Macro let_extract

macro_rules! let_extract {
    ($($p:ident)::+ ( $($i:ident),* $(,)* ) , $t:expr, match { $($body:tt)* }) => { ... };
    ($($p:ident)::+ { $($i:ident),* $(,)* } , $t:expr, match { $($body:tt)* }) => { ... };
    ($($p:ident)::+ { $($k:ident : $v:ident),* $(,)* } , $t:expr, match { $($body:tt)* }) => { ... };
    ($($p:ident)::+ ( $($its:tt)* ) , $t:expr, $els:expr) => { ... };
    ($($p:ident)::+ { $($its:tt)* } , $t:expr, $els:expr) => { ... };
}

Extract the fields of a single variant from an enum, binding them into the current scope.

If the binding fails due to the variant being incorrect, execute the error expression. Alternatively, a partial match block may be written to handle the remaining cases.

Examples

Given the following enum:

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

If the extract succeeds, the variable names written bind into the current scope.

let a = Foo::A(10);
let_extract!(Foo::A(x), a, panic!());
assert_eq!(x, 10);

If it fails, the expression passed in as the 3rd argument is executed instead, and can cause execution to diverge.

let a = Foo::A(10);
let_extract!(Foo::B(x, y), a, return);
unreachable!();

Alternatively, it can provide default values for the variable bindings, from left to right.

let a = Foo::A(10);
let_extract!(Foo::B(x, y), a, (40, 50));
assert_eq!(x, 40);
assert_eq!(y, 50);

The other cases can each be handled specifically with a match-like block.

let a = Foo::A(10);
let_extract!(Foo::B(x, y), a, match {
    Foo::A(x) => return,
    Foo::C{..} => unreachable!(),
    Foo::D{..} => unreachable!(),
});
unreachable!();

Struct variants are also supported, and can be written either as { field } or { field: binding }.

let d = Foo::D { z: 10 };
let_extract!(Foo::D{ z }, d, unreachable!());
assert_eq!(z, 10);

let_extract!(Foo::D{ z: apples }, d, unreachable!());
assert_eq!(apples, 10);