Rust by Example

Domain Specific Languages (DSLs)

A DSL is a mini "language" embedded in a Rust macro. It is completely valid Rust because the macro system expands into normal Rust constructs, but it looks like a small language. This allows you to define concise or intuitive syntax for some special functionality (within bounds).

Suppose that I want to define a little calculator API. I would like to supply an expression and have the output printed to console.

macro_rules! calculate {

(eval $e:expr) => {{

{

let val: usize = $e; // Force types to be integers

println!("{} = {}", stringify!{$e}, val);

}

}};

}

fn main() {

calculate! {

eval 1 + 2 // hehehe `eval` is _not_ a Rust keyword!

}

calculate! {

eval (1 + 2) * (3 / 4)

}

}

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Output:

1 + 2 = 3

(1 + 2) * (3 / 4) = 0

This was a very simple example, but much more complex interfaces have been developed, such as lazy_static or clap.

Also, note the two pairs of braces in the macro. The outer ones are part of the syntax of macro_rules!, in addition to () or [].