on_demand

This Rust crate provides a macro to generate aux macros for on-demand usage.

[dependencies]
on_demand = "0.1"

Examples

use on_demand::generate_on_demand_macro;

fn foo() {
    generate_on_demand_macro!(a: usize = None, {
        println!("a");
        1
    });
    generate_on_demand_macro!(b: usize = None, {
        println!("b");
        let a_data = on_demand_get_a!();
        2 + *a_data
    });
    generate_on_demand_macro!(c: usize = None, {
        println!("c");
        let a_data = on_demand_get_a!();
        let b_data = on_demand_get_b!();
        3 + *a_data + *b_data
    });

    let c_data = on_demand_get_c!();
    assert_eq!(*c_data, 6);
}

After calling generate_on_demand_macro to the variable (for example, a), three new macros on_demand_get_a, on_demand_get_a_mut and on_demand_into_a are generated. When on_demand_get_a is called, it determines whether a has been calculated. If it is, then returns the reference to its data, otherwise calls the expression given as the second parameter of generate_on_demand_macro, then assigns the returned value to a. The other two generated macros do the similar job, but returns the mutable reference to, or takes ownership of a.

In all, this means calculating a lazily using the expression.