Playing around with

buildMap

but please can someone explain why this gives a compilation error (unless I uncomment the null):

fun main() {
    buildMap {
        if (true) {
            println("test")
            // null
        } else {
            put("foo", "bar")
        }
    }
}

https://pl.kotl.in/sohJmyPiJ

It cannot infer type without it correctly, because if returns Unit as inferred type, but else set it to String

@gildor why though? Technically it could infer the whole

if

expression as

Any?

You can specify type explicitly, it will solve the problem: buildMap<String, String>

Yeah this works, also adding

Unit

after the

put

in the

else

branch works. Interestingly, adding

Unit

after the whole

if

is not sufficient, so it's not about the return value of the lambda expected by

buildMap

.

it’s not about the return value of the lambda expected by

Yes, it’s not about return value, it’s about builder interface inference

Yeah but I'd be interested in why the builder inference prevents this

if

from being legal. I expected the builder inference to look for

put

and infer the type based on that regardless of other structures and expressions. But I know nothing of the builder inference frankly 😄

I think it’s just a matter of implementation limitation, you right, it can find all possible branches and infer type, but it apparantly doesn’t do this

By the way, if I uncomment the null, then Android Studio gives a warning that

null

is unused.

@Mark I think the cleanest option when builder inference is moody is to explicitly specify the types in

buildMap

, as mentioned by Andrey. I wouldn't try to add

null

or

Unit

in the right places to fix the inference

@Joffrey totally agree, I just wanted to point out that small bug with the warning message.

@Mark thanks for the report! Could you create an issue please? https://youtrack.jetbrains.com/issue/KT-51143 Will try to fix it soon.

Fixed, thanks! https://youtrack.jetbrains.com/issue/KT-51143