Good fences make for good neighbors

If you work in an environment as professional as mine, automatic testing is probably a given.

Odds are, also, that the expression “Design by Contract” (or DBC) isn’t. Maybe that you even never heard about it.

So, what is it?

DBC, in short

If you want to know more about “Design by Contract” and “Assertive Programming” in general,  “The Pragmatic Programmer” (Thomas, Hunt) has a section about each to get you started.

Design by contract is a form of assertive programming where you ensure that functions/methods only operate on sound input and produces the output expected.

You can also identify “class or state invariants”: conditions that should hold before and after each function/method execution, and enforce them.

How do you implement it?

Some languages, like Clojure, have built-in support for DBC (I have briefly mentioned this technique in a previous entry on my blog), for example:

(defn function [arg1 arg2]
    {:pre [(is-foo? arg1) (is-bar? arg2)}]
     :post [(looks-right? %) (sparks-joy? %)]
    ;; your code here

will throw an exception if the checks on arg1 or arg2 fail, or if the function result will not "look right" or does not "spark joy".

In other languages, you can still make do. The first time I used DBC was with Objective C on an iPad application. Since the language doesn’t support it, I used a mix of assertions and preprocessor macros.

If I had a function and I wanted to make sure it would add a single value to a non-empty array, I would have done something like:

void appendValue(NSMutableArray *arg) {
#ifdef DBC
    NSUInteger origSize = [arg count];
    assert(origSize > 0); 

// Do your thing

#ifdef DBC
    assert(origSize + 1 == [arg count]);

It’s clunky – I know – but it works, and proved invaluable thorough development.

Plain Java can get something similar using the assert construct but is limited by the extensive use of mutability (just as in the previous example) and lack of preprocessing.

Why use it?

First of all, if you are thinking now: “Well, I do TDD or UT, I have full coverage: I don’t need DBC!” you would be factually wrong.

DBC is complementary to unit testing, and it provides value whether you have a testing infrastructure in place or not.

DBC tests the application while it runs, where UT – even a comprehensive end-to-end test – will only check a selected scenario.

One of the main strengths of UT (at least design-wise) is that it’s a form of black-box testing: it ensures that the test subject produces the expected outcome in a set scenario, without delving into the object’s state.

DBC, on the other hand, ensures that your application is internally coherent at runtime as it checks the internal state, and it’s an inherently low-cost approach.

Consequently, you can easily practice DBC in situations where UT and TDD have a prohibitively high cost/benefits ratio, like legacy code or poorly designed applications.

Why is DBC not popular?

If DBC is so good, why isn’t it as popular as TDD or UT?

One of the reasons – I think – is that Test-Driven Development and Unit Testing are both safety nets and design techniques.

DBC – on the other hand – helps the developer to think about the scope of each function but does not lead to clean code.

I suspect – however – that this is only part of the reasons.

I think the main reason why we don’t use it is that – as developers – we tend to be complicit with our code.

It’s the same reason we don’t do assertive programming: we are deeply uncomfortable with the idea of making our code crash, even if we are sure it’s not doing the right thing.

We root for our code, beyond reason.

We think:


Is my method unexpectedly receiving an empty list?!

Well, let’s still give it a chance!

Go on anyway, "boolean checkNonEmptyList(List arguments)": I believe in you!

Exemplifying the first week of my current job, I was doing pair-programming with a senior team member. The guy is reviewing my code.

At some point, he sees a (rather tame) assertion I had put in one of my methods. He looks at me aghast and mutters:

We don’t put assertions in the code. We don’t do this.

As of now and that I know, assertions are still taboo in my workplace (a workplace which is – to be crystal clear – a place full of very respectable and capable developers).

Checks are still made of code

I’ll play the devil’s advocate at this point.

There is at least a good reason to look at DBC with distrust: the checks you are adding in your code are – of course – still code.

And like any code, it will contain bugs and it will fail on occasion, so the idea of adding a check that may actually add a bug to your code does not win a popularity contest.

Sure, you can be careful all you want but, eventually, your application will crash due to a bogus contract.

Is then DBC still worth it? YES. The number of bugs you’ll catch with DBC fairly exceeds the bugs you will introduce, and finding one of the latter contributes to your understanding of your code.

Yet, this is probably a (more reasonable) explanation of why this technique is less used and why I respectfully disagree with those who would leave all DBC checks on in production by default.


“Design by Contract” should be in every programmer’s toolbox: it forces you to think about the scope of your code, provides runtime testing, and is cheap to implement.

Like everything in this world, it isn’t devoid of downsides, but the benefits far outweigh the risks.

My “Advent of Code 2020” with Clojure

This is the last of a three parts series of posts about the “Advent of Code”, you can check this post if you want to know about the initiative or this other one for my general experience with the 2020 edition.

