capture.rs

A macro for adding explicit capture clauses to a (closure-) expression.

Using this macro, it becomes possible to be explicit about what variables a closure captures, and by which capture mode it does so.

Syntax

capture!($([move IDENT,]
           [ref IDENT,]
           [IDENT IDENT,])*
         in EXPR)

Semantic

The macro will expand to nested block expressions with a let binding per capture clause:

• The move x clause rebinds a name by itself, which is effectively a no-op.
• The ref x clause rebinds a name by a shared reference to it.
• The ref mut x clause rebinds a name by a mutable reference to it.
• The IDENT x clause rebinds a name to the result of calling .IDENT() method on it. This can for example be used to capture by clone, which would look like this: clone x.

These bindings will be in scope for the final EXPR expression, which will usually be a by-value capturing closure.

Current limitations and future changes

• The macro syntax makes it somewhat verbose. A future version of this package might make use of expression-attributes instead to become more lightweight.
• The macro might become more specialized to always produce a by-value capturing closure, rather than allowing arbitrary expressions.

Example

Using the macro:

#![feature(phase)]
#![feature(unboxed_closures)]

#[phase(plugin)]
extern crate capture;

fn main() {
    let (x, y, z) = (1u32, 2u32, 3u32);
    let g = capture!(move x, ref y, clone z in move |:| x + *y + z);

    assert_eq!(g(), 6);
}