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());

Multiple Assignments

When there is more than one assignment within a matching pattern all assignments are returned in a tuple. Since assignments in a pattern may not be ordered linearly, when there are multiple assignments they will be returned in lexicographic order.

use variant::variant;

let val = (Some(10), Some(true));
let (a, b) = variant!(val, (Some(b), Some(a))).expect("tuple");
assert_eq!((a, b), (true, 10));

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)));

Or Patterns

The variant macro does not support Or patterns at any level. Since there is no simple expected way to signal to the user what values are returned in the case where only some assignments may match. If a pragmatic solution to this problem is found then adding this feature in the future may be possible.

Macros

variant