Haxe 3.2.0 @:callable example used in http://haxe.io/releases/3.2.0/

build.hxml

-js Main.js
-main Main

Main.hx

package;

@:callable abstract Function<T>(T) from T {}

class Main {
	
	static function main() {
		var a:Function<Int->Int> = function(v) return v * v;
		a(31);
	}
	
}

Main.js

(function (console) { "use strict";
var Main = function() { };
Main.main = function() {
	var a = function(v) {
		return v * v;
	};
	a(31);
};
Main.main();
})(typeof console != "undefined" ? console : {log:function(){}});