Typescript 4.0 is introducing two new operators:
In Rust:
fn main() {
let mut a = true;
a &&= true;
a ||= false;
}
In rare circumstances I wished to use this.
Typescript 4.0 is introducing two new operators:
In Rust:
fn main() {
let mut a = true;
a &&= true;
a ||= false;
}
In rare circumstances I wished to use this.
Oddly enough, I wanted this here: https://github.com/rust-analyzer/rust-analyzer/pull/4587/files#diff-2d3440bb91de25b42566a44695daf55cR135
Yes, it's not a super-common need, but once in a while I find a use for those operators. But I'm curious to know why Rust doesn't have them
Operators like +=
are required to implement operations efficiently for non-primitive types like bigints, where a += b
can be more efficient than a = a + b
. But for ||
and &&
, the first operand is necessarily a bool
, so ||=
is not as important to have as +=
. My point is that although they can save typing an identifier in some cases, I don't think these operators would carry their weight.
Just for comparison: Rust does have &=
and |=
operators which work with bool
, as well as numeric types and user-defined types. Of course, these don't have the short-circuiting behavior of &&
and ||
, so they are usually not appropriate when the RHS is expensive to evaluate. (For example, you can't use them to simplify @matklad's example, above.)
As with any time people suggest a new punctuation token:
Unfortunately, this would be breaking for some macros (at least without "interesting" compat hacks between the actual token model and macro_rules!
). (Or a new statement type <place> && = <expr>
which would work but would also be.. "interesting.")
So I doubt the small motivation outweighs the cost of adding the new token (or otherwise making it work). At the very least it'd be a "next edition" kind of thing.
Well, if the LHS is costly to compute (e.g. it is a long expression) then this will save a lot. Normal compiled languages have common sub-expression elimination that can probably lift out the common LHS and just use a pointer to the location to mutate. However, JavaScript is not compiled.
I would be interested to see how this is implemented though, especially when LHS &&= RHS;
where LHS
involves property access that can potentially have side effects because the equivalent LHS = LHS && RHS
is going to trip those side effects twice.
Maybe this is why it takes so long to show up.
I guess that it creates a new variable if necessary, e.g.
a.b().c &&= d();
// becomes
const _temp = a.b();
_temp.c && (_temp.c = d());
If c
is a property, both its getter and its setter might be called.
There is a note in the announcement that says beware of property access with side effects.
It also says it is smart enough to handle complicated expressions on the LHS...
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