There are many things we talk about when assessing code and trying to construct an analysis of its quality metrics. We talk about modularity in systems at the large and small levels, we talk about coupling, we talk about performance metrics, we talk about “Best Practices.” We talk about style, statefulness, paradigm, elegance, smells, and so many different things.

However, we rarely talk about Reasoning. It’s rarely something mentioned in interviews, code reviews, architectures, patterns, classes, or anywhere outside of perhaps formalist maths training.

I would like to speak to this concept because I think it’s something we oft touch on in everything we do as programmers. Though we frequently interact with the concept, reasonability is rarely given a label or defined in any sincere sense. Every time we look at code and automatically find ourselves struggling to make out the actual semantics, behaviour, and precise functionality of a piece of code we’re touching on the concept of Reasonability. Every time you pick up a piece of code you’ve never seen before and can find all of its actual behaviours writ explicitly within that single piece, we are touching on the concept of Reasonability.

Reasoning is something we do every day when we have to look at some code and decide what it will do, and what it should do. Every time we are writing a piece of code and trying to make its behaviour as clear as possible within its own scope, we are focusing on making that code easy to reason about.

Reasonable code is what you get when you needn’t look across multiple files, you needn’t look across a large section, or ponder with difficulty over many logical decisions within the code itself.

Let’s step back for a moment though.

What is Reasoning to begin with? It’s so rarely spoken about that I think it’s worth giving a clear definition of, such that we may begin to utilize it as a skill explicitly - as opposed to by accident or undefined habit.

I would give this definition for Reasoning where it relates to logical analysis:

Reasoning: The act of mentally deducing by parts, the complete behaviour of a logical system, such that given any input you can accurately assess all outputs.

By this definition, I would lay claim that any section of code - from a single block to an entire system is a Logical System. As such, we constantly find ourselves reasoning through these systems - bit by bit - all day every day as programmers. I believe the reason this is so infrequently mentioned or spoken of is because the act is not only an obvious necessity to complete any given task within the scope of being a programmer, but more importantly it generates no visible outputs. With the entire act being invisible both to the participant and any observer, it has resulted in making one of the key activities we as programmers all practice, one which we have nearly no formal or conceptual analysis of.

However, with the above simple definition of Reasoning, I think we can begin to construct an entirely new approach to assessing code quality. One that takes into account something we do far more frequently than nearly any other activity where it regards interacting with code. The best part? While it’s somewhat subjective, while it’s something we cannot assess computationally because it depends on our individual anecdotal experiences interacting with code, it’s the easiest metric to test code against of any. You need no tools but your own mind and a text editor.

To begin to assess the reasonability of code, all you need to do is take a piece of code, look through it and measure how long it takes you to reason about every facet of it. Practice doing this with your code, with code written by others, practice doing it in the small and in the large. If you are attentive during this process to the results you’ll begin to notice patterns. These patterns will stand out in regards to when code is very quickly reasoned through, and when code takes a long time to reason through, and when you find code that you simply cannot completely reason through.

These patterns are great to learn and recognize! These are not design patterns! These are characteristics of code you will notice, when present affect how easily you can reason through it.

Here now we get to the nitty gritty. I would like to list the characteristics I have personally found makes code more Reasonable, such that I can complete the act of Reasoning as defined above on the code with greater ease, speed, and accuracy than code where these characteristics are not present.

Small scope

When a code section touches very few things outside of its own scope, up to and including nothing outside of its own scope, this makes reasoning through the sections entire behaviour far quicker and the understanding I have of the code vastly more accurate.

Dictating instead of deciding

When a code section has very few logical decisions, but rather is actively dictating what it will do without querying data to decide one way or another, the code is often far more reasonable.

Explicit data use

When a code section is very explicit and specific in what data it is taking as input, what it is doing to said data, and what data it is outputting, my ability to reason through the completeness of its behaviour is often far more accurate than when data is implicitly touched. For instance, if the section of code makes a call to another section which implicitly changes data, it takes more time for me to reason through the full systemic effect of the original section of code.

Short stacks

When a code section calls into methods which call into methods which call into methods until you have a very high chain of behaviours being executed, the code becomes very difficult to reason about. This goes back to the point about small scope - each section of code called by a single section is yet another piece of the scope you must understand to fully reason about the original section of code.

Explicit data ownership

When a code section makes clear that it owns the data it is interacting with so that its data is completely inaccessible to other sections, it becomes natural to reason through any changes you might make as you can be confident you need not look at any other code section with concern for how you may be altering its behaviours. An example is using data that is only local or data that is values rather than references are two simple ways to ensure the data you are interacting with will not affect other code sections.

Explicit outputs

When a code section uses its data in an explicit manner and makes explicit what data it owns and doesn’t own, it will often cause all outputs of the section to be explicit as well. This is extremely helpful in reasoning about what the code sections consumers are relying on and expecting.

The value in focusing on making your code reasonable is quite significant. You will end up with code that is easy for new developers to pick up and start editing. The code will be easier to integrate with other code, easier to migrate to new technologies, and easier to recognize correctness or incorrectness.

No one language will cause these characteristics, and no one framework, library, or technology will force them to occur. No matter what you are writing your code with, you can effect these characteristics into it and practice attentiveness to how well you and others can reason about your code, how well you and others can reason about your system. As a result it becomes easy to reason about how you will implement new changes - from bug fixes to enhancements, your ability to see where and how you will effect these changes will improve drastically.

The characteristics I listed above are in no way comprehensive - or even necessarily always accurate! The important point is to pay attention to the process of reasoning about your code. To write it, and then attempt to sit and look the horse in the mouth- and be self-critical of how well you can construct a comprehensive and accurate mental model for its entire set of behaviours. The more you practice this, the more you will begin to recognize which characteristics make it quicker and easier to accomplish for you and others.