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0 pointsratww3y ago0 comments

Monads themselves aren't really contagious, it's the actions that would otherwise have side-effects that are, and also the fact they have to be executed in sequence.

This is also true for other things in other paradigms, such as async functions in javascript.

This is a good thing, however. In imperative programming, you have invisible temporal coupling. In pure-FP you have the same coupling, but it's exposed.

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garethrowlands3y ago

While I broadly agree with you, I think the post you're responding to has a point. You can't, in general, get a value back out of a monad, so if you call a monadic function, you may well have to return a monad. The obvious example is IO: there's no (safe) way get the `a` from `IO a`, so IO is kinda contagious.

Then again, there are lots of monads, such as `Maybe` and `List`, where you can get values out. These aren't contagious at all.

I agree with you that this is a good thing. Effects show up in the type signature - and it's all about those effects and managing them.

ratwwOP3y ago

Yes, this is what I mean. IO is contagious for reasons unrelated to it being a Monad.

endgame3y ago

I think that applicatives and monads feel more contagious than they really are, because at first people tend to write functions that consume values of type `f a` too readily. This is because it takes some time to become comfortable with `fmap` and friends, so the new Haskell programmer often doesn't write as many pure functions.

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