can someone explain or point documentation what is the reasoning behind that using return in list.map { return xxx } is not actually return of value of the lambda function? and what is the use case where this return behavior would be useful?

You mean the

return

in

map

actually returns to a outer scope?

Yes, "non-local returns" it is

The reasoning is in consistency. In Kotlin,

return

is consistent in that it returns from the enclosing

fun

by default. From lambdas you don’t usually return — you program them (usually) in functional style where their value is the last expression.

I made a silly bug because I tried to write

list.map { it -> 
    return if (condition) {
        Foo()
    } else Bar()
}

It makes sense to add some kind of inspection that would recognize mistake of the first type. It should be easy to detect.

Agreed