9.2 Arguments

Most of the time, arguments to macros are expressions represented as an instance of enum Expr. As such, they are parsed but not typed, meaning they can be anything conforming to Haxe's syntax rules. The macro can then inspect their structure, or (try to) get their type using haxe.macro.Context.typeof().

It is important to understand that arguments to macros are not guaranteed to be evaluated, so any intended side-effect is not guaranteed to occur. On the other hand, it is also important to understand that an argument expression may be duplicated by a macro and used multiple times in the returned expression:

import haxe.macro.Expr;

class Main {
  static public function main() {
    var x = 0;
    var b = add(x++);
    trace(x); // 2
  }

  macro static function add(e:Expr) {
    return macro $e + $e;
  }
}

The macro add is called with x++ as argument and thus returns x++ + x++ using expression reification, causing x to be incremented twice.


Take the Haxe Survey!