TL;DR: using Clojure, I loved the flexibility of the language: solving the problems with a functional paradigm added another (welcome) layer to the challenges. The lack of ready tools has been a minor disappointment, though.

As usual, I hardly even consider non-LISP languages for any leisurely coding, so I picked Clojure for my first participation in the “Advent of Code”.

My choice assumed (correctly) that the competition was comprehensive enough to allow developers with languages as slow as Java to have a chance: anything else would have made “Advent of Code” even more niche than it already is.

The next paragraphs summarize my experience with the “Advent of Code” using Clojure. Enjoy!

Join the dynamically typed side!

You chose Clojure because it’s not Java, then why coding in it as if it was Java?

It’s been a while since the last time I coded in Clojure, and, being a Java developer by day, I felt some (unexpected and unwelcome) uneasiness passing generic vectors and maps around.

At some point, I gave up on the temptation, and I did reify a protocol.

Around day 20, after creating the umpteenth protocol (because what sense does it make to create the C protocol if there are not the A protocol, the B protocol, and since we are at that, the D and E protocols?), I trashed the whole thing.

I restarted from scratches, this time using generic data structures, and never looked back.

Darth Vader "Join Me"

Now, I’m not saying that protocols (and classes, etc…) don’t have their place, but a competition based on daily, ever-changing problems, ain’t it.

And with Clojure, you have a choice.

Functional thinking

aka “that thing I don’t (yet) have”

I guess that if you really miss your C programming and want to make a trip on memory lane, you can start using atoms and do blocks like crazy and, besides producing ugly code, no harm will be done.

If you aim to use Clojure idiomatically, though, you’ll have to solve familiar problems in counter-intuitive ways.

This is a challenge I picked up willingly.

It certainly made me slower but provided some of my best return-of-investment from the competition in technical terms.

It also set my mind at ease about some worries I had about performances: transposing a table using map and apply operations is not as slow as I imagined it to be.

Planning for failure

Mh… It’s not working… Should I have written some tests?

I don’t think so: the scope of each problem was so small that the overhead of testing or TDD didn’t seem to pay for itself.

Also, I can be boring up to a point.

On the other end, not having a safety net (especially with a dynamically typed language) can be a source of minor headaches.

One solution that complements unit testing, is Design By Contract

Have you heard about "Design by Contract"?

I am by no means knowledgeable on the matter, I picked what I know from the excellent book “The Pragmatic Programmer” (Thomas, Hunt – new or old edition, it doesn’t matter).

It boils down to having runtime checks – mostly around functions – to ensure each method receives just what you expect and produces the right outcome.

You can do that in any language but – it turns out – Clojure conveniently supports it with the :pre and :post conditions in the defn special form.

In this (rather uncalled for) use of them:

(defn mangle-list [numbers]
    {:pre [(every? number? numbers) 
           (not (empty? numbers))]
     :post [(number? %)]}
    (reduce * numbers))

the :pre condition makes sure that the input is a non-empty sequence of numbers, and the :post ensuers that the output is a number.

It won’t make a difference with simple examples like this one, but when you are knee-deep in code, pre and post conditions can save you a lot of time.

Tools are not at hand

Let’s profile this thing with VisualVM! (5 minutes later…) Nevermind.

Debugging and profiling with Clojure is not as ready and convenient as with Java. This is a fact.

I’m not saying it can’t be done, I’m saying – again – that it’s not convenient, not at hand (cf. “Simple made Easy” by the man himself), that’s all.

Lack of tools is the most relevant grief I have with the language, even though the language itself is (mostly) not to blame.

That said, if you want to set a breakpoint, CIDER works beautifully (and I absolutely love CIDER, mind you!), but if you want to break on an exception you are out of luck.

On the other hand, you can use VisualVM for profiling, but at the first glance, if you don’t exclude the Clojure namespace first – between that and REPL-related threads – you have to wade through a lot of garbage to find what you are looking for.

Spiderman: "Nevermind, I'll just go..." meme


I had other possible choices for the competition.

Gambit or Guile would have been some excellent candidates if execution speed was of the essence, but I am frankly not proficient enough with them, and they don’t have the JRE with its goodies backing them up.

Emacs Lisp holds a place in my heart, despite being ancient, but the awkward support for dictionaries (or as they call them: “tables”) would have made everything so much harder.

The possibility of picking an entirely new language (Elixir is next on my TODO-list) crossed my mind and was – fortunately – crossed itself. I really dodged a bullet there (not because of Elixir specifically!).

I chose Clojure, fortunately, and – despite some minor hiccups – the combination of REPL-driven programming, support for a rich set of collections, and dynamical approach turned out to be a winning move.

