Continuation Passing Style

I have been doing some work with a library that creates guards around various web endpoints. They have different kinds of authentication and authorization rules, but are all written in a continuation passing style. The idea of the continuation passing style is that you give some construct a function to ‘continue’ execution with once it does something. If you’ve ever written an event handler, that was a continuation. The usages all look somewhat like

secureActionAsync(parseAs[ModelType]) { (userInfo, model) => user code goes here }

There was some discussion around whether we wanted to do it like that or with a more traditional control flow like

secureActionAsync(parseAs[ModelType]) match {
    case Allowed(userInfo, model) => user code goes here then common post action code
    case Unallowed => common error handling code

The obvious issue with the second code sample is the need to call the error handling code and post action code by hand. This creates an opportunity to fail to do so or to do so incorrectly. The extra routine calls also distract from the specific user code that is the point of the method.  

There were some additional concerns about the testability and debuggability of the code in the continuation passing style. The debugging side does have some complexity but it isn’t any more difficult to work through than a normal map call which is already common in the codebase. The testability aspect is somewhat more complex though. The method can be overwritten with a version that always calls the action, but it still needs to do the common post action code. The common post action code may or may not need to be mocked. If it doesn’t need to be mocked this solution works great. If the post action code needs to be mocked then putting together a method that will set up the mocks can simplify that issue.

This usage of a continuation helps collect the cross cutting concern and keeps it all on one place. You could wrap up this concern other ways, notably with something like like AspectJ. The issue with doing it with something like AspectJ is that it is much less accessible than the continuation passing style. AspectJ for this problem is like using a bazooka on a fly, it can solve it but the level of complexity introduced isn’t worth it.

Being a Wizard

A somewhat obscure question got asked in a chat channel at work that I knew the answer to, which helped out some other engineer. The question wasn’t anything that abnormal – it was about a weird error message coming from an internal library. Searching through the library’s code wasn’t immediately helpful since the unique part of the error message didn’t appear in the code. The reason I knew the answer wasn’t because it was easy, but because I had spent an hour investigating it the day before.

Sometimes when you see someone have an apparently impressive insight, that doesn’t necessarily mean they are better than you, they may just have had an experience which makes the answer obvious to them. This applies to all sorts of other technical activities. During the Hackathon I did a similar thing. One of the other devs on the team was integrating the portion of the code I was working on and having trouble. It was immediately obvious to me why, because I had put in the time earlier to figure it out the hard way. Your mind is a powerful pattern matching system. It immediately recognizes this:


happycatOr thisftc


If you think back to when you first started learning calculus, the terminology and symbols of it were complicated and foreign, but after a while you gained a certain familiarity with them and after a while they became second nature.

You may go to work and make some business web app in one particular technology stack, but there are all sorts of concepts that go with it that aren’t the business or the tech stack. You’re synthesizing things like design patterns, test driven development, RESTful web services, algorithms, or just the HTTP stack and everything that goes with that. These are all the transferable skills that can help you “cast a spell” and jump past a problem.

When I sat down to learn Scala, it wasn’t that big a task since most of the language features had equivalents I was familiar with in other languages. That let me skip forward to the nuances of those implementations and the few language features I was less familiar with. Getting experience with those ideas in the abstract let me appear as a wizard going forward since I jumped ahead on the learning curve and look the wizard. Some of the common feelings of impostor syndrome are the worry to be found out like another wizard.


Book Chat: Zero Bugs and Program Faster

Zero Bugs and Program Faster by Kate Thompson is a book that’s hard to describe. None of it is a really novel way of looking at creating software but it’s all of those things that you would expect to describe when you think about how to do programming well. It’s a breezy and fun read that is divided into enough small sections that you can read it in however much time you have available.

The book is structured in two parts. The first part is a series of short vignettes about programming. Some of which are more direct, like the chapter on ACID; some are more abstract, like the chapter entitled “The Many Sides of the Elephant.” I appreciated the dual chapters of “Do It Now” and “Do It Later” that are about how you can’t always do it now but you shouldn’t always defer it either. None of it was a mind shattering revelation but it was all solid advice about programming.

The second part is extracts from various programs to demonstrate a lot of different ideas. The code samples in the second part were generally significantly older, mostly in assembly or C. The low level nature of the examples made it more difficult for me to appreciate. Seeing Altair assembly from the 70s that’s notable for being clever and concise won’t help me build a better web service today.

If you are the sort of person who is reading lots of programming books, you will appreciate the book, however you may not get much from it. If you aren’t the kind of person who reads lots of programming books some of the more oblique points may be obscured. I don’t have anything bad to say about it but don’t know who I would recommend the book to.

Planning Anecdote

The team I joined at my new job had been doing about 50 points per sprint with 4 devs and the manager when I joined. There were some additional staff changes, but when everything smoothed out again we ended up doing about 80 points per sprint with 4 devs and the manager. The new team did not seem better than the old team, but during the turbulence of the staff changes we changed how we prepared work to be done and managed to get better organized, which enables us to do more.

The change was a different way of collecting the relevant dependencies in the tickets. For example, instead of giving a vague reference to an endpoint that needed to be called, we made sure to give an explicit url, plus links to the swagger definition for the endpoint or a listing of explicit endpoints to be created and the related models. We also linked together all of the tickets that were related to make it easier to juggle which services were ready to release or where we had dependencies between tickets. It doesn’t seem like a significant change, but it resolved a bunch of the little dependencies where something wasn’t clear and you needed to hunt down an answer.

The other half of the anecdote is that due to scheduling conflicts we missed out on time to do some of the preparation for a sprint and ended up going ahead without as much preparation; we dropped back down to about 50 points completed. Our manager didn’t seem to have gotten any credit for us having gone from 50 to 80 but certainly caught flack for the sprint where we went back to 50.

Maybe this anecdote will inspire you to experiment with that little change that didn’t seem to be worth trying since it wasn’t clear how it would really help. Lining up a little change is easier than doing something big.