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# 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 = $e; // Force types to be integers
println!("{} = {}", stringify!{$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 `[]`.
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