[reactive] Temporal mechanics

[Svein Ove Aas]

Specifically, the implementation of Time. I have some options, and
would like your opinion.

Facts:
- There are two serious options for joining (eg. join :: m (m a) -> m
a) futures, and by extension events/reactives/behaviour. Picking the
largest time value, or summing them. Both are useful in different
scenarios.
- There are essentially two variants of time; absolute and relative.
Absolute time might be represented as nanoseconds since 1970, while
relative would be relative to some arbitrary starting point.
Currently, and in conal's reactive, they are not differentiated.

After giving it some thought, I believe that it would be reasonable to
split Time in two types; one for representing absolute time, one
relative; of course parametrizing the event/etc. types on the Time
type the same way it's done in reactive.

The join operators would be implemented as using a Monoid instance on
the time types, which act a bit differently, depending.

When mappending absolute times, the max of the two times would be
used, with mempty of course MinBound.
For relative times, the times would be summed; mempty = 0.

One thing I'm not entirely sure about is the interface between
absolute and relative time. Probably, the best option is to have a
specialized join function for absolute :. relative -> absolute; other
requirements will likely surface over time.


This setup lends itself well to many common cases. When mapping one
event occurence to an event consisting of multiple reactions spread
over time, the reactions are naturally relative to the original
occurence; using a sum function to join them, you won't have to care
about exactly when that original happens.

What do you think? Sounds reasonable?


Lastly one ability that would take implementation effort all out of
its proportion to its usefulness is the ability to subtract two
absolute times, giving a relative time, and have that time provide
partial information at any point before both absolute times are fully
defined. If any of you would like that, and have a good reason, now is
the time to speak up; doing it later would only increase the needed
effort (though not beyond the bounds of Time.hs).

-- 
Svein Ove Aas