Using boolean variables as flags exposes accidental implementation and pollute the code with Ifs.

TL;DR: Don't use boolean variables, they force you to write Ifs. Create polymorphic states instead.

Problems

• Extensibility

• Comparison in some languages

Solutions

• If Boolean maps to a real-world entity are safe. Otherwise, model as a State to favor Extensibility. This also follows the Open/Closed Principle.

Examples

• Flags

Exceptions

• Real-world true/false rules

Sample Code

wrong

fun processBatch(
    useLogin: Boolean,
    deleteEntries: Boolean,
    beforeToday: Boolean
) {
    ...
}

Right

fun processBatch(
    useLogin: LoginStrategy,
    deleteEntries: DeletePolicy,
    beforeToday: OffsetDateTime
) {
    ...
}

Conclusion

Take extra care when declaring something as a boolean. Flags are difficult to maintain and extend. Learn more about the domain. Try migrating to the state design pattern. Use polymorphism instead of ifs or when (pattern matching).

More Info

• FlagArgument by Martin Fowler

Credits

• Code Smell 07 - Boolean Variables by Maxi Contieri