gistfile1.js

(function(global){
	var undef;
	function Promise(){}
	Promise.prototype.constructor = Promise;
	
	global.promise = function(cb) {
		var publicAPI, queue = [], old_ret, promise_fulfilled = false;
		
		function fulfill(val) {
			var ret_val = val;
			if (typeof ret_val != "undefined") old_ret = ret_val;
			
			try {
				return val;
			}
			finally {
				for (var i=0,len=queue.length; i<len; i++) {
					if (typeof ret_val != "undefined") old_ret = ret_val;
					
					ret_val = queue[0].call(publicAPI,ret_val);
					
					if (typeof ret_val == "undefined") { ret_val = old_ret; }
					else if (ret_val.constructor !== Promise) old_ret = ret_val;	// lame, V8 "instanceof" currently broken
										
					queue.shift();
					
					if (ret_val.constructor === Promise) {	// lame, V8 "instanceof" currently broken
						promise_fulfilled = false;
						ret_val.then(function(P){ promise_fulfilled = true; return (old_ret = fulfill(P.value)); });
						break;
					}
				}
			}
		}
				
		publicAPI = new Promise();
		publicAPI.then = function(cb){
			queue[queue.length] = function(val){ return cb.call(publicAPI,{value:val}); };
			if (promise_fulfilled) fulfill(old_ret);
			return publicAPI;
		};
		
		cb.call(publicAPI,{fulfill:function(val){
			promise_fulfilled = true;
			fulfill.call(publicAPI,val);
		},value:undef});
		
		return publicAPI;
	};
})(window);

gistfile2.js

var Load = function(src,cb) {
	var xhr = new XMLHttpRequest();
	xhr.open("GET",src);
	xhr.onreadystatechange = function(){
		if (xhr.readyState == 4) {
			cb(xhr.responseText);
		}
	};
	xhr.send();
};

gistfile3.js

promise(function(P){
	Load("abc.js",P.fulfill);
})
.then(function(P){
	return promise(function(P2){
		setTimeout(function(){
			P2.fulfill(P.value);
		},2000);
	});
})
.then(function(P){
	return P.value.toUpperCase();
})
.then(function(P){
	return promise(function(P2) {
		setTimeout(function(){
			P2.fulfill(P.value);
		},1000);
	})
	.then(function(P){
		return P.value+"!";
	});
})
.then(function(P){
	// do nothing -- silently pass along P.value
})
.then(function(P){
	console.log(P.value);
}); 

gistfile4.js

promise(function(P){
	P.fulfill("Hello");
})
.then(function(P){
	return promise(function(P2){
		P2.fulfill(P.value+" World");
	})
	.then(function(P){
		return P.value.toUpperCase();
	});
})
.then(function(P){
	console.log(P.value+"!");
});