[Haskell-cafe] Design and state...

Magnus Therning magnus at therning.org
Wed Mar 21 10:57:17 EDT 2007

On Wed, Mar 21, 2007 at 07:19:20 -0700, Stefan O'Rear wrote:
>On Wed, Mar 21, 2007 at 09:28:15AM +0000, Magnus Therning wrote:
>> Recently I've been hacking together a small Haskell module on top of
>> ptrace(2).  It's comming along nicely, but it's very low level.  The
>> signals arrive in a list:
>>   wait :: Int -> IO [Signal]
>Aside: Why lazy? It seems like
>wait :: Int -> IO Signal
>would be better, and almost as easy to use:
>sequence_ . repeat $ wait >>= processSignal

If I do it lazy then I can end the list when a "terminating signal"
arrives (e.g. on interrupt).  AFAICS the only option when using an
infinate list is to throw an error.  (Am I right about this?)

One could argue that I need to deal with errors anyway so what you
suggest might turn out to be more elegant in the end.

(Another cute thing is that your suggestion removes the need to use
unsafeInterleaveIO.  See [1] for that discussion.)

>> and processing of the signals can be done using e.g. mapM_:
>>   wait childPid >>= mapM_ processSignal
>> (Or would it be better using IORefs and "OO" in the way described in "IO
>> Inside"[1]?)
>> What I am struggling with is how to allow the user of the module to have
>> state as well.  I can see how "OO"[1] can be used to solve it, but is
>> there a more elegant, maybe more idiomatic (Haskellic??), way?
>In some sense a PID is *already* a handle to external mutable state,
>so the original motivation for IORefs applies.
>data Child = Child { pid :: CInt, debug_state :: IORef StateBlock }
>wait :: Child -> IO ()

Yes, good point.  Thanks.


[1]: http://www.haskell.org/pipermail/haskell-cafe/2007-January/021367.html

Magnus Therning                             (OpenPGP: 0xAB4DFBA4)
magnus at therning.org             Jabber: magnus.therning at gmail.com
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