I am trying to make a wrapper for a macro. The trouble is that I don't want to repeat the same rules in both macro. Is there a way to do that?
Here is what I tried:
macro_rules! inner {
($test:ident) => { stringify!($test) };
($test:ident.run()) => { format!("{}.run()", stringify!($test)) };
}
macro_rules! outer {
($expression:expr) => {
println!("{}", inner!($expression));
}
}
fn main() {
println!("{}", inner!(test));
println!("{}", inner!(test.run()));
outer!(test);
outer!(test.run());
}
but I get the following error:
src/main.rs:8:31: 8:42 error: expected ident, found test
src/main.rs:8 println!("{}", inner!($expression));
^~~~~~~~~~~
If I change the outer macro for this, the code compile:
macro_rules! outer {
($expression:expr) => {
println!("{}", stringify!($expression));
}
}
What am I doing wrong?
macro_rules! is both cleverer and dumber than you might realise.
Initially, all input to a macro begins life as undifferentiated token soup. An Ident here, StrLit there, etc. However, when you match and capture a bit of the input, generally the input will be parsed in an Abstract Syntax Tree node; this is the case with expr.
The "clever" bit is that when you substitute this capture (for example, $expression), you don't just substitute the tokens that were originally matched: you substitute the entire AST node as a single token. So there's now this weird not-really-a-token in the output that's an entire syntax element.
The "dumb" bit is that this process is basically irreversible and mostly totally invisible. So let's take your example:
outer!(test);
We run this through one level of expansion, and it becomes this:
println!("{}", inner!(test));
Except, that's not what it looks like. To make things clearer, I'm going to invent some non-standard syntax:
println!("{}", inner!( $(test):expr ));
Pretend that $(test):expr is a single token: it's an expression which can be represented by the token sequence test. It is not simply the token sequence test. This is important, because when the macro interpreter goes to expand that inner! macro, it checks the first rule:
($test:ident) => { stringify!($test) };
The problem is that $(test):expr is an expression, not an identifier. Yes, it contains an identifier, but the macro interpreter doesn't look that deep. It sees an expression and just gives up.
It fails to match the second rule for the same reason.
So what do you do? ... Well, that depends. If outer! doesn't do any sort of processing on its input, you can use a tt matcher instead:
macro_rules! outer {
($($tts:tt)*) => {
println!("{}", inner!($($tts)*));
}
}
tt will match any token tree (see the Macros chapter of the Rust Book). $($tts:tt)* will match any sequence of tokens, without changing them. This of this as a way to safely forward a bunch of tokens to another macro.
If you need to do processing on the input and forward it on to the inner! macro... you're probably going to have to repeat the rules.
I had some success with the $($stuff: expr),+ syntax.
macro_rules! println {
( $($stuff: expr),+) => {
avr_device::interrupt::free(|cs| {
uwriteln!(unsafe { &SERIAL_STATIC}.borrow(cs).borrow_mut().as_mut().unwrap(),
$($stuff),+)
})
}
}
Related
I am hoping to write a Rust macro which forwards its entire argument to a second macro — even when that argument contains exciting parenthesization.
Here is what I have tried so far:
macro_rules! parse {
(done) => (println!("done!"));
(if ($cond:tt) {$then:tt}) => (println!("if! "); parse!($cond); parse($then));
}
macro_rules! forward {
($($e:tt)*) => (parse!($($e)*; done));
}
fn main() {
forward!(if (done) {done} );
}
This doesn't work, and produces the error:
error: no rules expected the token `if`
What am I doing wrong here?
Edit: Beyond simply forwarding the arguments to forward, I was hoping to "paste" the tokens ; done on to the end of forward's arguments. Is there a way to make this work while preserving that behavior?
The problem is the ; done in forward. What's going on here is that the macro expansion code matches literal input tokens against arms one at a time. If an arm doesn't match, it gives up and tries the next one. When it runs out of arms to try, it has to fail and explain why.
But which token in the input was the problem? Answering that when there are potentially multiple arms involved is hard, so instead it just picks the first token and says "this was the problem".
Whenever you see a macro expansion complaining about the first token in the input not matching, it's quite possible it's something later in the input that tripped it up.
Fixing that (and fixing the parse invocation that's missing its !) gives:
macro_rules! parse {
(done) => (println!("done!"));
(if ($cond:tt) {$then:tt}) => (println!("if! "); parse!($cond); parse!($then));
}
macro_rules! forward {
($($e:tt)*) => (parse!($($e)*));
}
fn main() {
forward!(if (done) {done} );
}
Using a macro that breaks out of a loop works, but I want to pass in a label to be able to define which outer loop to break out of.
Passing the argument in as an expression gave a syntax error, the only way I managed to get this to work was to pass in a block however this isn't very elegant, e.g.:
my_macro({ break 'outer; });
Is there a way to pass:
my_macro('outer);
... that can be written in the macro as break $my_label; that expands into break 'outer; ?
Passing it as the versatile tt (token tree) works:
macro_rules! my_break {
($label:tt) => { break $label; }
}
fn main() {
'outer: loop {
println!("Start of outer");
loop {
println!("Start of inner");
my_break!('outer);
println!("Not reachable");
}
println!("End of outer");
}
println!("End of main");
}
Playground
To future readers, there's an accepted RFC adding a lifetime specifier for macro parameters.
