[Haskell-cafe] Intercept stdin in Haskell

Andrew Gibiansky andrew.gibiansky at gmail.com
Sun Jan 5 23:14:25 UTC 2014

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
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