My favourite interview question | Hacker News Reader

My favourite interview question

weblog.raganwald.com

126 pointsjoksnet32 comments

What I’m about to say will be blindingly obvious to the Enterprise crowd...The rules must be considered as carefully as the entities. Enterprise developers have known this for years: that’s why you see rules engines, table-driven designs, and visual workflow editors in many Enterprise applications.

What a refreshing statement at a time when we're bombarded with so many annoying "SQL is Dead" posts.

Try designing an enterprise production/distribution system where:

  - set-ups must be minimized
  - backlog must be minimized
  - inventory must be minimized
  - trucks must be full
  - warehouse space is limited
  - deliveries must be on time, but not too early
  - sales people must have stock on hand
  - plant absorption must be maximized
  - bills must be paid on time
  - down time must be zero
  - working capital must be put to the best use
  - stockholders must be satisfied

Say what you want about enterprise programmers, but they get stuff built that handles all of these while academicians fuck around with linear algebra and OO castles for years.

After this, Monopoly sounds like a day off. Great post!

Of course many of the concrete methods enterprise developers use come from these 'academicians' so don't be too quick to judge them.

We all stand on their shoulders... it's fine to give em a good kick every so often but remember who is holding us up.

I agree that we all stand on the shoulders of giants, just not the same ones you cite.

As a trained mathematician, I wish I could agree with you and cite all the wonderful applications of classical thinking that I've used to solve real enterprise problems, but alas, I can't. It was only after I abandoned the constant search for "elegant" solutions to real world problems that I was able to embrace good old fashioned "blocking and tackling" to solve the problem at hand.

I've lost track of how many times I was challenged to "use the simplex method" to attack multiple simulaneous mutually exclusive constraints when the most effective approach ended up being, "Put all the data into a relational database and keep spitting it out different ways until our people figure it out." The scientist in me wishes it wasn't true, but the pragmatist knows better.

Sorry to dump on the academic world so much, but I suppose I wouldn't if I could cite more success stories.

The Monopoly example just struck a nerve; a classic example of the difference between theory and practice.

The relational database model is actually a very good example of mathematical rigor, and comes out of the world of research.

Good point. Sorry I missed that. (Although I have to admit I don't remember that last time I actually took anything to 4th Normal Form. It just seems like too much hammer for most problems at hand.)

Ah, you just have to do it differently. You can't approach an enterprise problem with "real math"; real math is usually pretty brittle in practice, in the sense that the way the axiomatic systems tend to get set up you can ram the slightest deviation from them through to prove anything, thus breaking the system wide open. Practical programming does need to be more robust than that. Even Haskell requires unsafePerformIO and while end users should not use it, some important libraries certainly do. Even algorithms work tends to be not very useful.

On the other hand, the way in which you think of mathematical entities can be very important and useful, particularly in thinking of things in terms of constraints and preconditions and the resulting guarantees the code can make. Again, not in a flat-out "code proof" sense, but as a way of approaching the code. This has allowed me to write some surprisingly capable code in a multi-team setting by carefully considering the minimal requirements I can impose on the other teams to get a desired result, then making sure I enforce those requirements carefully (since I am in a language where I can't do it with a type system), and then on those basic promises build another layer of the system that is the thing I am actually working on. If I came at this with a hacky hacky chop chop approach it would fail... just as the previous effort did, ahem.

Also, in programming as in math, a thing is what it does, profoundly. This can take a long time to come to grips with. A typical programming entity "is" quite a bit more than your typical math entity, though, for better and for worse.

Preconditions, postconditions, and constraints also make great testing fodder, by which I mean, they make great practical testing fodder. Writing code in practice is a lot easier if you have some assurance that those things are actually true.

I don't necessarily even sit down and sketch this all out, again, because being too rigid causes your rigid assumptions to break, but this stuff is always in the back of my head, and it will get you quite far once you master it....

... and I say that in general. edw519 I wouldn't care to second guess how you're doing things at this point. :)

Not as refreshing, when you realize this article was written in 2006.

What's changed? Perhaps the technology a little, but as for the real world problems, not nearly as much. Give the frameworks for thinking a little more credit than that.

The thinking/mindshare of the NoSql crowd has changed, that's all - your other points are certainly valid then as they are now.

May be completely besides the point, but my first OO instinct upon approaching the question is to model rules etc. as objects themselves. Only the most naive OO modeler would think that OO objects must be nouns within some physical analogue of the system - to my mind, anyhow.

