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

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