Add a lawful parallel Applicative instance with derivatives #2
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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 theE
returned 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 correctE
to return.