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A Primer on Promises in ES6

(Post was originally written as a response to a question about how to resolve "callback hell" when asynchronous calls need to be executed sequentially.)

A popular system for managing sequential asynchronous operations, called "Promises", was integrated into ES6 (the latest release version of ECMAScript/Javascript). It's definitely a good idea to make sure you understand basic callbacks before diving into the Promises API, but it can really streamline asynchronous sequences once your callbacks start to get hellish.

It's based on an abstract data type called a "thenable". The basic premise is pretty simple -- a "promise" contains an operation that will be executed asynchronously, and has a .then() method which can be used to specify an operation to follow once the promise is fulfilled. You can create a new promise like this:

// Promise.new() creates a new promise.  It accepts a function as an argument.

	#!/bin/python3
	# https://www.hackerrank.com/challenges/journey-to-the-moon/problem

	import math
	import os
	import random
	import re
	import sys
	from itertools import combinations

	#!/bin/python3
	# https://www.hackerrank.com/challenges/bfsshortreach/problem

	import math
	import os
	import random
	import re
	import sys

	from itertools import chain

	#!/usr/bin/env python3
	"""
	[Created: March 2018]
	Oh hello, I'm here to destroy everything you love about Python.

	This is kind of ridiculous, but I wanted to create an easy way to
	define struct-like classes in Python and it seemed like a good way
	to learn some more about the python data model.
	It's more of a syntax hack than anything else, but it was fun to
	make.

	#!/usr/bin/python3
	# Requires Python 3.7+ or CPython 3.6 for ordered dict
	# Lightweight type-checking argument parser requiring minimal boilerplate
	# Just a coding exercise for myself, but if you like it please use it!
	# Todo:
	# - Boolean flags and grouping (e.g. "-o -a", "-xAd")
	# - Required and optional arguments (e.g. "--mode quiet -n 2")

	import sys
	from collections import namedtuple