Introduction

Functions are the primary tool that programmers can use to build abstractions. Sometimes, however, programmers want to perform abstractions over things that are not runtime values. Macros provide a syntactic abstraction. For an example of how this can be useful, consider the following two code fragments, which both pattern-match on their input and return early in one case, and do nothing otherwise:

# enum t { special_a(uint), special_b(uint) };
# fn f() -> uint {
# let input_1 = special_a(0), input_2 = special_a(0);
match input_1 {
    special_a(x) => { return x; }
    _ => {}
}
// ...
match input_2 {
    special_b(x) => { return x; }
    _ => {}
}
# return 0u;
# }

This code could become tiresome if repeated many times. However, there is no straightforward way to rewrite it without the repeated code, using functions alone. There is a solution, though: defining a macro to solve the problem. Macros are lightweight custom syntax extensions, themselves defined using the macro_rules! syntax extension. The following early_return macro captures the pattern in the above code:

# enum t { special_a(uint), special_b(uint) };
# fn f() -> uint {
# let input_1 = special_a(0), input_2 = special_a(0);
macro_rules! early_return(
    ($inp:expr $sp:ident) => ( //invoke it like `(input_5 special_e)`
        match $inp {
            $sp(x) => { return x; }
            _ => {}
        }
    );
);
// ...
early_return!(input_1 special_a);
// ...
early_return!(input_2 special_b);
# return 0;
# }

Macros are defined in pattern-matching style: in the above example, the text ($inp:expr $sp:ident) that appears on the left-hand side of the => is the macro invocation syntax, a pattern denoting how to write a call to the macro. The text on the right-hand side of the =>, beginning with match $inp, is the macro transcription syntax: what the macro expands to.

Invocation syntax

The macro invocation syntax specifies the syntax for the arguments to the macro. It appears on the left-hand side of the => in a macro definition. It conforms to the following rules:

It must be surrounded by parentheses.
$ has special meaning.
The ()s, []s, and {}s it contains must balance. For example, ([) is forbidden.

Otherwise, the invocation syntax is free-form.

To take as an argument a fragment of Rust code, write $ followed by a name (for use on the right-hand side), followed by a :, followed by a fragment specifier. The fragment specifier denotes the sort of fragment to match. The most common fragment specifiers are:

ident (an identifier, referring to a variable or item. Examples: f, x, foo.)
expr (an expression. Examples: 2 + 2; if true then { 1 } else { 2 }; f(42).)
ty (a type. Examples: int, ~[(char, ~str)], &T.)
pat (a pattern, usually appearing in a match or on the left-hand side of a declaration. Examples: Some(t); (17, 'a'); _.)
block (a sequence of actions. Example: { log(error, "hi"); return 12; })

The parser interprets any token that's not preceded by a $ literally. Rust's usual rules of tokenization apply,

So ($x:ident -> (($e:expr))), though excessively fancy, would designate a macro that could be invoked like: my_macro!(i->(( 2+2 ))).

Transcription syntax

The right-hand side of the => follows the same rules as the left-hand side, except that a $ need only be followed by the name of the syntactic fragment to transcribe into the macro expansion; its type need not be repeated.

The right-hand side must be enclosed by delimiters, and must be an expression. Currently, invocations of user-defined macros can only appear in a context where the Rust grammar requires an expression, even though macro_rules! itself can appear in a context where the grammar requires an item.

Multiplicity

Invocation

Going back to the motivating example, recall that early_return expanded into a match that would return if the match's scrutinee matched the "special case" identifier provided as the second argument to early_return, and do nothing otherwise. Now suppose that we wanted to write a version of early_return that could handle a variable number of "special" cases.

The syntax $(...)* on the left-hand side of the => in a macro definition accepts zero or more occurrences of its contents. It works much like the * operator in regular expressions. It also supports a separator token (a comma-separated list could be written $(...),*), and + instead of * to mean "at least one".

# enum t { special_a(uint),special_b(uint),special_c(uint),special_d(uint)};
# fn f() -> uint {
# let input_1 = special_a(0), input_2 = special_a(0);
macro_rules! early_return(
    ($inp:expr, [ $($sp:ident)|+ ]) => (
        match $inp {
            $(
                $sp(x) => { return x; }
            )+
            _ => {}
        }
    );
);
// ...
early_return!(input_1, [special_a|special_c|special_d]);
// ...
early_return!(input_2, [special_b]);
# return 0;
# }

Transcription

As the above example demonstrates, $(...)* is also valid on the right-hand side of a macro definition. The behavior of * in transcription, especially in cases where multiple *s are nested, and multiple different names are involved, can seem somewhat magical and intuitive at first. The system that interprets them is called "Macro By Example". The two rules to keep in mind are (1) the behavior of $(...)* is to walk through one "layer" of repetitions for all of the $names it contains in lockstep, and (2) each $name must be under at least as many $(...)*s as it was matched against. If it is under more, it'll be repeated, as appropriate.

Parsing limitations

For technical reasons, there are two limitations to the treatment of syntax fragments by the macro parser:

The parser will always parse as much as possible of a Rust syntactic fragment. For example, if the comma were omitted from the syntax of early_return! above, input_1 [ would've been interpreted as the beginning of an array index. In fact, invoking the macro would have been impossible.
The parser must have eliminated all ambiguity by the time it reaches a $name:fragment_specifier declaration. This limitation can result in parse errors when declarations occur at the beginning of, or immediately after, a $(...)*. For example, the grammar $($t:ty)* $e:expr will always fail to parse because the parser would be forced to choose between parsing t and parsing e. Changing the invocation syntax to require a distinctive token in front can solve the problem. In the above example, $(T $t:ty)* E $e:exp solves the problem.

A final note

Macros, as currently implemented, are not for the faint of heart. Even ordinary syntax errors can be more difficult to debug when they occur inside a macro, and errors caused by parse problems in generated code can be very tricky. Invoking the log_syntax! macro can help elucidate intermediate states, invoking trace_macros!(true) will automatically print those intermediate states out, and passing the flag --pretty expanded as a command-line argument to the compiler will show the result of expansion.

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