[Haskell-cafe] Intercept stdin in Haskell

John Lato jwlato at gmail.com
Mon Jan 6 00:11:37 UTC 2014

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 at 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 at 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 at 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 at haskell.org
>>> http://www.haskell.org/mailman/listinfo/haskell-cafe
> _______________________________________________
> Haskell-Cafe mailing list
> Haskell-Cafe at haskell.org
> http://www.haskell.org/mailman/listinfo/haskell-cafe
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://www.haskell.org/pipermail/haskell-cafe/attachments/20140105/1bb79809/attachment-0001.html>

More information about the Haskell-Cafe mailing list