I would like to propose a new syntax to simplify e...

# language-evolution

[melatonina]

I would like to propose a new syntax to simplify expressions involving nullable types and avoid awkward nested ?.let { } expressions, in case it has not been discussed before.
Whenever, inside of an expression, a ? (question mark) is appended to the name of a variable (mutable var or non mutable val) of nullable type, the whole expression containing it would evaluate to null if that variable is null, otherwise the variable is cast to the respective non-nullable type.

fun f(a: A): B = TODO()
fun g(a: A?): B? = f(a?)

fun h(a: Int?, b: Int): Int? = (a? + b)*2
fun i(a: Int?, b: Int?, c: Int?): Int? = (a? + b?)*c?

I don't know how to call this feature. Maybe "short-circuiting null operator"? As suggested by @uli, this feature is symmetric to the ?. operator, and would work on all arguments of a function like ?. does on the this argument.
Would this syntax cause conflict with other language features? Would you consider its implementations?

[melatonina]

This is the original discussion, in the wrong channel: https://kotlinlang.slack.com/archives/C0B9K7EP2/p1615799229038800?thread_ts=1615799229.038800&cid=C0B9K7EP2

[uli]

There is a good hint in the name you choose. Discussing this name brings us immediately to the question do we want to shirt circuit. I.e. not evaluate parameters 2 if parameters 1 is already null? Seems a little unexpected?

[uli]

With only one single ?-expression on the receiver there was no short circuiting

[uli]

So i am totally in favor of the idea. But i see some important design questions

[Imran/Malic]

In Arrow continuations there is `nullable`https://github.com/arrow-kt/arrow/blob/85862e5629f19c1858891904044a864308d81ebe/ar[…]re-data/src/test/kotlin/arrow/core/computations/NullableTest.kt
But this would be great to have something in that direction at language level.

[melatonina]

@uli, thanks making those observations. I should check the case of other method invocations, but, as far as assignment is involved, safe-calls (?.) do perform short-circuiting. The documentation says:

// If either `person` or `person.department` is null, the function is not called:
person?.department?.head = managersPool.getManager()

But you are right: these issues should be discussed and defined properly.

[melatonina]

@Imran/Malic Yes, Arrow nullable was mentioned in the original thread by @kioba. Thanks for mentioning it here too.

[Imran/Malic]

Oh I am sorry for repeating that part 🙏🏽 @melatonina. I haven’t checked the previous conversation

[melatonina]

A variant of this proposal would be liming the scope of the short-circuit only to the function call where the variable? expression appears. So:
(f(a?) + 2) * g("hello")
would only short-circuit
(f(a?) + 2)
and probably cause a compile-time error. While:
(f(a?) + 2)? * g("string")
would short-circuit the whole expression.
This would be an option. I'm not sure which one I'd like more.

[uli]

The interesting case is: f(effect1()?, effect2()?) because effect1 and effect2 are independent

[uli]

And in person?.department?.head there is no short circuit. That's why you need two question marks. person?.department.head would fall because ? does not short circuit

[melatonina]

@uli I see what you mean, now: what I meant is that the right hand of the assignment is not evaluated.

[uli]

@melatonina you are right. In the case of optional left hand side expressions ? does short circuit. thanks for pointing that out. i was not immediately aware.