I'm reading a book about Rust, and start playing with Rust macros. All metavariable types are explained there and have examples, except the last one – tt. According to the book, it is a “a single token tree”. I'm curious, what is it and what is it used for? Can you please provide an example?
That's a notion introduced to ensure that whatever is in a macro invocation correctly matches (), [] and {} pairs. tt will match any single token or any pair of parenthesis/brackets/braces with their content.
For example, for the following program:
fn main() {
println!("Hello world!");
}
The token trees would be:
fn
main
()
∅
{ println!("Hello world!"); }
println
!
("Hello world!")
"Hello world!"
;
Each one forms a tree where simple tokens (fn, main etc.) are leaves, and anything surrounded by (), [] or {} has a subtree. Note that ( does not appear alone in the token tree: it's not possible to match ( without matching the corresponding ).
For example:
macro_rules! {
(fn $name:ident $params:tt $body:tt) => { /* … */ }
}
would match the above function with $name → main, $params → (), $body → { println!("Hello world!"); }.
Token tree is the least demanding metavariable type: it matches anything. It's often used in macros which have a “don't really care” part, and especially in macros which have a “head” and a “tail” part. For example, the println! macros have a branch matching ($fmt:expr, $($arg:tt)*) where $fmt is the format string, and $($arg:tt)* means “all the rest” and is just forwarded to format_args!. Which means that println! does not need to know the actual format and do complicated matching with it.
Is it possible to write a macro that expands an expression into multiple indexed arguments, which can be passed to a function or another macro?
See this simple self contained example.The aim is to have unpack3 expand v into v[0], v[1], v[2].
macro_rules! elem {
($val:expr, $($var:expr), *) => {
$($val == $var) || *
}
}
// attempt to expand an array.
macro_rules! unpack3 {
($v:expr) => {
$v[0], $v[1], $v[2]
}
}
fn main() {
let a = 2;
let vars = [0, 1, 3];
// works!
if elem!(a, vars[0], vars[1], vars[2]) {
println!("Found!");
}
// fails!
if elem!(a, unpack3!(vars)) {
println!("Found!");
}
}
The second example fails, is it possible to make this work?
Possible solutions could include:
Changing use of macro grammar.
Using tuples, then expanding into arguments after.
Re-arranging the expressions to workaround macro constraints.
Note, this may be related to Escaping commas in macro output but don't think its a duplicate.
This is impossible in two different ways.
First, to quote the answer to the question you yourself linked: "No; the result of a macro must be a complete grammar construct like an expression or an item. You absolutely cannot have random bits of syntax like a comma or a closing brace." Just because it isn't exactly a comma doesn't change matters: a collection of function arguments are not a complete grammar construct.
Secondly, macros cannot parse the output of other macros. This requires eager expansion, which Rust doesn't have. You can only do this using recursion.
Is it possible to build an enum inside a Rust macro using fields that are defined as macro parameters? I've tried this:
macro_rules! build {
($($case:ty),*) => { enum Test { $($case),* } };
}
fn main() {
build!{ Foo(i32), Bar(i32, i32) };
}
But it fails with error: expected ident, found 'Foo(i32)'
Note that if the fields are defined inside the enum, there is no problem:
macro_rules! build {
($($case:ty),*) => { enum Test { Foo(i32), Bar(i32, i32) } };
}
fn main() {
build!{ Foo(i32), Bar(i32, i32) };
}
It also works if my macro only accepts simple fields:
macro_rules! build {
($($case:ident),*) => { enum Test { $($case),* } };
}
fn main() {
build!{ Foo, Bar };
}
But I've been unable to get it to work in the general case.
It's absolutely possible, but you're conflating totally unrelated concepts.
Something like $case:ty does not mean $case is something which looks like a type, it means $case is literally a type. Enums are not made up of a sequence of types; they're made up of a sequence of variants which are an identifier followed (optionally) by a tuple structure body, a record structure body, or a tag value.
The parser doesn't care if the type you give it happens to coincidentally look like a valid variant, it's simply not expecting a type, and will refuse to parse one in that position.
What you need is to use something like $case:variant. Unfortunately for you, no such matcher exists. The only way to do something like this is to manually parse it using a recursive incremental parser and that is so out of scope of an SO question it's not funny. If you want to learn more, try the chapter on incremental TT munchers in the Little Book of Rust Macros as a starting point.
However, you don't appear to actually do anything with the cases. You're just blindly substituting them. In that case, you can just cheat and not bother with trying to match anything coherent:
macro_rules! build {
($($body:tt)*) => {
as_item! {
enum Test { $($body)* }
}
};
}
macro_rules! as_item {
($i:item) => { $i };
}
fn main() {
build!{ Foo, Bar };
}
(Incidentally, that as_item! thing is explained in the section on AST coercion (a.k.a. "the reparse trick").)
This just grabs everything provided as input to build!, and shoves it into the body of an enum without caring what it looks like.
If you were trying to do something meaningful with the variants, well, you're going to have to be more specific about what you're actually trying to accomplish, as the best advice of how to proceed varies wildly depending on the answer.