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 an have the output printed to console.

1. macro_rules! calculate {
2. (eval $e:expr) => {{
3. {
4. let val: usize = $e; // Force types to be integers
5. println!("{} = {}", stringify!{$e}, val);
6. }
7. }};
8. }

10. fn main() {
11. calculate! {
12. eval 1 + 2 // hehehe `eval` is _not_ a Rust keyword!
13. }

15. calculate! {
16. eval (1 + 2) * (3 / 4)
17. }
18. }

Output:

1. 1 + 2 = 3
2. (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.