Haxe Code Cookbook
Haxe programming cookbookMacrosWorking with compiler flags

Working with compiler flags

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This snippet demonstrates how to get, set, check for, and list compiler flags.

Compiler flags can be assigned values. For example, -D key=value defines a compiler flag called key with a value of value.

Compiler flag values representing numbers can be evaluated to Float and Int. Everything else is considered a String constant. If no value is assigned to a compiler flag key, the compiler defaults it to 1. If a value of an undefined compiler flag key is requested, the compiler returns null.

It is possible to get and set compiler flags, check if they are defined, and list all defined compiler flags.


class Main {
  static function main() {
    trace("Warning: " + getDefine("warning"));
    if (!isDefined("foo")) {
      setDefine("foo", "bar");

  // Shorthand for retrieving compiler flag values.
  static macro function getDefine(key : String) : haxe.macro.Expr {
    return macro $v{haxe.macro.Context.definedValue(key)};

  // Shorthand for setting compiler flags.
  static macro function setDefine(key : String, value : String) : haxe.macro.Expr {
    haxe.macro.Compiler.define(key, value);
    return macro null;

  // Shorthand for checking if a compiler flag is defined.
  static macro function isDefined(key : String) : haxe.macro.Expr {
    return macro $v{haxe.macro.Context.defined(key)};

  // Shorthand for retrieving a map of all defined compiler flags.
  static macro function getDefines() : haxe.macro.Expr {
    var defines : Map<String, String> = haxe.macro.Context.getDefines();
    // Construct map syntax so we can return it as an expression
    var map : Array<haxe.macro.Expr> = [];
    for (key in defines.keys()) {
      map.push(macro $v{key} => $v{Std.string(defines.get(key))});
    return macro $a{map};


Assume the following build file:

-main Main
-neko main.n
-D warning="Don't let the bed bugs bite!"

Running neko main.n from the compiler output directory will result in:

Warning: "Don't let the bed bugs bite!"
{haxe3 => 1, haxe_ver => 3.201, dce => std, sys => 1, hxcpp_api_level => 321, true => 1, foo => bar, neko => 1, warning => "Don't let the bed bugs bite!"}

The last line may differ depending on the Haxe version and compilation options, because it is the string representation of the map containing all defined compiler flags.

Learn about conditional compilation here: https://haxe.org/manual/lf-condition-compilation.html

Learn about available global compiler flags here: https://haxe.org/manual/compiler-usage-flags.html

Mark Knol
Last modified:
Category:  Macros