is there a compiler flag or something I can set to make sure

Question

is there a compiler flag or something I can set to make sure that an expression that is supposed to return `Unit` has to return `Unit` By being able to return anything when a `Unit` is expected I have created a bug in my code where some piece of side effecting code wasn t run to return `Unit` but the compiler didn t pick it up because you can return anything if `Unit` is expected disappointed

Sam · Answer

Can you share a small code example I think I have some idea of what you mean but it s not totally clear to me

David Smith · Answer

```class Effect lt A B gt val f A gt B fun effect Effect lt Unit Unit gt = Effect println launch missiles fun lt A B gt runEffect eff Effect lt A B gt a A B = eff f a fun wontLaunchMissiles eff Effect lt Unit Unit gt Unit = run effect ```

David Smith · Answer

I want the compiler to tell me to write `run runEffect effect ` because I have returned an `Effect lt Unit gt ` not a `Unit`

David Smith · Answer

FYI this is obviously contrived but we have an effects system that we use in house and it effectively does something like this

David Smith · Answer

I understand that for certain types of coding there is a reasonable argument for not forcing `Unit` to be returned but for a functional style this return type indicates that you have done some side effect and so by not returning it explicitly you are losing the ability for the compiler to tell you that you ve made a mistake I wondered if anyone knew of some linting rule or experimental compiler flag or something to disable this implicit `Unit` return

Sam · Answer

I think basically this is the same issue as <https youtrack jetbrains com issue KT 12719> i e `effect ` returns a non unit value so the IDE compiler should always warn if it s unused

Sam · Answer

The rule about using the last statement of a lambda function as its return value just doesn t apply to Unit functions see <https kotlinlang org docs lambdas html lambda expression syntax> But I think this problem generalises beyond lambda functions anyway A warning would be useful anywhere `effect ` is called without using its result

David Smith · Answer

yeah it looks like KT 12719 would do it However that is a harder problem to solve so might not happen any time soon disappointed I don t think a ktlint rule would be possible because it can t know the return type of the thing you want to be warned isn t unit disappointed

David Smith · Answer

ok thanks anyway

Sam · Answer

Oh looks like < C88E12QH4|detekt> can help

David Smith · Answer

oh not heard of Detekt

David Smith · Answer

I think I might have a go using that however unfortunately this rule requires an annotation It will still be useful to put in places where I know something should always be used

Sam · Answer

I think you can also configure the detekt rule to inspect everything by setting `restrictToAnnotatedMethods` to false if you want The configuration for that rule looks pretty flexible thumbsup

David Smith · Answer

ah cool