I've been working on a board-game program lately and I've created a "referee" object that's responsible for enforcing rules, handling workflow, turn taking, etc. Works really well for me.

Why would you want to do that? They have no state. You're almost never going to do anything with them except invoke them from specific places. This sounds like complexity for its own sake.

First up, there's nothing wrong with immutable objects; indeed, immutable objects have benefits that mutable objects don't. So not having mutable state is not a good argument against modeling them as objects in an object-oriented language.

Secondly, rules do have state, in so far as one rule is distinct from another. Perhaps a rule has preconditions (which could be represented as a binary tree and evaluated through recursion) that must be true before the rule is "active". Perhaps rules have effects, a list of state changes that they impose on the rest of the game world. You could model these as functions, or alternatively again as trees, evaluated through recursion. If the rule system needs to be flexible enough, they could be serializable and deserializable from a text or binary format.

If the game is a simple one, with a fixed set of rules that never change, there's little need for this. But if you look at a game like Monopoly, there's a lot of complexity that comes up in real-life games that isn't so simple to encapsulate in rule lists, such as making complex deals, haggling, borrowing money from other players, even receiving charitable donations from other players to prevent the game ending prematurely.

Move further into business rule systems and the need for flexibility increases again. I've architected systems which needed a generic rule-based system for data validation and workflow transitions; and these rules had themselves rules for when they applied, as older rules needed to be grandfathered in to older data, but expired for new data.

No one here or on the original page seemed to consider they should design a Monopoly game on the net to be "fun." Especially since Hacker News is about and for entrepreneurs, it would seem natural to turn an idea like that into money. Monopoly is a game. It should be fun. It had better be fun, or the design doesn't matter because it won't sell.

What makes the game fun? Part of it is the interaction between players, laughing, joking, scheming, and posturing. How do you bring that to a game played over the net? Is text chat sufficient, or can you easily do voice? What do you display to the players? Certainly part of the board needs to be displayed. When I play, I like to see what people have for properties and money. Will I be able to see that Alice has a stack of $500 bills or that Bob has nothing larger than a $50?

The original question is a very cool thought experiment, imho. My initial reaction was that to design this, I'd want to sit and play some games of Monopoly to see what to include and to see what the game is really all about. I've played hundreds of games of Monopoly, but that's not the same as designing a fun game to play over the net. Running through this little thought experiment helps raise many questions about how the game will work. Does each player have a complete client, or is this a server based system (or is it somewhere in between)? How do you handle dice rolls? How quickly can people start a game?

You need to know what you're going to create before you start applying tools to create it.

I didn't get into what I would call the "product management" aspect of the question in that post, but if it doesn't bore you to hear it, I have participated in interviews where this question was asked of non-technical people, specifically a product manager and a web designer.

I don't remember if "fun" was specifically discussed, but the discussions were more focused on what users would do and see, very much in keeping with your second paragraph.

The product manager was also asked to consider the exercise from a competitive perspective. What could be done to differentiate a new site from an existing, popular site? is the strategy to be simplere and easier? To cater to power users running into limitations of the existing site? And so on.

Not only do I think the "project management" aspect is interesting, I think it is essential!

I develop s/w in an R&D environment. My users are very specific about how they use information, but know almost nothing about s/w development. Likewise I know little of their specific domain, but can develop whatever tools they need. While I can craft and code algorithms that should dazzle them, if it doesn't solve their problem they won't use it. For me, design has to account for the users' experience as well as developing clever and efficient algorithms to solve problems.

I really love the original question, Reg. It was fun.

This was exactly my immediate reaction, but mostly because I never really found Monopoly to be all that fun in the first place. Instead, my friend and I pooled together our monopoly dollars and made a monopoly casino.

There's not really a good digital equilvalent to this, but that's the tricky thing about monopoly: the things that set it apart from other games are the hardest to transfer to a digital analogue.

This reminds me of the ongoing quest to make a popular online version of Cosmic Encounter. I grew up playing the paper version with every expansion. There were so many aliens to be (and we frequently played with two/person) that the dynamic was different every game. But to add a new power to the digital version can require a whole new set of rules, and so they always fall startlingly short. [as a brief example, the Filch power, when coupled with it's power card, allowed you to cheat by secretly taking cards from the discard pile and whatnot until you were caught - this is nearly impossible to accurately mimic digitally]

Interesting that you'd turn monopoly into a casino because I turn monopoly into poker.

