try_match 0.1.0

An expression macro that does pattern matching and returns the result in `Result`.


Provides an expression macro try_match that performs pattern matching and returns the bound variables via Ok(_) iff successful.


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(x) = Var1(42)    => x),     Ok(42));
assert_eq!(try_match!(Var2    = Var2::<u32> => "yay"), Ok("yay"));

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

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

// An anonymous struct if there are multiple bound variables
let vars = try_match!(Var1((a, b)) = Var1((12, 34))).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((_0, _1)) = Var1((12, 34))).unwrap();
assert_eq!((a, b), (12, 34));

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

let _ = try_match!(Var1((_0, _2)) = Var1((12, 34)));
let _ = try_match!(Var1((_0, _9223372036854775808)) = Var1((12, 34)));


  • Macros cannot be used in a supplied pattern.

Related Work: matches

matches! is similar but only returns bool indicating whether matching was successful or not.

let success1 = matches!(Var1(42), Var1(_));
let success2 = try_match!(Var1(_) = Var1(42)).is_ok();

License: MIT/Apache-2.0