Playing around with `buildMap` but please can some...

# getting-started

[Mark]

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

[gildor]

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

[Joffrey]

@gildor why though? Technically it could infer the whole if expression as Any?

[gildor]

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

[gildor]

Technically it could infer the whole if expression as Any?

I think because it’s not really an expression it’s BuilderInference

[Joffrey]

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.

[gildor]

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

[Joffrey]

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 😄

[gildor]

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

[Mark]

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

[Joffrey]

@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

[Mark]

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

[Victor Petukhov]

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

[Mark]

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