try_match

Provides an expression macro try_match that matches a pattern on a given expression and returns the bound variables in Ok(_) if successful.

Basic Usage

Explicit Mapping

use try_match::try_match;

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

// The right-hand side of `=>` if successful
assert_eq!(try_match!(Var1(42),    Var1(x) => x),     Ok(42));
assert_eq!(try_match!(Var2::<u32>, Var2    => "yay"), Ok("yay"));

// `Err(input)` on failure
assert_eq!(try_match!(Var2::<u32>, Var1(x) => x),     Err(Var2));
assert_eq!(try_match!(Var1(42),    Var2    => "yay"), Err(Var1(42)));

// Supports `if` guard
assert_eq!(try_match!(Var1(42), Var1(x) if x < 20 => x), Err(Var1(42)));

Implicit Mapping

=> and the part that comes after can be omitted (requires implicit_map feature, which is enabled by default; you can disable it to skip the compilation of the internal procedural macro):

// `()` if there are no bound variables
assert_eq!(try_match!(Var1(42), Var1(_)), Ok(()));

// The bound variable if there is exactly one bound variables
assert_eq!(try_match!(Var1(42), Var1(x)), Ok(42));
assert_eq!(try_match!(Var1(42), Var1(x) if x < 20), Err(Var1(42)));

// An anonymous struct if there are multiple bound variables
let vars = try_match!(Var1((12, 34)), Var1((a, b))).unwrap();
assert_eq!((vars.a, vars.b), (12, 34));

It produces a tuple if you name the bound variables like _0, _1, _2, ...:

let (a, b) = try_match!(Var1((12, 34)), Var1((_0, _1))).unwrap();
assert_eq!((a, b), (12, 34));

try_match!(Var1((12, 34)), Var1((_0, _1)) if _0 == _1).unwrap_err();

It's an error to specify non-contiguous binding indices:

let _ = try_match!(Var1((12, 34)), Var1((_0, _2)));

let _ = try_match!(Var1((12, 34)), Var1((_0, _9223372036854775808)));

Quirks

When using implicit mapping, bind variables defined inside macros are not recognized because at the point of try_match's macro expansion, inner macros are not expended yet.

This macro moves a value out of the place represented by the input expression to return it on failure. Make sure to pass a reference if this is not desired.

let array = [Some(UncopyValue), None];
// ERROR: Can't move out of `array[0]`
let _: &UncopyValue = try_match!(array[0], Some(ref x)).unwrap();

let _: &UncopyValue = try_match!(&array[0], Some(x)).unwrap();

Applications

Iterator::filter_map

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

Iterator::map + Fallible Iterator::collect

let array = [Var1(42), Var2, Var1(10)];
let filtered: Result<Vec<_>, _> = array
    .iter()
    .map(|x| try_match!(x, &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> {
        try_match!(self, Var1(_0)).ok()
    }

    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) = try_match!(foo, &Var1([_0, _, _, _1])).unwrap();

    // Once RFC 1303 is stabilized, you can do instead:
    // let &Var1([i0, _, _, i1]) = foo else { panic!("{:?}", foo) };

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

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

Related Work

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

if_rust_version! { >= 1.42 {
    let success1 =   matches!(Some(42), Some(_));
    let success2 = try_match!(Some(42), Some(_)).is_ok();
    assert_eq!(success1, success2);
} }

bind_match::bind_match! and extract::extract! use the same syntax (except for implicit mapping) but return Some(expr) on success instead.

License: MIT/Apache-2.0