Behaviors are sometimes recomputed unnecessarily

[HeinrichApfelmus]

One of the "small efficiency problems" I alluded to in the package description is that behaviors are sometimes recomputed unnecessarily.

Example:

let b = bexpensive <*> bin
in (liftA2 (+) b b) <@> e1

The value corresponding to bin will be computed twice. While this doesn't make a difference semantically, it may be expensive.

There is no fundamental reason for this omission, I was just too lazy to implement it. The main reason for that is that I will have to revamp all the internals in a future release anyway. The future release won't happen very soon, though, that's why I'm mentioning the issue here.

By the way, the offending code is the ApplyB case of the following function:

-- compile a behavior
-- FIXME: take care of sharing, caching
compileBehaviorEvaluation :: Behavior Linear a -> Run a
compileBehaviorEvaluation = goB
    where
    goB :: Behavior Linear a -> Run a
    goB (ref, Pure x)            = return x
    goB (ref, ApplyB bf bx)      = goB bf <*> goB bx
    goB (ref, ReadBehavior refb) = readBehaviorRef refb

At the moment, it doesn't include any provisions for observable sharing.

---

Let me know if this turns out to be a problem for you; I will fix it if there is demand.

[ehird]

Looks like your example has an error, unless you intended to never reference b.

[HeinrichApfelmus]

Oops, thanks. Fixed.

[HeinrichApfelmus]

The internal changes in reactive-banana-0.5 eliminate this issue.

So, unless I messed up observable sharing or something, this problem is gone for good. It's time to close, yeah!