Static extensions in Haxe can be used to mimic multiple inheritance similar to mixins in Javascript. It’s a powerful feature and should be used carefully.

StringTools class is a good example of station extension. Refer to my earlier post for more info on that.

Quick Example:

package ;

class ArrayUtils {
	public static inline function last<T>(a:Array<T>):T {
		return a[a.length - 1];
	}
}

using keyword brings the defined class into the context.

package ;

using ArrayUtils;

class Example {

	public function new() {
		var arr:Array<Int> = [100, 200, 500];
		trace(arr.last()); //500
	}
}

Another example with multiple static extensions:

package ;

class ArrayUtils2 {
	public static inline function first<T>(a:Array<T>):T {
		return a[0];
	}
}

package ;

using ArrayUtils;
using ArrayUtils2;

class Example {

	public function new() {
		var arr:Array<Int> = [100, 200, 500];
		trace(arr.last());  //500
		trace(arr.first()); //100
	}
}

A few things to note:

• If the same method is defined in multiple static extension classes, using expressions are checked from bottom to top. For example if the method last is also defined in ArrayUtils2 then it will take precedence as ArrayUtils2 is the bottom class in the above example.
• Built in class fields would take priority over the static extension fields. For example if you define toString method in ArrayUtils class above it wouldn’t work as toString is a built in array method and it takes precedence.
• Compiler metadata @:noUsing can be used in static extension class if you want to omit any field being included in the context. Check the example below.

package ;

class ArrayUtils {
	public static inline function last<T>(a:Array<T>):T {
		return a[a.length - 1];
	}

	@:noUsing
	public static inline function second<T>(a:Array<T>):T {
		return a[1];
	}
}

You can’t use the method second as the compiler will not include it in the context when used.