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.