Is it possible for sweet.js macros to define other macros? - sweet.js

Question

I have attempted to define a sweet.js macro that allows other repeating macros to be defined more easily, but I have found a syntax error here:
SyntaxError: [patterns] Ellipses level does not match in the template
11: { $($b...)... }
This is the macro that produced this syntax error:
macro repeatingMacro{
rule{
$a {
$b...
} {
$c...
}
} => {
//the output of this macro should be another macro with repeating patterns
macro $a {
rule{
{ $($b...)... }
} => {
{ $($c...)... }
}
}
}
}
If this macro were correctly defined, then it would allow other macros to be created, like this one:
repeatingMacro cond {
$a... { $b... }
}
{
if($a...){
$b...
}
}
var x = 1;
var y = 2;
cond {
(x > y) {
alert(x);
}
(x < y) {
alert(y)
}
}
This code can be edited online here.
In other words, is it possible to define a macro that will automatically transform this macro:
macro cond {
rule {
$x... { $y... }
} => {
if($x...){
$y...
}
}
}
...into this macro?
macro cond {
rule {
{ $($x... { $y... })... }
} => {
$(if($x...){
$y...
})...
}
}

Answer 1

The immediate problem you are running into is that if you need to emit literal ellipses ... in the template of a macro you need to escape it by doing $[...].
But at a higher level, are you sure you need to go to the effort of macro defining macros here? Sweet.js actually has a nice declarative feature called macroclass (documented here) that makes doing things like this really simple. In fact, cond is the example we use:
// define the cond_clause pattern class
macroclass cond_clause {
pattern {
rule { $check:expr => $body:expr }
}
}
macro cond {
rule { $first:cond_clause $rest:cond_clause ... } => {
// sub-pattern variables in the custom class are
// referenced by concatenation
if ($first$check) {
$first$body
} $(else if ($rest$check) {
$rest$body
}) ...
}
}
cond
x < 3 => console.log("less than 3")
x == 3 => console.log("3")
x > 3 => console.log("greater than 3")