1 unstable release
0.1.0 | Feb 23, 2021 |
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#2833 in Rust patterns
11KB
90 lines
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.