“That’s wrong!”, interjected a member of the audience.
I was taken aback. I had suggested the following definition of the Single Responsibility Principle.
“A class should do the smallest possible thing; that is, it should have a single responsibility.”
At the time, all I could respond with was: “well, that’s one definition”. In fact, this quote was taken verbatim from Practical Objected-Oriented Design in Ruby by Sandi Metz. The purpose of my talk was to use Sandi’s excellent book to generate discussion about better OO design in Rails applications. Fortunately for me, those present at the Sydney Ruby meetup (or “rorosyd” as it is affectionately known) responded well and all ended well.
However, the experience left me pondering.
Several weeks later, I had the privilege to listen to Kevlin Henney talk about deconstructing the SOLID design principles (pdf) at the YOW! software development conference at UNSW in Sydney.
Kevlin is an Englishman who delights in the proper use of the English language. Very early in his talk he questioned the use of the word principle in relation to SOLID design. Kevlin suggested that the word pattern was more accurate. As he illustrated his point by referring to the Oxford English dictionary, it was hard to mount an argument against his criticism.
Focussing on the so-called Single Responsibility Principle, Kevlin started with the Wikipedia entry. After all, it appears first in a Google search and how could it possibly be wrong? Interestingly, the entry is both potentially misleading and illuminating. It defines SRP thus:
“Every class should have a single responsibility, and that responsibility should be entirely encapsulated by the class. All its services should be narrowly aligned with that responsibility.”
This is not so different to Sandi’s definition. However, the same Wikipedia entry also notes that Robert C. Martin introduced the term, going on to say that:
“Martin described it as being based on the principle of cohesion, as described by Tom DeMarco in his book Structured Analysis and Systems Specification.”
Here is where an exploration of the language behind the description of SRP gets interesting. Kevlin referred to Glenn Vanderburg’s article on cohesion. Glenn relates that he has had most success in explaining the term cohesion in terms of it’s etymology. In short, cohesive things belong together as opposed to those that need an adhesive to glue them together.
Bearing this in mind, let’s read what Robert C. Martin says in 97 Things Every Programmer Should Know:
“One of the most foundational principles of good design is:
Gather together those things that change for the same reason, and separate those things that change for different reasons.
This principle is often known as the single responsibility principle, or SRP. In short, it says that a subsystem, module, class, or even function, should not have more than one reason to change.”
In other words, responsibility is equated to having only one reason to change. As someone else present at my rorosyd talk said in response to the first interjection, “it depends what you mean by responsibility”. Reflecting on the language used, however, leaves me thinking that the term “single responsibility” is not as helpful as the word “cohesion” when discussing good software design.
The whole of Kevlin’s talk was both thought-provoking and entertaining. He concluded by suggesting five alternatives for the SOLID principles that all begin with the letter C. In the place of SRP as the first “pattern” was Cohesion by Usage.
Kevlin’s talk has caused me to reflect on the importance of language in the context of software design.
What have I learned?
It also occurs to me that we need to guard against the increasing tendency in this day and age to only read short chunks of text. TL;DR is no excuse for those striving to be better developers.
In closing, if you’ve read this far, I’ll leave you with a recommendation. If you ever get the chance to see and hear Kevlin Henney talk about software, grab it with both hands.
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