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 <jwlato@gmail.com> 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 <andrew.gibiansky@gmail.com> 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 <andrew.gibiansky@gmail.com> 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 <donn@avvanta.com> 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

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