Multiple Occurrences of an Event at the same Time

Question

Conal Elliott's paper defines Event as

type Event a = [(T , a)] -- for non-decreasing times

This would allow more than one occurrence at a time. In my FRP library I would like to implement the function:

sample :: Reactive a -> Future () -> Future a

This would sample the reactive when the future fires. Since the reactive can have more than one value at the time the future fires how should I implement it? Always use the last, the first, or a nonempty list?

This is how the sample function should behave:

sample
    (MkReactive "a"
        (MkEvent
            (MkFuture 2
                (MkReactive "b" ...)
            )
        )
    (MkFuture 1 ())
= MkFuture 1 "a"

Answer 1

In the paper Push-pull functional reactive programming (if duplode is correct), the fundamental sampling combinator switcher is described as:

The semantics of b₀ `switcher` e chooses and samples either b₀ or the last behavior from e before a given sample time t

sample should behave the same way to be compatible with the combinators in that paper. Since a Reactive is an Event with an initial value, and a Future is a time-value pair, sample r (MkFuture t ()) should return a MkFuture t v where v is the last value in r from strictly before t, or the initial value of r if there is no such value. There's no issue with multiple values at the same time: any values at time t are ignored since they're not strictly before t, and of the values before t, "take the last one" is unambiguous.