Exercises in Programming Style–Constructivist

You can become a serverless blackbelt. Enrol to my 4-week online workshop Production-Ready Serverless and gain hands-on experience building something from scratch using serverless technologies. At the end of the workshop, you should have a broader view of the challenges you will face as your serverless architecture matures and expands. You should also have a firm grasp on when serverless is a good fit for your system as well as common pitfalls you need to avoid. Sign up now and get 15% discount with the code yanprs15!

NOTE : read the rest of the series, or check out the source code.

If you enjoy read­ing these exer­cises then please buy Crista’s book to sup­port her work.


Fol­low­ing on from the last post, we will look at the Constructivist style today.


Style 20 – Constructivist


  • Every single procedure and function checks the sanity of its arguments and either returns something sensible when the arguments are unreasonable or assigns them reasonable values.
  • All code blocks check for possible errors and escape the block when things go wrong, setting the state to something reasonable.


So far we have neglected error handling on purpose, but not anymore. The next few styles will focus on error handling and illustrate the subtle differences between a few different strategies.

In today’s style, we’ll be mindful of errors but provide sensible fallback values wherever possible so that the program can continue.

Take the extractWords function below as an example, when we’re given invalid input we return a sensible fallback value (in this case the empty array) instead of throwing an exception.

Similarly, if we’re not able to open the input file (perhaps the file doesn’t exist), rather than throwing an exception and terminating the program flow, we return an empty array instead so the program may continue.


We’ll apply the same approach to dealing with anomalies in the removeStopWords function here. If we are unable to read the file with all the stop words for whatever reason, we’ll return the input words in its entirety as if the stop words file was empty.


And the same goes to the other functions.


Finally, we string everything together in a pipeline.



When applied well, the Constructivist style to dealing with errors can have a positive impact to user experience in user experience.

For example, browsers take a very Constructivist approach to rendering HTML pages – it’ll do the best job it can even when images, css, js scripts or other resources the page depends on is missing.

Similarly, Netflix’s Hystrix library has a built-in support for fallback commands so that you can gracefully degrade the user experience during exceptional circumstances.

For example, when you execute a HystrixCommand to load the movie list for a user:

  1. try to load movie list based on user profile (preferences, watch history, etc.)
  2. UserProfile service is non-responsive
  3. fallback to fetch generic movie list for UK
  4. generic movie list for UK not available due to DB outage
  5. fallback to hardcoded movie list from the service’s config file

which is a whole lot better than the whole app crashing whenever any of the dependent services is not responsive! (apparently that’s how things were at Netflix in the early days)


On the other hand, when you give fallback values to the user without notifying them it can also cause a lot of confusion.

For example, if I had a typo in the path to the stop words file, this will be the output of my program:

the – 4507

to – 4243

of – 3728

and – 3658

her – 2225

i – 2070

a – 2012

which would not be what I was expecting.

Fortunately, in this case we do inform the user of the error so that they’re able to reason about the unexpected output from the program.


We could have done better though, by informing the user that “due to error opening the stop words file, all words from the input file will be counted”.

There are many other situations where an explicit error might be better. A few of the JS examples from Gary Bernhardt’s WAT talk springs to mind! 


You can find the source code for this exer­cise here.

Liked this article? Support me on Patreon and get direct help from me via a private Slack channel or 1-2-1 mentoring.
Subscribe to my newsletter

Hi, I’m Yan. I’m an AWS Serverless Hero and I help companies go faster for less by adopting serverless technologies successfully.

Are you struggling with serverless or need guidance on best practices? Do you want someone to review your architecture and help you avoid costly mistakes down the line? Whatever the case, I’m here to help.

Hire me.

Skill up your serverless game with this hands-on workshop.

My 4-week Production-Ready Serverless online workshop is back!

This course takes you through building a production-ready serverless web application from testing, deployment, security, all the way through to observability. The motivation for this course is to give you hands-on experience building something with serverless technologies while giving you a broader view of the challenges you will face as the architecture matures and expands.

We will start at the basics and give you a firm introduction to Lambda and all the relevant concepts and service features (including the latest announcements in 2020). And then gradually ramping up and cover a wide array of topics such as API security, testing strategies, CI/CD, secret management, and operational best practices for monitoring and troubleshooting.

If you enrol now you can also get 15% OFF with the promo code “yanprs15”.

Enrol now and SAVE 15%.

Check out my new podcast Real-World Serverless where I talk with engineers who are building amazing things with serverless technologies and discuss the real-world use cases and challenges they face. If you’re interested in what people are actually doing with serverless and what it’s really like to be working with serverless day-to-day, then this is the podcast for you.

Check out my new course, Learn you some Lambda best practice for great good! In this course, you will learn best practices for working with AWS Lambda in terms of performance, cost, security, scalability, resilience and observability. We will also cover latest features from re:Invent 2019 such as Provisioned Concurrency and Lambda Destinations. Enrol now and start learning!

Check out my video course, Complete Guide to AWS Step Functions. In this course, we’ll cover everything you need to know to use AWS Step Functions service effectively. There is something for everyone from beginners to more advanced users looking for design patterns and best practices. Enrol now and start learning!