Make `launchAff_` operate only on `Aff Unit`

[i-am-the-slime]

Problem

I’m finding launchAff_ to be quite dangerous. When I have an interface that requires an Effect Unit but I need to perform some network calls I usually provide the callback with launchAff_ . Now I ran into a situation where I used launchAff_ on an Aff (Either Error Unit) which is not what I intended. Do you think it would hurt much if launchAff_ only worked on Aff Unit values? That way it would be similar to the warning about implicitly discarding values in a do block.

Describe the solution you'd like

launchAff_ has the signature

launchAff_ ∷ Aff Unit → Effect Unit

Additional context
I'm aware that this should cause a major version bump since it's a breaking change. I'm hopeful however, that when this change requires somebody to change their code it will improve its clarity or even catch potential bugs.

[garyb]

+1 from me! It's the same kind of thing discard is meant to prevent in a monadic context.

[JordanMartinez]

main :: Effect Unit
main = launchAff_ $ void do
  problemNotSolved

[i-am-the-slime]

@JordanMartinez I don't understand, can you elaborate?

[garyb]

I don't really think that's an issue, using unsafeCoerce there would have the same effect 😉, it's the same situation as discard too, void is supposed to let you do that.

[JordanMartinez]

Ah, I misread the opening issue. The issue is that launchAff_ will convert things to Aff Unit when you may not want it to. By changing the type signature, it brings this issue immediately to the developer. The developer will either think "Oh no! I don't want launchAff_ to discard that value! Let me fix that" (as is the case that opened this issue) or "Ah... I don't care about that value. Nothing a void can't fix." So, it's a minor annoyance (and breaking change) for most developers but otherwise makes the resulting code much safer.

Please disregard my former comment.

[wclr]

launchAff_ is just a shortcut for using void, to me, it's not justified why such things exist at all. Trying to make things simplier easier we get less predictable code and behaviour. So I would consider deprecating and removing such things.

[cryogenian]

👍 for removing launchAff_