What are some best practices for writing clear and maintainable Prolog code?

2 Follow a coding style

To make your Prolog code more readable and maintainable, you should follow a coding style that defines the formatting, indentation, spacing, and punctuation. This will help you organize your code, avoid errors, and make it easier for others to understand and modify it. Common elements of Prolog coding styles include using lowercase letters for predicates, variables, and constants (except for true and false), underscores to separate words in names, parentheses to group arguments and avoid ambiguity (especially when using operators or infix predicates), a consistent indentation level for clauses and subgoals (such as four spaces or one tab), blank lines to separate different predicates or logical blocks of code, and comments to explain the purpose, logic, or assumptions of your code (but not too many). Single-line comments should be marked with % and multi-line comments should be marked with /* */.

3 Use modular and reusable code

One of the advantages of logic programming is the ability to write modular and reusable code that can be combined and extended. To reach this goal, it’s important to use predicates that are general and declarative, rather than specific and procedural. Additionally, predicates should be deterministic, meaning they have one solution for a given set of arguments, or at least indicate when they are non-deterministic. Furthermore, it’s important to use predicates that are pure, meaning they do not have side effects such as modifying the program or environment. To avoid this, avoid using predicates such as read/1 or write/1 unless they are necessary or clearly marked. Instead, use predicates that return values which can be used by other predicates.

4 Use built-in and library predicates

Prolog provides a rich set of built-in and library predicates that can help you write more concise and efficient code. Instead of reinventing the wheel, you should use these predicates whenever possible, as they can save you time and effort, and avoid errors. For example, instead of writing your own predicate to check if a list is empty, use the built-in predicate var/1, which returns true if its argument is an uninstantiated variable. Or instead of writing your own predicate to sort a list, use the library predicate sort/2, which sorts a list and removes duplicates. To use library predicates, you need to import the corresponding module using the directive :- use_module(Module).

5 Test and debug your code

Finally, testing and debugging Prolog code regularly and systematically is essential for writing clear and maintainable code. Prolog offers various tools and techniques to do this, such as the interactive interpreter (also known as the top-level or console) to run queries and check the results of predicates. The trace/0 predicate can be used to view the execution steps of a query and backtracking points. Additionally, the assert/1 and retract/1 predicates can help experiment with different scenarios and test hypotheses. The print/1 and write/1 predicates can be used to print values and messages to the standard output stream for monitoring program state. Lastly, the debug/0 and nodebug/0 predicates can be used to enable or disable debugging mode. In this mode, commands such as creep, skip, leap, and fail can be used to control execution flow and inspect variable values.