Crate helper_fn[−][src]
Expand description
helper_fn
A macro to create closure-like helper functions.
Usage
This attribute can be applied to a function item to allow it to inherit variables from the parent scope. The helper function must be declared before its usage and after the variables it inherits.
use helper_fn::helper_fn;
let mut num = 5;
#[helper_fn(num: i32)] // captures num from the surrounding scope
fn get_num() -> i32 {
num
}
assert_eq!(get_num!(), 5); // note that this is a macro invocation, not a function call
Variables can be captured by value (using move or copy semantics), by reference, or by mutable reference:
#[helper_fn(copied: i32, moved: Vec<i32>, &by_ref: Foo, &mut by_mut_ref: Bar)]
If there are scoping issues, you can use the use_helper_fn
macro:
use helper_fn::{helper_fn, use_helper_fn};
let mut num = 5;
// hoist the definitions
use_helper_fn! {
get_num(num),
get_num_times_two(num) as get_num_times_2,
increment_num(&mut num),
};
assert_eq!(get_num!(), 5);
assert_eq!(get_num_times_2!(), 10);
increment_num!();
assert_eq!(get_num!(), 6);
assert_eq!(get_num_times_2!(), 12);
#[helper_fn(num: i32)]
fn get_num() -> i32 {
num
}
#[helper_fn(num: i32)]
fn get_num_times_two() -> i32 {
// reuse the definition from the parent scope
// has to be a different name to avoid conflict
use_helper_fn!(get_num(num) as get_num_);
get_num_!() * 2
}
#[helper_fn(&mut num: i32)]
fn increment_num() {
*num += 1;
}
Rationale
Closures are often used as helper functions that require the surrounding scope; here’s a simple example:
let mut num = 5;
let get_num = || {
// Lots of complex stuff
num
};
assert_eq!(get_num(), 5);
This works just fine, but if you mutate num
in between calls, it won’t compile:
let mut num = 5;
let get_num = || {
// Lots of complex stuff
num
};
assert_eq!(get_num(), 5);
num += 1; // cannot assign to `num` because it is borrowed
assert_eq!(get_num(), 6);
One workaround is to use a local fn
item and pass variables in:
let mut num = 5;
fn get_num(num: i32) -> i32 {
// Lots of complex stuff
num
}
assert_eq!(get_num(num), 5);
num += 1; // this is ok
assert_eq!(get_num(num), 6);
This can work, but it can become quite verbose with multiple variables.
With helper_fn
, you can get the best of both worlds:
use helper_fn::helper_fn;
let mut num = 5;
#[helper_fn(num: i32)]
fn get_num() -> i32 {
num
}
assert_eq!(get_num!(), 5); // you don't need to pass in `num`
num += 1; // this is ok
assert_eq!(get_num!(), 6); // note that these calls are macro invocations
Macros
Attribute Macros
See the crate level documentation.