Rust Programming By Example

Multiple pattern rules

Let's write a macro that will simplify the implementation of the traits to overload operators. This macro will have two rules: one for the + and one for the - operators. Here's the first rule of the macro:

macro_rules! op {
    (+ $_self:ident : $self_type:ty, $other:ident $expr:expr) => {
        impl ::std::ops::Add for $self_type {
            type Output = $self_type;

            fn add($_self, $other: $self_type) -> $self_type {
                $expr
            }
        }
    };
    // …

In this pattern, we use other types of syntactic elements: ident, which is an identifier, and <span&gt;expr, which is an expression. The trait (::std::ops::Add) is fully qualified so that the code using this macro won't need to import the Add trait.

And here's the rest of the macro:

 (- $_self:ident : $self_type:ty, ...