Crate variant

The variant macro matches an expression against a given pattern returning a Result. If the pattern matches, then the Ok branch is returned including any assignments from the pattern (or unit if none are given). If the match fails then Err is returned with either a given error, or defaulting to Box<dyn std::error::Error> otherwise.

Simple Example

use variant::variant;

let val = Some((0, 1));
let res = variant!(val, Some((i, _))).expect("i");
assert_eq!(res, 0);

let res = variant!(val, Some((10, j)));
assert!(res.is_err());

Guards

Conditional guards work the same as with matches!.

use variant::variant;

struct Foo {
    a: usize,
    b: Option<bool>,
}

let val = Foo { a: 20, b: None };
let res = variant!(val, Foo { a, .. } if a == 20).expect("a");
assert_eq!(res, 20);

let res = variant!(val, Foo { b, .. } if b.is_some());
assert!(res.is_err());

Custom Errors

use variant::variant;

#[derive(Debug)]
enum MyError {
    Bad,
    Worse,
    Expensive,
}

let val = Some(1);
let res = variant!(val, Some(i), MyError::Bad).expect("i");
assert_eq!(res, 1);

let res = variant!(val, Some(50), MyError::Worse);
assert!(matches!(res, Err(MyError::Worse)));

// We can also send an error returning closure with the following syntax
let res = variant!(val, Some(50), else || MyError::Expensive);
assert!(matches!(res, Err(MyError::Expensive)));

Macros

variant