is_Of and has_: Predicate Naming

Prolog is a declarative language, we have the joy of telling the computer what is, and run our programs by allowing the computer to deduce what is true. As programmers we often have to read code for ourselves and figure out what is true. This naming convention can help us do that by making our intentions explicit.


In “The Art of Prolog”, Richard O’Keefe lays out his advice for the order of arguments in predicates. To overly simplify, inbuilt binary predicates such as succ/2 would expect, as it’s primary use-case, the first argument to be the input case, and the second to be the output case: succ(3, N). This is Prolog, so of course it will also work with succ(N, 3).

This is a general convention that experienced Prolog programmers begin to intuit. This is why we can read `father(bob, bill).` as “the father of bob is bill”, same as “the successor (succ) of 3 is N”. In predicate logic we’d read it as “bob father bill”, which isn’t really the most grammatical of English and is rather confusing. The predicate logic reading suggests that "bob is the father of bill". If we're honest, we can't be sure if it's “the father of bob is bill” or “bob’s father is bill”.

So let’s use a naming convention found in some Knowledge representation circles: `is_Of` and `has_`. Applied to `father/2` we’d then have `isFatherOf(bob, bill)` and `hasFather(bill, bob)`. Now we can read them in both the input/output style: “is father of bob is bill” or better in the predicate logic style “bob is father of bill”, either way it means the same thing without ambiguity.

This particular naming convention is particularly useful when writing your own facts in your data/knowledge-base. Code your intention: `is_Of` and `has_`.