Sorry for taking so long to have a look at this, and sorry that I can only triage this at the moment.

I think the idea is interesting and useful.

Note that the implementation of fromPull does not need a LatchWriter, the purpose of the latter is only to connect a Pulse and a Latch. In this case, everything can be implemented within the Latch record. The reason why your code does not work is probably that the _seenL value is not updated.

I think a correct implementation would simply be to apply cachedLatch to an IO action (lifted into the EvalL monad).

Could this concept simply replace fromPoll? fromPull has the same type signature, and pulling only wen necessary seems preferable to polling on every input event.

Note that the implementation of fromPull does not need a LatchWriter, the purpose of the latter is only to connect a Pulse and a Latch. In this case, everything can be implemented within the Latch record. The reason why your code does not work is probably that the _seenL value is not updated.

I think a correct implementation would simply be to apply cachedLatch to an IO action (lifted into the EvalL monad).

I believe if you implement this feature only with cachedLatch, the IO action will be performed as many times as the latch is evaluated. For example,

b <- fromPull (print 5)
e1 <- b <@ someEvent
e2 <- b <@ someEvent

reactimate (print 10 <$ e1)
reactimate (print 20 <$ e2)

Here, ideally the action print 5 would only be performed once. But because it's sampled by two different events, this would print

5
10
5
20

when someEvent fires.