Add a lawful parallel Applicative instance with derivatives #2
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| if (O.isSome(a)) { | ||
| return consume(f(a.value))().then(ret => { | ||
| resolveFa(ret); | ||
| return promiseFa.then(retFa => pipe(retFa, E.apSecond(ret))); |
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This is where the magic happens that ensures that the correct return value is used, right now. But it also prevents fab's disposal from running until fa's disposal has completed. Ideally, we'd allow both disposals to run in parallel, just as both acquisitions did.
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So the version I've currently pushed is lawful, but sequential in its disposal (parallel only in acquisition). In practice however, I find that most services have a much faster disposal Task than acquisition Task. So I think I prefer this lawful, predictable, abstract-able, but sequentially disposing version of ApplyPar over the unlawful, surprising, abstraction-breaking, but parallel disposing version of ApplyPar. |
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Okay, yeah. I'll include this changes in #1 |
The parallel Apply instance currently included isn't lawful, as demonstrated by the tests added in this PR.
The problem is that when:
ap'd Brackets fails; andthe sequential Apply favours the
E(Left "error" value) returned from the disposal, but the parallel Apply favours theEreturned from the consumption. This seems like it might be easy to fix, but it turns out that there doesn't seem to be a way to reliably detect whether it was a Bracket's consumption that failed, or whether it was its disposal. Without this knowledge, the parallel Apply implementation can't choose the correctEto return.