My “Advent of Code 2020”, in review

If you don’t know what the “Advent of Code” is, you can check my previous post on the topic.

Getting in the top 100 requires a kind preparation, and mindset I don’t have.

It’s the kind of skill you train when you routinely compete in online events where the super-fast win and, while I respect it, it’s not the approach I like when coding.

I golf to be deliberate and thoughtful, and coding clean instead, so I took the competition at my own pace, with only one exception: I tried to solve each problem in its own day.

Here is my take-away on the competition itself, which I’ll hopefully follow with an entry about doing “Advent of Code” with Clojure.

Even one problem a day can be too much

Having a regular job and a family, I often started the day’s problem at 10PM, only to finish it at 3AM.

Maybe other people are faster than I am, but for as long as I managed to ride the wave of each daily problem, it’s been taxing.

It’s fun (most of the time)

You wake up and you are being treated with a new, absurd MacGyver-esque problem. You wrap your head around it, find an elegant solution, and give yourself a pat on the back.

Some problems required more time to crack than others, it’s hard work sometimes, but the lasting feeling is rewarding.

This is what happens on a sunny day.

It’s less fun when you remain stuck

Day 23 turned out ugly. I got seriously stuck for two days on a problem that – I realized afterward – was embarrassingly simple.

From the statistics on the website, I know all other participants are moving forward. The leaderboard doesn’t lie: I’m among the few who are finding that problem a challenge.

At some point, I literally felt like the last sucker: I personally questioned my value as a developer. And I was still stuck, so I had to bite the bullet and check Reddit for other people’s solutions.

It turned out the problem was among the simplest ones, and right because the solution was so simple, I had pre-emptively discarded it a priori.

I won’t lie: It did hurt. A LOT.

Finishing the remaining two problems 2 days late didn’t really deliver much joy or a sense of accomplishment, but I tried to put the experience in perspective.

In hindsight, there are some considerations that help to mend my ego a bit, but that can have some sort of general value.

First, I may have been trailing after the group, but it was still a small group of people who don’t just code but also love so much doing so, it solves coding problems while on vacation. So yes, I may even have been “the last”, but I’m definitely not the last sucker.

Second, of the almost 150000 people who started participating only 8% managed to finish it: so even if I was among the slowest 10% of people to solve day 23, I’m in the top 8% who didn’t give up!

The best day was the worst day

More importantly, however, day 23 has been the most enlightening day.

Maybe the other ones may have been more pleasant, and they may have even managed to let me think and improve a bit, but thedebacle of day 23 highlighted a fundamental flaw in my way to approach problems in general, and hopefully managed to get the point across.

It did also show my shortcomings in responding to failures, in general: letting myself feeling down, grudgingly pressing on, losing the sense of perspective are all pains I could have made without.

I picked up the (wonderful) “Thanks for the Feedback: The Science and Art of Receiving Feedback Well” (Douglas Stone, Sheila Heen) again to re-read the chapter about “giving yourself a second score”.

The idea is that failures are not always under our control, but the way we react to them is and, like in this case, so it’s the way I react to my failure to react to failure.

I also refreshed a bit another favorite book of mine: “The Subtle Art of Not Giving a F*ck, A Counterintuitive Approach to Living a Good Life” (Mark Manson).

I didn’t re-read it all, I just picked two sentences:

“Pain is part of the process”


“Failure is the way forward”

And only God knows how much pain and failure I face every time I code…

About the “Advent of Code”

Ok, it’s too late now, but say it’s Christmas: vacations are looming, plenty of time to focus on your project, but what if you don’t have one?

Eric Wastl must have felt this pain: he’s the mind behind the Advent of Code competition, a friendly and geeky website where each day of the advent a new computing problem is unlocked.

The puzzles are bound together with a backstory: maybe you are rescuing Santa and saving Christmas or – like this year – trying to make it to your resort for a vacation.

It doesn’t matter: whatever the backstory could be, you will end up facing new zany situations every day, the solution of which will require (you guessed!) computing and logical skills.

The puzzles have two parts: the first one is usually straightforward, while the second part can be tricky: it may require some specific optimization techniques, some math, or just intuition.

Each problem/day solved, a new part of the ASCII-art calendar is also unveiled: a thing of irresistible geekish and childish beauty!

The narrative is witty and fun, and the problems are engaging (most of the time). Old calendars remain open, so you can still solve all the old challenges.

Even if it’s not the advent anymore, you can still wake up, read a problem, and set yourself the goal of finishing it within the day, maybe competing among friends on a private board (the site has those).

It’s a great project: I’m impressed about how much care has been put into the narrative (even if it’s absurd…) and the details, so whether you are bored or are looking for some new kata to practice a new language, make sure to check it out!