| # 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. |
| |
| ```rust,editable |
| macro_rules! calculate { |
| (eval $e:expr) => { |
| { |
| let val: usize = $e; // Force types to be unsigned integers |
| println!("{} = {}", stringify!{$e}, val); |
| } |
| }; |
| } |
| |
| fn main() { |
| calculate! { |
| eval 1 + 2 // hehehe `eval` is _not_ a Rust keyword! |
| } |
| |
| calculate! { |
| eval (1 + 2) * (3 / 4) |
| } |
| } |
| ``` |
| |
| Output: |
| |
| ```txt |
| 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`](https://crates.io/crates/lazy_static) or |
| [`clap`](https://crates.io/crates/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 `[]`. |
