6.1 Conditional Compilation

Haxe allows conditional compilation by using #if, #elseif and #else and checking for compiler flags.

Define: Compiler Flag

A compiler flag is a configurable value which may influence the compilation process. Such a flag can be set by invoking the command line with -D key=value or just -D key, in which case the value defaults to "1". The compiler also sets several flags internally to pass information between different compilation steps.

This example demonstrates usage of conditional compilation:

class Main {
  public static function main() {
    #if !debug
    trace("ok");
    #elseif (debug_level > 3)
    trace(3);
    #else
    trace("debug level too low");
    #end
  }
}

Compiling this without any flags will leave only the trace("ok"); line in the body of the main method. The other branches are discarded while parsing the file. These other branches must still contain valid Haxe syntax, but the code is not type-checked.

The conditions after #if and #elseif allow the following expressions:

Any identifier is replaced by the value of the compiler flag by the same name. Note that -D some-flag from command line leads to the flags some-flag and some_flag to be defined.
The values of String, Int and Float constants are used directly.
The boolean operators && (and), || (or) and ! (not) work as expected, however the full expression must be completely contained by parentheses.
The operators ==, !=, >, >=, <, <= can be used to compare values.
Parentheses () can be used to group expressions as usual.

The Haxe parser does not parse some-flag as a single token and instead reads it as a subtraction binary operator some - flag. In cases like this the underscore version some_flag has to be used.

Working with compiler flags

Compiler flags are available at compile time, the following methods only work in macro context:

To see if a compiler flag is set, use haxe.macro.Context.defined("any_flag").
To get the value of a compiler flag, use haxe.macro.Context.definedValue("any_flag").
To get a map of all compiler flags with its value, use haxe.macro.Context.getDefines().

Haxelibs

By default, each used haxelib version is automatically added as flag, e.g. when you add -L actuate, the compiler adds -D actuate=1.8.7. To test if a library exists in current context, use #if actuate. To check a specific haxelib version, use the operators, for example #if (actuate <= "1.8.7").

Built-in Compiler Flags

An exhaustive list of all built-in defines can be obtained by invoking the Haxe Compiler with the --help-defines argument. The Haxe Compiler allows multiple -D flags per compilation.