When I'm only doing average or not so well as I want to let on I make sure I have my 1000s on the top of a large stack. When I'm clearly leading I spread all my money into individual stacks so that people can clearly see that I'm far in the lead. The same goes for properties. Especially when I have stuff built on them. Anything with the most value is up in people's faces unless I'm slightly behind the curve where I obfuscate what I have to not seem a threat. (Games of monopoly often turn into horse trading in the late stages.)

Translating that online would be hard. Fun but hard.

"If the candidate uses up all of the interview time trying to obtain perfect requirements, we have a problem. In the software development I do, the requirements are never perfect. I don’t demand that a candidate try to create an agile, iterative process on the spot, but I look for someone who knows when to say 'close enough, let’s move forward.'"

I think this is a great reason why most interviewing techniques fail. The interview is not a reliable model of the work environment. This is especially true given the fact that people love "gotcha" interview questions which are essentially brain teasers which require you to know some arcane trick to get the question right.

I have seen great candidates spend a lot of time trying to understand a simple problem in an interview. These are people who under normal working conditions would never waste tons of time trying to get a perfect spec before starting work. But, because of the types of interviews people try and pass over as being "reliable indicators of future performance" candidates get stuck trying to find the trick in each question. If you have a candidate spend their entire interview trying to tease out a spec it sounds like a better indicator of the low quality of the interviewer rather than the candidate to me...

When people start heading that direction in an interview, be sensible and say, "Do you feel like you need a complete spec before going on? I promise there's no trick I'm trying to get you to fix" and you'll get the response you're actually looking for.

My favourite interview question is "When can you start?"

If the candidate uses up all of the interview time trying to obtain perfect requirements, we have a problem

I don't know how long your interview is, but if its just an hour and if the game is as ambiguous as you seem to say it is, I think its the fault of the interviewer, not the interviewee. The interviewee doesn't know what your schedule is -- unless you tell them, "You must code monopoly by the end of this hour, warts and all". But otherwise, I think its reasonable to take the full hour on requirements. In real life, an hour on requirements doesn't describe if there should be a splash screen or not.

What if your candidate isn't familiar with Monopoly because, say, they didn't grow up in the west?

Simply select a different game that's culture-appropriate. If you're interviewing in Asia, perhaps Go would be more appropriate. (And I believe the original author has a thing or two to say about that game as well. :)

Or don't even pick a game; I love "design an elevator control system". Most anyone looking for a programming position has interacted with one at some point in their life, and while you could easily mire down in details, it's not so complicated that you couldn't give it a fair hearing in the context of an interview.

There's a million things like this in our everyday lives that make great fodder for mental exploration in an interview.

The rules of Go are well defined, the interesting thing about Monopoly is that it's 'loose' definition replicates many of the real world problems that arise in a devs life.

One of my first Uni projects was to write an elevator control system and as you say, it's very revealing.

I've had similar interview questions before about different types of boats (I know barely anything about boats). The secret here of course is to demonstrate how you can deal with unknowns, turn it into a problem you understand and then solve it. If anything, you have greater opportunity to demonstrate yourself here.

Perhaps hand them the rule-sheet?

Available online, even:

http://www.hasbro.com/common/instruct/DeluxeMonopoly.pdf

Reading through a rule-sheet, even carefully, is not going to give you the type of understanding of game dynamics playing even 10 games would give -- and which would be fairly useful in this task.

Take any problem where there are rules and n exceptions to those rules. All salesmen get cost * x% commission on an item EXCEPT when it's a a) our annual year end sale then they get y% or b) when it's an internet sale then they get z%.

That's the pattern to all this, rules and exceptions and how easy it is to change those rules (because they change constantly).

So do you put all that into your SalesPerson object or is there a better way to abstract the logic of commissions?

In my experience, it's already hard to get a good read of a candidate within 45-60mn without adding external entities.

Monopoly is not just an external entity, it's a pretty big one with tight cultural ties. If the candidates you interview were not born in the same country as you, the best case scenario is that they played Monopoly in a different language and the worst case scenario is they never played it.

I prefer to focus my interviewing on questions that rotate around pure coding (if that's what the candidate is interviewing for) and even then, I hardly find the time to dig as deep as I'd like.

I know this is kind of beside the point, but my response would ne pretty simple - I've never played Monopply, and have no idea of its structure at all.

This has been posted before, with extensive discussion:

http://news.ycombinator.com/item?id=2063092

;-)

Reply on news.ycombinator.com