Macro json::extract

macro_rules! extract {
    (
      let decoder = $dec:ident;
      let buffer  = $buf:expr;
      $($name:ident: $modus:ident. $typ:ty = $key:expr => $action:expr),+
    ) => { ... };
    (
      let buffer = $buf:expr;
      $($name:ident: $modus:ident. $typ:ty = $key:expr => $action:expr),+
    ) => { ... };
    (
      let decoder = $dec:ident;
      $($name:ident: $modus:ident. $typ:ty = $key:expr => $action:expr),+
    ) => { ... };
    (
      $($name:ident: $modus:ident. $typ:ty = $key:expr => $action:expr),+
    ) => { ... };
}

Macro to support extraction of subsets of JSON object fields.

Optionally extract! accepts a Utf8Buffer to use when decoding object keys. Also decoder can be omitted in which case the macro expands to a lambda function expecting the decoder as argument (this form is useful for nested object! invocations).

Example:

let p = extract! {
    let decoder = d;
    x: req. u32 = "x" => Decoder::from_json,
    y: req. u32 = "y" => Decoder::from_json
}
println!("x={}, y={}", p.x, p.y)