155 lines
5.2 KiB
Markdown
155 lines
5.2 KiB
Markdown
% Rust Macros Tutorial
|
|
|
|
# Introduction
|
|
|
|
Functions are the programmer's primary tool of abstraction, but there are
|
|
cases in which they are insufficient, because the programmer wants to
|
|
abstract over concepts not represented as values. Consider the following
|
|
example:
|
|
|
|
~~~~
|
|
# 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 reasonable function that could be written to solve this problem. In such a
|
|
case, it's possible to define a macro to solve the problem. Macros are
|
|
lightweight custom syntax extensions, themselves defined using the
|
|
`macro_rules!` syntax extension:
|
|
|
|
~~~~
|
|
# 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 {
|
|
$sp(x) => { return x; }
|
|
_ => {}
|
|
}
|
|
);
|
|
);
|
|
// ...
|
|
early_return!(input_1 special_a);
|
|
// ...
|
|
early_return!(input_2 special_b);
|
|
# return 0;
|
|
# }
|
|
~~~~
|
|
|
|
Macros are defined in pattern-matching style:
|
|
|
|
# Invocation syntax
|
|
|
|
On the left-hand-side of the `=>` is the macro invocation syntax. It is
|
|
free-form, excepting the following rules:
|
|
|
|
1. It must be surrounded in parentheses.
|
|
2. `$` has special meaning.
|
|
3. The `()`s, `[]`s, and `{}`s it contains must balance. For example, `([)` is
|
|
forbidden.
|
|
|
|
To take as an argument a fragment of Rust code, write `$` followed by a name
|
|
(for use on the right-hand side), followed by a `:`, followed by the sort of
|
|
fragment to match (the most common ones are `ident`, `expr`, `ty`, `pat`, and
|
|
`block`). Anything not preceded by a `$` is taken literally. The standard
|
|
rules of tokenization apply,
|
|
|
|
So `($x:ident => (($e:expr)))`, though excessively fancy, would create 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 `$` need only be followed by the name of the syntactic fragment
|
|
to transcribe.
|
|
|
|
The right-hand side must be surrounded by delimiters of some kind, and must be
|
|
an expression; currently, user-defined macros can only be invoked in
|
|
expression position (even though `macro_rules!` itself can be in item
|
|
position).
|
|
|
|
# Multiplicity
|
|
|
|
## Invocation
|
|
|
|
Going back to the motivating example, suppose that we wanted each invocation
|
|
of `early_return` to potentially accept multiple "special" identifiers. The
|
|
syntax `$(...)*` accepts zero or more occurrences of its contents, much like
|
|
the Kleene star 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 Kleene star in transcription,
|
|
especially in cases where multiple stars 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 `$(...)*` is to walk through one "layer"
|
|
of repetitions for all of the `$name`s 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 will be repeated, as appropriate.
|
|
|
|
## Parsing limitations
|
|
|
|
The parser used by the macro system is reasonably powerful, but the parsing of
|
|
Rust syntax is restricted in two ways:
|
|
|
|
1. The parser will always parse as much as possible. 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.
|
|
2. The parser must have eliminated all ambiguity by the time it reaches a
|
|
`$name:fragment_specifier`. This most often affects them when they occur in
|
|
the beginning of, or immediately after, a `$(...)*`; requiring a distinctive
|
|
token in front can solve 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, using `trace_macros!(true)` will automatically print those
|
|
intermediate states out, and using `--pretty expanded` as an argument to the
|
|
compiler will show the result of expansion.
|
|
|
|
|