mystor / lambda-macro

A lambda! macro for rust

lambda-macro

Lambdas in rust are confusing. Currently, when you type |x| { x + i }, the lambda cannot live past the lifetime of the variable it closes around: i. That means that if i is a parameter to a function, you can't return a lambda from that function.

To get around this restriction, the values refered to in the lambda need to be "moved" into the closure. procs do this, but can only be run once, as they clean up the moved variables after they are run.

This macro gets around that by instead moving the variables closed over into a heap-allocated struct, and adding a call method to that struct.

Usage

Load the macro into your program:

#![feature(phase)]

#[phase(plugin)]
extern crate lambdas;

...

This imports the lambda! macro, which takes the general form of the following:

lambda!([i: int, j: int]  // closed over variables and types
        (k: int, l: int)  // lambda parameters and types
        -> int            // lambda return types
        { i + j + k + l } // lambda body
): Box<Fn<(int,int), int> + 'static> // Produces a boxed struct implementing the Fn trait

A simple example would be a curried adder function

fn adder(i: int) -> Box<Fn<(int,), int> + 'static> {
    lambda!([i: int] (j: int) -> int {
        i + j
    })
}

This could be called as follows:

fn main() {
    let add1 = adder(1);
    println!("{}", add1.call((5,))); // 6
}

This is a bit inconvenient, so there is also a call! macro, which allows us to do the following:

fn main() {
    let add1 = adder(1);
    println!("{}", call!(add1, 5));
}

Implementation

The lambda! macro expands into something like the following:

lambda!([i: int] (j: int) -> int {
    i + j
})
=>
{
    // The struct containing the closed-over variables
    struct Closure(int);
    // The call method for the Closure
    impl Fn<(int,), int> for Closure {
        extern "rust-call" fn call(&self, args: (int,)) -> int {
            let (j,) = args;
            let &Closure(i) = self;
            { i + j } // Your closure body
        }
    }
    // Boxing the value on the heap
    box() Closure(i)
}

Limitations

With the #![feature(overloaded_calls)] you should be able to do:

fn main() {
    let add1 = adder(1);
    println!("{}", add1(5));
}

But that is currently not working (see rust#18349)

In addition, it would be nice if we could infer the types of closed over variables etc., but that is also currently impossible.