Macro enum_extract::extract

macro_rules! extract {
    (@ANON_TUPLE [$(,)*], [$($ids:ident)*], $($p:ident)::+, $t:expr) => { ... };
    (@ANON_TUPLE [_], [$($ids:ident)*], $($p:ident)::+, $t:expr) => { ... };
    (@ANON_TUPLE [_, $($more:tt)*], [$($ids:ident)*], $($p:ident)::+, $t:expr) => { ... };
    ($($p:ident)::+ { $($i:ident),* $(,)* } , $t:expr) => { ... };
    ($($p:ident)::+ ( $($its:tt)* ) , $t:expr) => { ... };
}

Extract the fields of a single variant from an enum, returning an Option<T> where T is either the single field, or a tuple of each of the fields in the order they are written.

Examples

Given the following enum:

enum Foo {
    A(i32),
    B(i32, i32),
    C { x: i32, y: i32 },
    D { z: i32 },
}

If the variant matches, it produces a Some of the fields in the matched variant.

let a = Foo::A(10);
assert_eq!(extract!(Foo::A(_), a), Some(10));

let d = Foo::D{ z: 20 };
assert_eq!(extract!(Foo::D{z}, d), Some(20));

If there is more than one field in the enum variant, it produces a Some of a tuple

let b = Foo::B(10, 20);
assert_eq!(extract!(Foo::B(_, _), b), Some((10, 20)));

let c = Foo::C{ x: 30, y: 40 };
assert_eq!(extract!(Foo::C{x, y}, c), Some((30, 40)));

// You can also control the order of the fields in the struct variant case!
assert_eq!(extract!(Foo::C{y, x}, c), Some((40, 30)));

If the pattern doesn't match, it produces a None

let b = Foo::B(10, 20);
assert_eq!(extract!(Foo::A(_), b), None);