Macros in Rust

Not quite like C macros

C's preprocessor operates on a lexical level.

Think find and replace - like sed.

#define MIN(a, b) (((a) < (b)) ? (a) : (b))
#define struct union
#define DO_A_THING(t) do { \
        do_blah(t); \
        do_foo(t); \
    } while(0)

Rust's macros operate on the AST

“An abstract syntax tree (AST), or just syntax tree, is a tree representation of the abstract syntactic structure of source code written in a programming language.”

pub enum Expr_ {
    ExprCall(@Expr, ~[@Expr]),
    ExprMatch(@Expr, ~[Arm]),
    ExprForLoop(@Pat, @Expr, P<Block>, Option<Ident>),
    ...
}

pub enum Item_ {
    ItemFn(P<FnDecl>, Purity, AbiSet, Generics, P<Block>),
    ItemTrait(Generics, ~[TraitDef], ~[TraitMethod]),
    ...
}

...

Why?

Return from the invoking function

macro_rules! try(
    ($e:expr) => (
        match $e {
            Ok(e) => e,
            Err(e) => return Err(e)
        }
    )
)

fn read_foo(r: &mut Reader) -> Result<Foo, IoError> {
    let bar = try!(r.read_be_i32());
    let baz = try!(r.read_bytes(10));
    Ok(Foo { bar: bar, baz: baz })
}

Why?

(Almost) freeform syntax

let map = make_map!(Start with collections::HashMap::new() and add
                    "foo" => 1,
                    "bar" => 2,
                    "baz" => 3);

Why?

Do work at compile time instead of run time!

test.rs:2:35: 2:37 error: argument never used
test.rs:2     println!("hello {}", "world", 10);
                                            ^~

Basic idea

It's simple for macro_rules!.

#[macro_export]
macro_rules! my_macro(() => (1))

Basic idea

It's a bit more complicated for procedural macros. Libraries define a macro_registrar function to register procedural macros with the compiler.

#[macro_registrar]
pub fn registrar(cb: |ast::Name, ext::base::SyntaxExtension|) {
    ...
}

pub enum SyntaxExtension {
    // e.g. #[deriving(...)]
    ItemDecorator(...),
    // e.g. try!(...)
    NormalTT(...),
    // e.g. macro_rules! foo(...)
    IdentTT(...),
}

Basic idea

Client code annotates the crate import to tell the compiler to load syntax extensions from it.

#[feature(phase)];

#[phase(syntax)]
extern crate my_macros;

fn main() {
    let _x = my_macro!();
}

Example!

Let's write a syntax extension that creates sorted arrays of strings:

#[feature(phase)];

#[phase(syntax)]
extern crate sort;

fn main() {
    assert_eq!(sort!("z", "", "hello", "a"),
               ["", "a", "hello", "z"]);
}

Example!