[GUI] Re: Events and finalizers

[Daan Leijen]

On Mon, 10 Mar 2003 21:36:00 +0100, Nick Name <nick.name@inwind.it> wrote:

> We don't have to waste time, just to solve the main issues and produce a
> clean and working implementation.

I love this sentence :-)

> I propose here to join our efforts and design a robust and generic
> stream library, wich has to be thread-safe, and subsume MVars, Channels
> and M.Chackravarty's Ports.

This doesn't sound like a good plan to me --this has a scope way beyond a standard library. It is not clear at all how to do this properly.

I do think it is an interesting subject, and I encourage you to pursue
your ideas and create a working implementation to show and discuss.
Maybe you can even work together with other people to find the right abstractions.

Don't get me wrong, I think it is great to discuss these things and it is fun to have this GUI specific list. I just think that certain items
shouldn't be part of the common GUI proposal. Let's keep these things
seperate.

In general, I think that we should only consider items that are
a) well understood
b) offer (just) enough functionality to implement more ambitious designs

Given (b), I would even go for *less* abstraction in order to provide the raw functionalty of the platform -- don't try to protect the user at
this level but offer programmers the freedom to create high level designs.

All the best,
  Daan.


> On Mon, 10 Mar 2003 18:12:19 +0100
> George Russell <ger@tzi.de> wrote:
>
>>
>> Well there are two approaches:
>> (1) you provide a more complicated function which in addition to the
>> new      event, returns an IO () action which unbinds it.  This is
>> what HTk      does, in general, though for many simple examples it is
>> of course not      required.
>> (2) in many but not all cases it is possible to work out that the
>> stream      can be automatically deregistered.  For example, as well
>> as finalizers      you can also forget the stream when the object is
>> destroyed.
>
> Exactly the same thing wich I am discussing with A.Reid in haskell-cafe;
> but finalizers don't "finalize" immediately in some cases, even if they
> might be satisfying. Moreover, there might be a different approach
> involving a list of "unsafePerformIO" values to uninstall the callbacks
> earlier, but I am not sure if it's worth the drawbacks.
>
> === AN IMPORTANT NOTE ===
>
> I want to explain better what you just stated: "a callback
> model also has to have a way of deinstalling the callback".
>
> If we return, together with the stream, the IO action used to unregister
> the callback, it could easily become of no use if we pass the stream to
> a new thread. We can't then know when that thread finishes to use the
> stream (we could pass the closing action together with the stream,
> anyway), so generally we HAVE to rely on garbage collection and
> finalizers. But this is the same thing of registering a callback wich
> multiplexes events to two threads, and the same issue.
>
> =====
>
> As usual, if there are two different approaches, each one with
> advantages and disadvantages, I prefer both, so the interface of a
> generic Stream library should provide both
>
> getStream :: IO [a]
>
> and
>
> getStreamAndTheOtherThing :: (IO [a],IO ())
>
> The former should use finalizers to deallocate any resource when
> appropriate, the latter could eventually use them anyway.
>
> However, George, there are other problems with streams, wich are leading
> me to a conclusion; before revealing it :) , here's some sample issue:
>
> - sometimes, the stream is produced with a synchronized model: you ask
> for a stream element only when you are ready to process it. This is used
> in gtk2 to avoid a backlog of mouse motion messages. We have to find a
> way (perhaps with a list of unsafeInterleaveIO results) to easily handle
> such cases; or maybe you already got it ?
>
> - if we want a library wich generically models a changing state, we have
> to deal with three different types, and combination of those: State,
> Input streams and Output streams. I provide now an example of any of
> these categories, and of any combination of these (directly taken from
> the comments I wrote last night on my source code :))  -- The standard input is only an Input, it has no notion of "current"
> -- value
>
> -- The standard output is only an Output
>
> -- The current time is only a State, it makes no sense to "listen" to
> -- it since it's a continuous value, but you can read it at a given
> -- moment
>
> -- An MVar has only State and Output
>
> -- A Chan has only Input and Output
>
> -- The mouse position has Input and State
>
> -- A Port (in M.C. terminology) has Input,Output and State
>
> CONCLUSION:
>
> I propose here to join our efforts and design a robust and generic
> stream library, wich has to be thread-safe, and subsume MVars, Channels
> and M.Chackravarty's Ports. I am doing such a work alone to use it in my
> future projects, and it grows up well, but of course one thing is me, a
> student, doing an hobbist work, and another thing is to work together.
> We don't have to waste time, just to solve the main issues and produce a
> clean and working implementation.
>
> An example of things to decide includes wether to use constructor
> classes or multiparameter type classes with functional dependencies
> (once stated that "we are GHC dependent anyway", to quote Axel)
>
> My experience (and your, I guess) is that such a library can be
> implemented correctly in a couple of days, so there shouldn't be any
> problem, after some week of planning. I propose this just to avoid each
> stream-lover to rewrite everithing again each time. What do you think?
>
> If so, the GUI task force will only have to be kind and use a generic
> layer for state handling, say a "Var" type with "getVar" and "setVar",
> so that we can provide an implementation wich also has streams when we
> are done.
>
> If anyone (not just George) is interested, the best thing to do is to
> post on the haskell mailing list to ask for interested people, and then
> do the job. What do you think?
>
>
>
> _______________________________________________
> GUI mailing list
> GUI@haskell.org
> http://www.haskell.org/mailman/listinfo/gui
>
>