quick, simple proposal

# language-proposals

[eduardog3000]

quick, simple proposal

Untitled.kt

[Marc Knaup]

What are the situations where you’d use that?

In years of Kotlin I’ve never even needed the current form x = x ?: y 🤔

[eduardog3000]

You've never done anything along the lines of if(x == null) x = y or the JavaScript pattern x = x || y?

[eduardog3000]

Ruby has ||= which is about the same as this idea. It's ultimately the same as x || x = y, so if x isn't nil it skips the assignment operator entirely. Kotlin could ultimately optimize this down to if(x == null) x = y, or something else more efficient.

[Marc Knaup]

I’ve never like the JavaScript pattern. The problem with it is that it has a different meaning than if (x === null) x = something .
If x is a property or has a delegate, there may be a setter behind it which is triggered by the re-assignment of the variable to itself, which can cause unexpected side-effects.

For the same reason you cannot automatically convert that pattern to the if form because the semantics are different.

[eduardog3000]

Ignoring JavaScript, Ruby's ||= solves your problem by being equivalent to x || x = y.

I'm not understanding how the last paragraph connects? Why couldn't ?= be equivalent to if(x == null) x = y in Kotlin, which means in reality that = still might be calling a setter, but only if x is null?

[eduardog3000]

Ignoring usefulness now, I'm just curious why you are saying (I think) that it can't work at all.

[Marc Knaup]

I’m not saying that it can’t work.
I’m saying that if (x === null) x = y and x = x ?: y do different things.

The latter translates to

if (x === null)
   x = y
else
   x = x

And the last statement here, x = x can cause side-effects if it is a property with a setter or a has a delegate. Both may executes additional code due to re-assignment of the same value.

[Leon K]

i strongly dislike this as it would encourage having x be mutable for no good reason.