#swift #guard

ward

Provides a ward! macro which returns the contents of an Option and otherwise returns early, and a guard! macro, which does the same, but with a syntax more similar to Swift’s guard syntax

6 stable releases

2.1.0 Sep 16, 2019
2.0.0 Sep 16, 2019
1.1.0 Sep 9, 2019
1.0.2 Sep 9, 2019

#452 in Rust patterns

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1,533 downloads per month

MIT license

9KB

ward

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This crate exports two macros, which are intended to replicate the functionality of Swift's guard expression with Option<T>.

The guard! macro was created to emulate the guard let statement in Swift. This macro is only really useful for moving values out of Option<T>s into variables. The ward! macro, on the other hand, doesn't force the creation of a variable, it only returns the value that the guard! variable would place into a variable. As such, it's a more flexible version of the guard! macro; and probably also somewhat more Rustic.

Examples

let sut = Some("test");

// This creates the variable res, which from an Option<T> will return a T if it is Some(T), and will
// otherwise return early from the function.
guard!(let res = sut);
assert_eq!(res, "test");

The ward! macro, by comparison, just returns the value, without forcing you to make a variable from it (although we still do in this example):

let sut = Some("test");
let res = ward!(sut);
assert_eq!(res, "test");

Both macros also support an else branch, which will run if the Option<T> is None:

let sut = None;
guard!(let _res = sut, else {
    println!("This will be called!");

    // Because sut is None, the else branch will be run. When the else branch is invoked, guard!
    // no longer automatically returns early for you, so you must do so yourself if you want it.
    return;
});
unreachable!();

Both macros also support an alternative "early return statement", which will let you e.g. break within loops:

// Not that you couldn't (and probably should) do this case with `while let Some(res) = sut`...
let mut sut = Some(0);
loop {
    let res = ward!(sut, break);
    sut = if res < 5 {
        Some(res + 1)
    } else {
        None
    }
}
assert_eq!(sut, None);

No runtime deps