[Haskell-cafe] Observer pattern in Haskell?
haskell at gimbo.org.uk
Mon Nov 9 09:23:42 EST 2009
I've been doing some GUI programming recently, using wx. To help
manage dependencies between state and UI elements, I looked for a
Haskell version of the Observer design pattern, and I found an
implementation written by Bastiaan Heeren of ou.nl .
It pretty much did what I wanted, though I made a few changes along
the way. Since this seems to be of general use (ie beyond wx), I've
proposed to Bastiaan that I package it up for release on hackage, and
he's happy for me to do so. Before I do so, I thought I'd ask for
The code is on github: http://github.com/gimbo/observer.hs where we
Control.Observer - a typeclass for observable objects.
Control.Observer.Synchronous - an implementation based on IORefs.
This is essentially the same as Bastiaan's original, except I've
changed the names, split it into two modules, commented it, added one
or two small things, and Cabalised it.
I've also made a branch where Control.Observer.Synchronous uses MVars
instead of IORefs, in an attempt to achieve thread safety:
Now, before I make a hackage release:
0. Is this completely insane, in a Haskell setting? Are there better
ways to do this that aren't laden with OO-worldview baggage?
1. Does anyone have any comments, on either version?
2. In particular, is the MVar version sensible? I'm aiming for mutual
exclusion between threads. I _think_ I've got it, but I'm perhaps not
familiar enough with the semantics of MVar to be certain. Advice
appreciated. If it _is_ sensible, then is there any reason not to
just use this, and discard the IORef version?
The current implementation is synchronous, in that any observer
functions are called immediately and synchronously (and in the same
thread as the change of subject value). I'm pondering extending the
package with an asynchronous version where the update just trips a
flag, and the observer function picks this up later - possibly in
another thread. The idea there is to help in cases where certain
operations have to be in a particular thread. But this will mean a
change to the typeclass too, I guess - or the addition of another one
for observers themselves. Again, any thoughts?
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