Boolean Variable Names

by Jeff Russo | June 30, 2020

Choosing good names for variables is an important art for software engineers to master. Variable names are documentation and can remove complexity from code. Using a bad name for a variable leads to confusion, bugs, and wasted engineer time. In this blog post I will discuss what makes a boolean variable name good or bad.

Good Boolean Names

Boolean names should be negatable and in the positive form. They should be clear and concise just as all variable names should be. As the scope of the variable increases, its level of description should increase as well.

The Elements of Style 1 is a concise guide to writing English. Some rules can be used as a guide for writing code as well. For example, rule 11, “Put statements in positive form,” can be applied to naming boolean variables. In code, there is one additional requirement: the statement must be positive as well. ‘enabled’ is both in the positive form and a positive statement. ‘disabled’ is in the positive form but is a negative statement. ‘notEnabled’ is both in the negative form and a negative statement.

In English, it is best to use enabled or disabled, and to avoid “not enabled”; they are more concise and stay in the positive form. The extra requirement of using a positive statement for boolean names is because the negations of boolean variables have meaning as well; in writing, “not disabled” is a double negative and is discouraged. In code ‘!disabled’ is also a double negative, and should be avoided.

A boolean name should give the reader more clarity as its scope expands. A boolean variable that exists for the scope of an if statement may have a short name (‘ok’). If this variable was in scope throughout the function, it would become meaningless.

Bad Boolean Names

As with many things in software engineering, it is easier to find mistakes than to do something correctly the first time. Here are some naming conventions to avoid:

  1. Names that are already a negation. ‘featureTurnedOff’ and ‘notProd’ may make sense in one use case, but they become confusing in a larger context. The following code is both hard to read and ambiguous (what if there is a third environment).

    
    confA.useProd := !confB.notProd
    
    
  2. Names that are not negatable. A name such as ‘sheets’ has ambiguous meaning. It probably has something to do with a spreadsheet, but it is the writer’s responsibility to prevent the reader from needing to do an investigation.

  3. Names that are in negative form. These typically appear when the default use is the negative form. For example, ‘disabled’ for a service typically disabled. Keep all the names in positive form to avoid a future headache explaining what the billing system does when it’s “not disabled”. Is it now enabled, or is it just not disabled?

  4. Overly general names. As discussed earlier, this depends on the scope of the name. Avoid using a name such as ‘on’. It leaves questions in the reader’s mind such as “Which thing is on?”. The exception to this is typical boolean names used across code bases; Go’s ‘ok’ is an example of this.

  5. Negatable adjectives without context. ‘bool customSize’ is clear, but without the ‘bool’ context, the type is unclear. Ambiguity can be removed by prepending ‘is’ or ‘has’ (‘isCustomSize’).

Conclusion

This blog post discussed a goal to aim at (good names) and several things to avoid (bad names). Combined, these will hopefully increase the readability of the software you write!

1 https://tinyurl.com/y3uxhzxc