[Haskell-cafe] Design and state...

[Stefan O'Rear]

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

> and processing of the signals can be done using e.g. mapM_:
> 
>   wait childPid >>= mapM_ processSignal
> 
[snip]
> (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 ()

...

HTH,
Stefan