Hey,
I’m not sure how people feel about Option types but there seem to be a few
hundred uses in the rust codebase. Wanted to share an idea we use in our custom
type system (“Hack”) at Facebook to make it a bit nicer to safely deal with
null references. We don’t use Option types directly. I don’t think this adds
any theoretical power or new possible efficiency gains, it’s mostly for making
the code a bit simpler.
Type declarations prefixed with a question mark represent references which
might be null. So ‘?Foo' is somewhat like a shorthand for 'Option<Foo>’.
function demo(?Car $maybe_car, Car $car) {…}
We use these like possibly-null pointers. Usually you write an if statement
prior to using a value.
function demo(?Car $maybe_car, Car $car) {
$car->start();
if($maybe_car) { $maybe_car->start(); }
}
Sometimes we use these in combination with a special function “invariant", an
assertion function that throws an exception if a condition is not met:
function demo(?Car $car) {
invariant($car, ‘Expected non-null car for the demo’);
$car->start();
}
If you forget to check for null one way or the other, the type-checker
complains. This is a static, ahead-of-time check.
function demo(?Car $car) {
$car->start(); // error
}
There are some natural annoying edge cases to be covered.
class Smash {
private ?Car $car;
function demo() {
if ($this->car) {
$this->smashCar();
$this->car->start(); // error
}
}
}
A downside of this approach vs. Option is that code written using pattern
matching over Option is more easily upgraded to code using pattern matching
over Result (or something else).
Anyway, we like this feature and I’d be happy to see it adopted elsewhere.
Tossing it out there as I don’t know anything about the Rust compiler or how
language design decisions get made for it :)
-Aran
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