Operator =?? (opposite to ??=)

[rostopira]

I propose to make =?? operator
Expression a =?? b works as

if (b != null)
  a = b;

If ??= exists, why this one doesn't?

[ThinkDigitalSoftware]

This can be accomplished by doing

a = b ?? other;

This is equivalent to

if(b != null)
    a = b;
else
    a = other;

This seems to work in any case I can think of. If you don't want to change a if b is null,
then use

a = b ?? a;

[lrhn]

The general problem is that of if (x != null) .... some operation containing x .....
That syntax works, but can feel cumbersome. In particular it often requires an extra variable because you need to both test and use the value of an expression.

We have added some extra syntax in some cases, where you can test a value and do something in a single expression. Those are (currently):

• e1 ?. selectors ↦ let tmp = e1 in tmp == null ? null : tmp.selectors
• e1 ?? e2 ↦ let tmp = e1 in tmp != null ? tmp : e2
• x ??= e2 ↦ let tmp = x in tmp != null ? tmp : x = e2

These are all defined to work on a very specific syntaxes, so it's clear to the user what the scope of the operation is.

I'd prefer to not add more specific operators, unless they are very generally useful (which the "specific" likely prevents), so I'd defer to #219.