6.12 Logging and Trace

Haxe provides developers with a powerful trace system. Simply call trace within functions:

trace("Hello world!");

In most Haxe targets trace will be printed to stdout. JavaScript uses console.log. Each trace is displayed with the filename and line number information where the trace occurred:

Test.hx:11: Hello world!

To trace without the default position information haxe.Log.trace (msg, null) can be used.

Custom trace

The trace can have a custom output by changing the Log.trace method where all trace calls are redirected.

class Main {
  static function main() {
    haxe.Log.trace = function(v:Dynamic, ?infos:haxe.PosInfos) { 
      //custom trace function here

The v argument is the first parameter of the trace call. It can be a String or any other value. The optional infos argument contains extra position parameter, see below.

The infos.customParams array contains all extra arguments that were given to the original trace. If no extra parameters are passed, it will be null.

As illustration, the previous example will be compiled as if it was calling the following:

haxe.Log.trace("hello", {
	fileName : "Test.hx", 
	lineNumber : 6, 
	className : "Test", 
	methodName : "main", 
	customParams : ["warning",123]
Removing traces

You can simply remove all trace informations by compiling your project with --no-traces argument. This will remove all trace calls as if they were not present in the program.

Extra Position Parameter

haxe.PosInfos is a magic type which can be used to generate position information into the output for debugging use. If a function has a final optional argument of this type, i.e. (..., ?pos:haxe.PosInfos), each call to that function which does not assign a value to that argument has its position added as call argument.

It is sometimes useful to define a custom method that does some traces in some case. The following usage is possible since in Haxe when the haxe.PosInfos optional parameter is not set, its default value will always be replaced by the compiler:

class Main {
  static function assert(cond:Bool, ?pos:haxe.PosInfos) {
    if (!cond)
      haxe.Log.trace("Assert in "+pos.className+"::"+pos.methodName,pos);

  static function main() {
    assert( 1 == 1 ); // nothing
    assert( 0 == 3 ); // trace "Assert in Test::main"
Tracing types

$type is a compile-time mechanism being called like a function, with a single argument. The compiler evaluates the argument expression and then outputs the type of that expression.

var myValue = "foo";
$type(myValue); // String