try_match

Provides expression macros to match a pattern on a given expression.

Basic Usage

Macros

use try_match::{try_match, match_ok, unwrap_match};

#[derive(Copy, Clone, Debug, PartialEq)]
enum Enum<T> { Var1(T), Var2 }

use Enum::{Var1, Var2};

// `try_match!` returns `Result`: `Ok(bindings)` on success or
// `Err(input_value)` otherwise
assert_eq!(try_match!(Var1(42), Var1(x)), Ok(42));
assert_eq!(try_match!(Var1(42), Var1(x) if x < 20), Err(Var1(42)));

// `match_ok!` returns `Option`
assert_eq!(match_ok!(Var1(42), Var1(x)), Some(42));
assert_eq!(match_ok!(Var1(42), Var1(x) if x < 20), None);

// `unwrap_match!` panics on failure:
assert_eq!(unwrap_match!(Var1(42), Var1(x)), 42);
unwrap_match!(Var1(42), Var1(x) if x < 20); // will panic

Bindings

// Returns `()` (wrapped by `Ok(_)`) if there are no bound variables
assert_eq!(unwrap_match!(Var1(42), Var1(_)), ());

// ... the bound value if there is exactly one binding
assert_eq!(unwrap_match!(Var1(42), Var1(x)), 42);

// ... an anonymous struct if there are multiple bindings
let vars = unwrap_match!(Var1((12, 34)), Var1((a, b)));
assert_eq!((vars.a, vars.b), (12, 34));

// ... or a tuple if the binding names are numeric
let (a, b) = unwrap_match!(Var1((12, 34)), Var1((_0, _1)));
assert_eq!((a, b), (12, 34));

// An optional `=>` clause specifies an explicit mapping
assert_eq!(unwrap_match!(Var1(42),    Var1(x) => x + 1), 43);
assert_eq!(unwrap_match!(Var2::<u32>, Var2    => "yay"), "yay");

Partial Application

// Omit the scrutinee expression to produce a closure
let _:                  Option<i32> = match_ok!(Var1(42), Var1(x));
let _: fn(Enum<i32>) -> Option<i32> = match_ok!(        , Var1(x));

Applications

Iterator::filter_map

let array = [Var1(42), Var2, Var1(10)];
let filtered: Vec<_> = array
    .iter()
    .filter_map(match_ok!(, &Var1(_0) if _0 > 20))
    .collect();
assert_eq!(filtered, [42]);

Iterator::map + Fallible Iterator::collect

let array = [Var1(42), Var2, Var1(10)];
let filtered: Result<Vec<_>, _> = array
    .iter()
    .map(try_match!(, &Var1(_0) if _0 > 20))
    .collect();

// `Var2` is the first value that doesn't match
assert_eq!(filtered, Err(&Var2));

Extract Variants

impl<T> Enum<T> {
    fn var1(&self) -> Option<&T> {
        match_ok!(self, Var1(_0))
    }

    fn is_var2(&self) -> bool {
        matches!(self, Var2)
    }
}

let enums = [Var1(42), Var2];
assert_eq!(enums[0].var1(), Some(&42));
assert_eq!(enums[1].var1(), None);

assert!(!enums[0].is_var2());
assert!(enums[1].is_var2());

Expect Certain Variants

fn this_fn_expects_var1(foo: &Enum<[u8; 4]>) {
    let (i0, i1) = unwrap_match!(foo, &Var1([_0, _, _, _1]));

    // Alternatively, you could use let-else (stabilized in Rust 1.65.0):
    // let &Var1([i0, _, _, i1]) = foo else { panic!("{foo:?}") };

    assert_eq!((i0, i1), (42, 45));
}

this_fn_expects_var1(&Var1([42, 43, 44, 45]));

Related Works

matcher::matches! (now incorporated into the standard library as core::matches!) is similar but only returns bool indicating whether matching was successful or not.

let success1 =  matches!(Some(42), Some(_));
let success2 = match_ok!(Some(42), Some(_)).is_some();
assert_eq!(success1, success2);

bind_match::bind_match! and extract::extract! behave in the same way as match_ok! except for the lack of implicit mapping and partial application.

variant::get_variant! from the extract_variant crate offers a similar functionality to match_ok!. It supports implicit mapping but uses different rules to handle multiple bindings.

License

MIT/Apache-2.0