIX's.

Donn _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

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