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garyb / purescript-indexed-monad

MIT License
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Makes ipure x y instead of x x #30

Closed mikesol closed 3 years ago

mikesol commented 3 years ago

I think (hope) this is legal - there seems to be precedent elsewhere for this: https://gist.github.com/gaku-sei/434d4de22dd66402728542cb9c3c7d35#file-indexedburger-re-L19. If this is swingable, it may solve a couple small issues in our codebase.

JordanMartinez commented 3 years ago

Pretty sure merging this PR would break the guarantees of an indexed monad.

ipure x x a means the "state" of the index does not change, which makes sense when you think about it. You're only lifting a value into a given indexed monad context. You're not using some API that changes the index's "state."

iwhen and iunless aren't as usable in indexed monads because indexed monads don't handle errors very well. Errors represent a fork in computations: one of two things could occur. What should be the state of the index at that point? Whenever the index's state is forked, one must 'unfork' it by handling the error immediately, which isn't always possible.

garyb commented 3 years ago

Yeah, Jordan has it right unfortunately! I understand the desire for this, but it basically would make ipure something like unsafeCoerce for the index, since you could just do something along the lines of void $ ipure unit and that will freely change the index without actually performing an action that would affect the condition being tracked by the index types.

mikesol commented 3 years ago

Makes total sense, thanks for the explanation!