[reactive] event merge problem?

[Conal Elliott]

Time in Reactive has some structure.

    type Time t = Max (AddBounds t)

Max adds a max-based Monoid instance.  It's a newtype and so doesn't add any
laziness/partiality.  AddBounds, however, is a data type:

    data AddBounds a = MinBound | NoBound a | MaxBound

Typically, the type parameter t in 'Time t' is bound to 'Improving Double',
which adds yet more partiality to times.

External inputs are represented by events whose occurrence times are wrapped
with NoBound.  The various matches against MaxBound then ought to fail
immediately, without any examination of the improving double inside the
NoBound.

That's my intention, anyway.  There may be a bug somewhere that thwarts this
trick.

I don't yet have a concrete example of the merge bug that Bob mentioned.
Once I have one, I'll see what I can learn about it.

  - Conal


On Tue, Dec 16, 2008 at 1:00 PM, Peter Verswyvelen <bugfact at gmail.com>wrote:

> But isn't just matching against MaxBound or MinBound enough to enforce the
> computation of the time value? And hence block when the event is
> not-never-occuring (hu that sounded weird)?
> 2008/12/16 Conal Elliott <conal at conal.net>
>
>> Yes indeed.  The issue is that if you know one of the events will never
>>> happen again, what you want to do is stop merging, and throw the
>>> never-occurring event in the bin.  Unfortunately, to pattern match against
>>> the never occurring event then blocks checking whether the match works or
>>> not :(.
>>
>>
>> I'm trying to understand what the problem is here.  I think the
>> not-yet-known values are all wrapped with a NoBound and so should pass right
>> by the MaxBound pattern matches without blocking.
>>
>>   - Conal
>>
>> On Tue, Dec 16, 2008 at 2:56 AM, Thomas Davie <tom.davie at gmail.com>wrote:
>>
>>>
>>> On 16 Dec 2008, at 11:50, Claus Reinke wrote:
>>>
>>>  Seems to be partially timer- and partially event-merge-related:
>>>>>> if I compile with -threaded, the pieces do not move at all on their
>>>>>> own, but as long as I keep pressing "down", the game appears
>>>>>> playable as intended; if I compile without -threaded, the pieces
>>>>>> move occasionally, but the effects of keyboard input are entirely
>>>>>> unpredictable.
>>>>>>
>>>>>
>>>>> I've noticed this behavior in since the last couple of days, So at a
>>>>>  guess a bug got introduced
>>>>> somewhere along the line.  The merge issue  is known -- at the moment,
>>>>> an occurrence doesn't
>>>>> appear in the output  until it's known whether or not the other event
>>>>> ever occurs, which  isn't
>>>>> ideal.  Unfortunately, the obvious fix for the issue creates a  large
>>>>> memory leak, I've been
>>>>> playing with a few ways to fix this.
>>>>>
>>>>
>>>> Thanks for the info, Tom,
>>>>
>>>> where can I find more details about this merge issue? Given that
>>>> FRPLs are traditionally synchronously concurrent languages, this
>>>> is rather surprising: when sampling anything, one knows about
>>>> everything's values _now_, which should be sufficient for merge,
>>>> no matter whether the other event might or might not happen later.
>>>>
>>>
>>> Yes indeed.  The issue is that if you know one of the events will never
>>> happen again, what you want to do is stop merging, and throw the
>>> never-occurring event in the bin.  Unfortunately, to pattern match against
>>> the never occurring event then blocks checking whether the match works or
>>> not :(.  The particular interesting bit of code is in PrimReactive.hs
>>> (Conal's comments here):
>>>
>>> -- | Merge two 'Future' streams into one.
>>> merge :: Ord t => Binop (FutureG t (ReactiveG t a))
>>>
>>> -- The following two lines seem to be too strict and are causing
>>> -- reactive to lock up.  I.e. the time argument of one of these
>>> -- must have been _|_, so when we pattern match against it, we-- block.
>>> --
>>> -- On the other hand, they patch a massive space leak in filterE.
>>>  Perhaps
>>> -- there's an unamb solution.
>>>
>>> Future (Max MaxBound,_) `merge` v = v
>>> u `merge` Future (Max MaxBound,_) = u
>>>
>>> u `merge` v =
>>>  (inFutR (`merge` v) <$> u) `mappend` (inFutR (u `merge`) <$> v)
>>>
>>> -- What's going on in this 'merge' definition?  Try two different
>>> -- future paths.  If u arrives before v (or simultaneously), then
>>> -- begin as u begins and then merge v with the rest of u.  Otherwise,
>>> -- begin as v begins and then merge u with the rest of v.  Because of
>>> -- the left-bias, make sure u fragments are always the first argument
>>> -- to merge and v fragments are always the second.
>>>
>>>
>>> Bob
>>
>>
>>
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>
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