How does Reactive's Monad instance handle simultaneous events?

In Push-Pull Functional Reactive Programming by Conal Elliot, Reactives are given a Monad instance in section 7.1.3. What happens if both the inner and outer reactive fire that the same time for join?

never :: Event a
MkFuture :: Time -> a -> Future a
pure :: a -> Future a -- at time zero
MkReactive :: a -> Event a -> Reactive a
MkEvent :: Future (Reactive a) -> Event a

join (MkReactive (MkReactive "foo" never) (MkEvent (pure (MkReactive "bar" never)))

Should "foo" be ignored or included in the result on the join?

Solution

Here is the (slightly paraphrased) join definition from section 7.1.3:

-- urr is the outer Reactive; ur is the inner one.
joinR :: Reactive (Reactive a) -> Reactive a
joinR ((a `Stepper` Ev ur) `Stepper` Ev urr) = a `Stepper` Ev u
    where
    u = ((`switcher` Ev urr) <$> ur) <> (join <$> urr)

The Monoid instance being used in the definition of u is the one for Futures, defined in section 4.5, which in case of simultaneous firings picks the left one. That being so, the inner firing is not discarded if there is a simultaneous firing in join. For the specific example in your question, sampling at time zero should still give out "foo". (I believe the monad law join . fmap return = id would be violated if that wasn't the case)