is there a compiler flag or something I can set to make sure that an expression that is supposed to ...

# getting-started

[David Smith]

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. 😞

[Sam]

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]

class Effect<A, B>(val f: (A) -> B)
fun effect(): Effect<Unit, Unit> = Effect { println("launch missiles") }
fun <A, B> runEffect(eff: Effect<A, B>, a: A): B = eff.f(a)
fun wontLaunchMissiles(eff: Effect<Unit, Unit>): Unit = run { effect() }

[David Smith]

I want the compiler to tell me to write run { runEffect(effect()) } because I have returned an Effect<Unit> not a Unit

[David Smith]

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]

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]

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]

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]

yeah, it looks like KT-12719 would do it. However that is a harder problem to solve so might not happen any time soon 😞
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 😞

[David Smith]

ok thanks anyway!

[Sam]

Oh, looks like #detekt can help! https://detekt.dev/docs/rules/potential-bugs/#ignoredreturnvalue

[David Smith]

oh not heard of Detekt

[David Smith]

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]

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 👍

[David Smith]

ah cool