Computing factorial with sweet.js

fac.sjs

macro fac {
	case infix { $acc:lit | fac 0 } => { return #{$acc} }
	case infix { $acc:lit | fac $n:lit } => {
		var acc = #{$acc}, n = #{$n};
		var res = acc[0].token.value * n[0].token.value;
		var n1 = n[0].token.value - 1;
		return withSyntax($res = [makeValue(res,n)],
		                  $n1 = [makeValue(n1,n)]) {
			return #{$res fac $n1}
		}
	}
	case { fac $n } => { return #{1 fac $n} }
}
 
console.log(fac 1);   // 1
console.log(fac 2);   // 2
console.log(fac 3);   // 6
console.log(fac 4);   // 24
console.log(fac 5);   // 120
console.log(fac 6);   // 720

The last rule actually involves 8 macro expansions:

    console.log(fac 6);
    ->
    console.log(1 fac 6);
    ->
    console.log(6 fac 5);
    ->
    console.log(30 fac 4);
    ->
    console.log(120 fac 3);
    ->
    console.log(360 fac 2);
    ->
    console.log(720 fac 1);
    ->
    console.log(720 fac 0);
    ->
    console.log(720);