# 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)
        }
    }

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 `[]`.