Crate variant

Expand description

The try_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 a default Box<dyn std::error::Error> otherwise.

The get_variant macro works in exactly the same way, except it returns Some if the pattern matches and None, otherwise.

Finally, the variant macro also works the same way, but panics if the pattern does not match.

Simple Example

use variant::{get_variant, try_variant, variant};

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

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

// Using get_variant instead
let opt = get_variant!(val, Some((i, _)));
assert_eq!(opt, Some(0));

let opt = get_variant!(val, Some((10, j)));
assert_eq!(opt, None);

// Using just variant
let var = variant!(val, Some((i, _)));
assert_eq!(var, 0);

// calling `variant!(val, Some((10, j)))` will panic.

Guards

Conditional guards work the same as with matches!.

use variant::try_variant;

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

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

let res = try_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, multiple assignments will be returned in lexicographic order.

use variant::try_variant;

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

Custom Errors

use variant::try_variant;

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

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

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

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

// Doesn't have to be a closure, any callable taking no parameters will do
fn make_err() -> MyError { MyError::Expensive }
let res = try_variant!(val, Some(50), else make_err);
assert!(matches!(res, Err(MyError::Expensive)));

Or Patterns