I'm trying to write a macro that will rewrite certain Rust control flow, but I'm having difficulty matching an if
expression. The problem is that the predicate is an expression, but an expr
is not permitted to be followed by a block
or {
.
The best I've got is to use tt
:
macro_rules! branch {
(
if $pred:tt
$r1:block
else
$r2:block
) => {
if $pred {
$r1
} else {
$r2
}
};
}
Which works fine with single-token or grouped predicates:
branch! {
if (foo == bar) {
1
} else {
2
}
}
But fails if the predicate was not grouped:
branch! {
if foo == bar {
1
} else {
2
}
}
error: no rules expected the token `==`
I also tried to use a repeating pattern of tt
in the predicate:
macro_rules! branch {
(
if $($pred:tt)+
$r1:block
else
$r2:block
) => {
if $($pred)+ {
$r1
} else {
$r2
}
};
}
But this produces an error because it's now ambiguous whether subsequent block should match the tt
too:
error: local ambiguity: multiple parsing options: built-in NTs tt ('pred') or block ('r1').
Is there a way to do this, or am I stuck with inventing special syntax to use in the macro?
You could use a TT muncher to parse the predicate:
macro_rules! branch {
{
if $($rest:tt)*
} => {
branch_parser! {
predicate = ()
rest = ($($rest)*)
}
};
}
macro_rules! branch_parser {
{
predicate = ($($predicate:tt)*)
rest = ({ $($then:tt)* } else { $($else:tt)* })
} => {
println!("predicate: {}", stringify!($($predicate)*));
println!("then: {}", stringify!($($then)*));
println!("else: {}", stringify!($($else)*));
};
{
predicate = ($($predicate:tt)*)
rest = ($next:tt $($rest:tt)*)
} => {
branch_parser! {
predicate = ($($predicate)* $next)
rest = ($($rest)*)
}
};
}
fn main() {
branch! {
if foo == bar {
1
} else {
2
}
}
}
Output:
predicate: foo == bar
then: 1
else: 2