Add explicit return type to some methods

[kmizu]

I think the code is more readable if a return type of a method is not omitted generally

[dottybot]

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[Blaisorblade]

Hi! Thanks for your PR, but how have you picked which methods to add a return type for? That's really not obvious to me, nor it is whether these methods need a return type.

[odersky]

I don't have a strong opinion but would argue that adding : Boolean to an is.. method is redundant. Similarly for : Unit. For the others, the change looks like an improvement in legibility.

[kmizu]

@Blaisorblade Hi. I think all public methods (private in some cases) should have explicit return type. in general. The reasons are below:

1. Return type inference may change method signature depending on implementation details. If these methods are called from different compilation unit, such accidental changes break binary compatibiliy and generate unneeded recompilation.
2. Depending on implementation details, very complex return type maybe inferred. Such a method is difficult to use.
3. If a return type is not inferred and written explicitly, the method user can call it without seeing its implementation.
4. adding explicit return type is low cost and have many merits.

In this PR, I picked several methods that the return types are inferred experimentally. If it's permitted, I'll try to add explicit return type to all public methods.

@odersky I think : Boolean to an is ... is not redundant. the consistency between the method implementation and the return type is guaranteed by : Boolean. Similarly for : Unit (in some cases, Anys were inferred accidentally).

[Blaisorblade]

Agree with most of those arguments, except that there's no binary compatibility to speak of (tho unneeded recompilation stands). Let's see what the others say — e.g. @allanrenucci, @smarter, what do you think?

[allanrenucci]

I usually only add explicit return type to public members. I can see value in adding explicit return type to private/local side effect-full methods that return Unit.

However in general, I would not add explicit return type to private/local methods (and local variables). These methods are usually fairly small and it makes the code more verbose and sometime less readable.

But that's my opinion 😄

[kmizu]

@allanrenucci I agree with you opinion (should not add explict return type to private/local methods).

[allanrenucci]

Hi @kmizu! We decided that we would accept a PR that adds explicit return type to all public methods. Note that you should be able to use scalafix to automate this task.

This PR adds explicit return type to local and private members which is not desired.

[OlivierBlanvillain]

It's been a month... @kmizu If you plan to continue working on this PR please reopen (or make a new one), I will close it for now.

[kmizu]

@OlivierBlanvillain I'm very sorry to forget it. Later, I'll continue working on this PR and reopen this PR (or make a new one).

[smarter]

Hi @kmizu, thanks for your work but note that meanwhile we've already added explicit result types everywhere in #5203 so this particular PR shouldn't be needed anymore.

[kmizu]

Hi, @smarter. I understand that this PR shouldn't be needed anymore. Thanks.