SOLID Principles to First Principles

Posted on 2018-9-14

Yet another article on SOLID. Why? Most articles focus on making these abstract principles more concrete. I'm going in the other direction. Hopefully hitting on things you could call "first principles". This is not an introduction.

In this article I will use subjective, loosely-defined, abstract terminology. That's the point. I aim to be very brief. I hope you revisit this article as your definitions evolve and gather new insights. I'll try to update it as mine do too.

First Abstraction

The first iteration takes the "known" definitions of the SOLID principles and produces short, analogous, higher-level statements.

SRP – Single Responsibility Principle

Your thing should do only one thing.

I can't think of many things at the same time. It's hard to use or change something that I don't understand.

OCP – Open/Closed Principle

It should be easy to do more, and hard to do less.

Suddenly doing less, breaks the contract I have with others. Inevitably people want me to do more, it would be nice if that were easy.

LSP – Liskov Substitution Principle

A special type of thing, is still a thing.

If you say you can do one thing, but also do other things. Those other things shouldn't interfere with you fulfilling your first duty.

ISP – Interface Segregation Principle

The less I know about you, the better.

There might be many aspects to you. But give me a finger, and I might take your whole hand.

DIP – Dependency Inversion Principle

I don't depend on you, I depend on something like you.

I don't want the police officer's home phone number, I want to call 911 and get help!

Second Abstraction

The second iteration aggregates and abstracts the ideas some more.

SRP + ISP + DIP

I can do more with less

LSP + ISP + DIP

A deal is a deal! Read the contract!

SRP + OCP + LSP + ISP + DIP

I want to be free to change.

and/or

I'm selfish. You're a black box to me.

Third Abstraction

Even more abstract...

Specify, but keep it short.

and

Build on top of things that don't change.

Finally...

After four iterations, my efforts produce this "first principle".

Connect the pieces using a minimal set of well-defined, and lasting support structures.

Final thoughts

Perhaps it was inevitable that my "first principle" ended up the way it did. My research on modularity brought me to the same conclusion, and I have been looking at the world through those glasses ever since. This idea is ubiquitous. You can see it in Rich Hickey's "Simple Made Easy" talk. Where he puts it as:

"Architectural agility dominates!"

Software design principles are often contradictory. There is tension between wanting "fewer components" and "smaller components". There are no truths, only trade-offs. This distilled principle is no different. Different interpretations of it will lead to contradictions. What it has going for itself is that it is, itself, a single principle.

*Updated wording of first principle based on feedback on HN.