A little JavaScript problem

In fact, this isn't about JavaScript, but that's the context I've discussed it in. I encourage you to think about it in more programming languages. (are there languages in which this can't be done?)

The problem: define functions range, map, reverse and foreach, obeying the restrictions below, such that the following program works properly. It prints the squares of numbers from 1 to 10, in reverse order.

var numbers = range(1, 10);
numbers = map(numbers, function (n) { return n * n });
numbers = reverse(numbers);
foreach(numbers, console.log);

/* output:



  • You must not use arrays. The square bracket characters, [ and ], are forbidden, as well as Array constructor.

  • You must not use objects. The curly braces, { and }, and the dot character (.) are forbidden. You may use curly braces for code blocks, but not for creating JavaScript objects.

  • Should go without saying, these functions must be generic and do what their name implies. They must not be hard-coded for the particular 1..10 example.

Feel free to define utilities; you don't have to restrict your program to these 4 functions. It does not matter how fast, small or elegant it is — if you can do it within the limitations above, I think you're an above-average programmer and I would probably hire you. (however: I'm not hiring)

Just for reference, my implementation (in ES6) is 8 lines of code, and it isn't totally unreadable. :-)

PS: please don't post your solutions, or links, here. I will not publish them. But feel free to email me in private if you really want me to look into it.


QUEEN, chess, and writing fast Lisp code

Back in 2008 I wrote an online chess game. It took me about a month to get it working, and then I gradually fixed bugs and added new features for another 3 months or so. It was online for about three years and I used to play chess there with my dad or friends. Then at some point I had to change my server, and it was a pain to get the server-side dependencies working again so I just dismissed my little service.

This year I've decided to put it back online, so I digged my old hard drives for the sources. As I said, it took like one month to write it -- so productive I was! A quick look at the code explains it, though. The server-side is Perl and it's basically write-only (awful) code. While I probably could sync it with the dependencies and get it back working, the feeling that there are a thousand bugs inside won't let me sleep at night, so I started rewriting the server side in Common Lisp (the client is somewhat OK, I can maintain it).

When you write chess software, even if it's not supposed to play chess but only allow two humans to play chess together, it must still have some notion about the rules of the game, for example to detect checkmate, or to prevent someone from making illegal moves. You need to implement a proper move generator even for something “as simple as” parsing chess moves in algebraic notation -- to read a move like "Ne5", which means “kNight moves to e5”, you must know where on the board is the knight of the current side that can move to e5 (and it better be only one). There's similar pain in generating the SAN notation for a move.

For Perl I wrote Chess::Rep back in 2008, and I knew I'd need something like this in Lisp, so I wrote QUEEN. It's the module I'm writing about here. The full game is not ready yet, but QUEEN is useful in itself so I published it and submitted it for inclusion into Quicklisp.

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The left-pad case

“Getting tired of people hating npm and JavaScript. Some of those people even make a living through JavaScript.”

— Atanas Korchev (@korchev) March 24, 2016

I felt this was addressed to me, following a number of tweets and retweets. I'm too critic. Point taken. Luckily I don't blog often, otherwise this site would be full of rage and “everything sucks” posts.

I must say I initially sympathised with Azer when I've read his post on “liberating” his modules. I'm a programmer, I love open-source, and I would definitely hate it if somebody asked me to unpublish uglify-js just because. Heck, I've created this trend — I don't remember any packages with the “ify” suffix before Uglify.

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Minesweeper and interviewing

Today I've read this article on interviewing and I decided to take the challenge:

You have one hour to implement as much of Minesweeper as possible. I've provided a JSBin for you though you're free to use whatever development tools/libraries/frameworks you're most comfortable with. You're allowed to use whatever internet resources you need, excluding plagiarism. I'll be available to answer any questions you may have. You're not expected to finish! In fact, no one ever has. Do your best and we'll chat about how things went afterward.

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Multiple inheritance and multiple dispatch in JavaScript

Multiple inheritance and multiple dispatch are core features in CLOS. The standard JavaScript object system supports neither, but they can be implemented. I thought I'd write this article, along with working code, for the JavaScript fellows who don't know the concept. Maybe someone smarter than me will come up with an efficient implementation.

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How to implement a programming language

I wrote a tutorial on how to implement a programming language in JavaScript. It got quite involved, and it took embarrassingly long to write. I wanted to write only about parsing, after a discussion with my dad about this problem; but once I had a parser I thought I'd add in a simple interpreter and once that was in place, I thought I'd turn it into continuation-passing style to show how we can workaround the lack of tail call optimization in JavaScript. But this opened the opportunity to talk about continuations. And of course, it was too slow so I thought I'd discuss compiling as well.

Long story short: I describe how to implement a non-trivial programming language. We get to a language that has decent performance, can interface easily with JavaScript and can offer first class continuations. To hell with the callback hell!

Read on and let me know what you think.


Find all expressions that evaluate to some value

Some friends came to visit last weekend, and we got to talk about the following problem, which seems to have made the news headlines (it was given at a math contest for ten year olds and apparently many math teachers couldn't figure out a solution):

We have numbers 1, 3, 4 and 6. Create an expression using any elementary operators (+, -, *, /), containing only these numbers, exactly once each, such that the result is 24.

(if you'd like to try finding a solution, stop reading now because there's a spoiler here)

I felt the problem itself is inappropriate for 10 y.o. kids and irrelevant for a maths contest; but it seemed like a good programming exercise, so I told'em “while you're banging your heads to guess a solution, I'll go and write a program to give me all solutions.”

I implemented it in Scheme, because it requires backtracking and using continuations for this is a joy. But in this blog post I'll work out the solution in JavaScript.

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Using UglifyJS for code refactoring

Somebody asked me the following question:

I want to do AST transformations on my javascript files. Specifically, i want to take all throw "string" expressions and replace them with throw new Error("string") expressions.

The problem is I want to make no other changes to my source. I want to not change anything: no comments, no indentation, no whitespace, etc. I just want match certain AST subtrees (a throw node with a string node as a child) and replace them with something different.

I thought I'd write this blog post to show how.

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Livenode — Live-code your NodeJS application

I'm working on a NodeJS application for Kendo UI. If you've been following me for a while, you probably know that I'm not the biggest fan of NodeJS—however, it seems that for this project Node is the best option, so I actually pushed for it.

A small annoyance that I'm trying to “fix” is the requirement to restart the server all the time in order to test changes. After some years with Common Lisp, I know there are better ways to do development than restarting the program — so here's Livenode, a module that allows to “live-code” a NodeJS application.

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Enterprise tools

Enterprise tools take an idea which is essentially simple and beautiful, and place a mountain of mud over it: acronyms, nouns, APIs, XML (and XSLT, DTDs, SOAP!, WSDL!) and the list is endless. Such that you don't see the beauty anymore; you don't see it simple anymore. You then live in constant fear that everything has to be enormously complex, ungodly hideous, and you thank your Enterprise Tools Vendor, your new God, for providing you with an IDE that makes life bearable, completely losing sight of the fact that it's the Enterprise Bullshit itself which adds in the complexity. You're locked to your Vendor, and you're doomed to never see beauty again.

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