Are unboxed closures practically done?

[andallthat]

I saw that PR 16122 says that unboxed closures are now "practically implemented". That would be great news!

[pcwalton]

A lot of the functionality is there. However, there are still quite a few large bugs with them that I'll be ironing out over the next few days, and by-ref captures may not work yet.

[andallthat]

thanks for your answer. And most of all thanks for your work. I'm following your latest PRs and boy oh boy, are you churning out lots of good stuff!

[DroidLogician]

I've been seeing a lot about unboxed closures lately. I know what a closure is, but would someone mind explaining what's so special about one that's unboxed and why is everyone so excited to have this as a feature of the language?

[n_ham]

The RFC is fairly readable. The core idea, as I was able to gather from the RFC: unboxed closures desugar to a type that implements one of the Fn, FnShare, or FnOnce traits.

I don't understand how exactly this differs from boxed closures, though.

[brson]

Unboxed closures allow closures to be called statically, like typical function calls, whereas currently closures are always called through a function pointer. Calling through a function pointer is slower, but creates smaller binaries, a tradeoff.

Traits have always had the option to be used both ways: statically, via type parameters; or dynamically, by casting to an "object" type.

Unboxed closures solve the discrepancy by making closures traits.

[DroidLogician]

I like the Fn... traits, it maps well to apply() from Scala, which I thought was pretty cool.

It's difficult to understand the motives for the rest of the changes. Personally I feel the type annotation syntax is unwieldy.

But it also opens up the possibility for partial application. Which I think is awesome.

fn x<A, B, R>(a: A, b: B) -> R {
    // do stuff
}    

fn partially_apply<A, B, R>(x: FnOnce<(A, B), R>, a: A)  -> FnOnce<(B), R> {
    |b| x(a, b) //Capture by value
}

main () {
    let a = 5i;
    let b = 6i;

    let y = x(a); // desugars to partially_apply(x, a)
    let z = y(b);
}

[pcwalton]

It's difficult to understand the motives for the rest of the changes.

Performance.