Evaluate code block in Kotlin (to hide variables inside scope) - kotlin

Question

In Scala, you can write
val x = {
... do some complex computations ..
42
}
to hide stuff inside of the code block.
The closest I came in Kotlin is:
val x = {
... do some complex computations ..
42
}()
Is there a better way?
EDIT:
isn’t run {} in the above example essentially the same
is calling run costly?
ANSWER:
using run {} inlines, whereas {}() does NOT (see my own answer below)

Answer 1

Use the run function. It takes a function as a parameter, runs it and returns the result.
val x = run {
... do some complex computations ..
42
}
The run function is inlined, so it will have no performance overhead.

Answer 2

Yes there is the run function.
val x = run {
...
42
}

Answer 3

And also you can use these methods too:
val a=1.also{
// your code
}
val b=2.apply{
// your code}
}
val c=3.let{
// your code
}
val d=4.runCatching{
// your code
}

Answer 4

To verify inlining when using run, I created a small example:
fun main() {
{
Math.random()
}()
run {
Math.random()
}
}
and viewed the produced bytecode:
public final static main()V
L0
LINENUMBER 2 L0
GETSTATIC TestKt$main$1.INSTANCE : LTestKt$main$1;
CHECKCAST kotlin/jvm/functions/Function0
INVOKEINTERFACE kotlin/jvm/functions/Function0.invoke ()Ljava/lang/Object; (itf)
POP
L1
LINENUMBER 6 L1
L2
L3
L4
ICONST_0
ISTORE 0
L5
LINENUMBER 7 L5
INVOKESTATIC java/lang/Math.random ()D
L6
L7
LINENUMBER 6 L7
L8
POP2
L9
LINENUMBER 9 L9
RETURN
L10
LOCALVARIABLE $i$a$-run-TestKt$main$2 I L5 L7 0
MAXSTACK = 2
MAXLOCALS = 1
So, indeed:
Calling via () calls a generated Function0-derived class that encapsulates the block as a lambda (label L0)
Calling via run fully inlines the block (label L5)