Prolog Coding Horror

I seemed to hear the whispered cry, "The horror! The horror!"
(Joseph Conrad, Heart of Darkness)

Why are you here?

As a Prolog programmer, you likely have a rebellious streak in you. In many cases, this is what it takes to guide you away from how an entire industry currently tries to solve problems. To focus on what lies beyond.

The point of this page is to show you where following this streak is likely not a good idea because the cost is high, and there is no benefit to it.

A small number of rules suffices to write great Prolog code. Breaking them will result in programs that are defective in one or more ways.

Video:

The horror: Losing solutions

A Prolog program that terminates and is acceptably efficient can be defective in two major ways:

It reports wrong answers.
It fails to report intended solutions.

Which of these cases is worse? Think about this!

Suppose a program is defective only in the first way. Is there anything you can do to still obtain only correct results? Then, suppose a program is defective only in the second way. What are your options to somehow still obtain all solutions that were intended?

The primary means to make your programs defective in the second way is to use impure and non-monotonic language constructs. Examples of this are !/0, (->)/2 and var/1. A declarative way out is to use clean data structures, constraints like dif/2, and meta-predicates like if_/3.

The horror: Global state

As a beginner, you will be tempted to modify the global database in Prolog. This introduces implicit dependencies within your programs. By "implicit", I mean that there is nothing in your program that enforces these dependencies. For example, if you use such predicates in a different order than intended, they may unexpectedly fail or yield strange results.

The primary means to make your programs defective in this way is to use predicates like assertz/1 and retract/1. A declarative way out is to use predicate arguments or semicontext notation to thread the state through.

The horror: Impure output

As a beginner, you will sometimes be tempted to print answers on the system terminal instead of letting the toplevel report them. For example, your programs may contain code like this:

solve :-
        solution(S),
        format("the solution is: ~q\n", [S]).

A major drawback of this approach is that you cannot easily reason about such output, since it only occurs on the system terminal and is not available as a Prolog term within your program. Therefore, you will not write test cases for such output, increasing the likelihood of introducing changes that break such predicates. Another severe shortcoming is that this prevents you to use the code as a true relation.

To benefit from the full generality of relations, describe a solution with Prolog code, and let the toplevel do the printing:

solution(S) :-
        constraint_1(S),
        etc.

Sometimes, you may want special formatting. In such case, you can still describe the output in a pure way, using for example the nonterminal format_//2. This makes test cases easy to write.

The horror: Low-level language constructs

Some Prolog programmers may see little reason to use more recent language constructs. For example, CLP(FD) constraints have only been widely available for about 20 years, which is a comparatively recent development for Prolog. If you think that low-level constructs have served you well, why bother learning newer material? The fact that millions of students were not well served by lower-level constructs need not concern you.

Unfortunately, sticking to low-level constructs comes at a high price: It makes the language harder to teach, harder to learn and harder to understand than necessary. It requires students to learn declarative and operational semantics essentially at the same time, which is too much at once in almost all cases.

The primary means to make Prolog harder to teach than necessary is to introduce beginners to low-level predicates for arithmetic like (is)/2, (=:=)/2 and (>)/2. A declarative way out is to teach constraints instead. See declarative integer arithmetic.

Horror factorial

To see some of these defects exemplified, behold the horror factorial:

horror_factorial(0, 1) :- !.
horror_factorial(N, F) :-
        N > 0,
        N1 is N - 1,
        horror_factorial(N1, F1),
        F is N*F1.

Observe the horror of losing solutions when posting the most general query:

?- horror_factorial(N, F).
   N = 0, F = 1.

The version without !/0 is almost as horrendous:

horror_factorial(0, 1).
horror_factorial(N, F) :-
        N > 0,
        N1 is N - 1,
        horror_factorial(N1, F1),
        F is N*F1.

The horror of low-level language constructs prevails:

?- horror_factorial(N, F).
   N = 0, F = 1
;  caught: error(instantiation_error,'(is)'/2)

If you accept this, you are

limited by using outdated language constructs.
mistaking relations for functions.
not caring about the most general query.
preventing declarative debugging by using impure constructs.

A way out: Purity

To stop the horror, stay in the pure monotonic subset of Prolog.

Start small. For example, instead of low-level integer arithmetic, use a more declarative alternative:

horror_factorial(0, 1) :- !.
horror_factorial(N, F) :-
        N #> 0,
        N1 #= N - 1,
        horror_factorial(N1, F1),
        F #= N*F1.

Still better than nothing. Then, remove the !/0:

n_factorial(0, 1).
n_factorial(N, F) :-
        N #> 0,
        N1 #= N - 1,
        n_factorial(N1, F1),
        F #= N*F1.

This version also works for the most general query:

?- n_factorial(N, F).
   N = 0, F = 1
;  N = 1, F = 1
;  N = 2, F = 2
;  N = 3, F = 6
;  ... .

That's quite good: A few simple changes have led to a quite general logic program.

Conclusion

In summary, I recommend you rebel where it makes sense, and only there.

It is ill-directed rebellion to cling to outdated features, for life is not lived backwards nor tarries in yesterday.

Use declarative constructs in your Prolog programs to make them more general while retaining acceptable performance.

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