What are the best Prolog programming practices and style guidelines?
OK, I know that this is very general question and that there were written some papers on the subject, but I have a feeling that these publications cover very basic material and I'm looking for something more advanced which would improve style and efficency. This is what I have in paper:
Sample subjects covered in these: tail recursion and differential lists, proper use of indexing, proper use of cuts, avoiding asserts and retracts, avoiding CONSing, code formatting guidelines (indentation, if-then-elses etc.), naming conventions, code documenting, arguments order, testing.
What would you add here from your own personal experience with Prolog? Are there any special style guidelines applicable only to CLP programming? Do you know of some common efficiency problems and know how to deal with them?
UPDATE:
Some interesting (but still too basic and too general for me) points are made here: Prolog programming guidelines of Lifeware Team
Just to highlight the whole problem I would like to qoute "Coding guidelines for Prolog" (Covington et al.):
As far as we know, a coherent and reasonably complete set of coding guidelines for Prolog has never been published. Moreover, when we look at the corpus of published Prolog programs, we do not see a de facto standard emerging. The most important reason behind this apparent omission is that the small Prolog community, due to the lack of a comprehensive language standard, is further fragmented into sub-communities centered around individual Prolog systems, none of which has a dominant position.
For designing clean interfaces in Prolog, I recommend reading the Prolog standard, see iso-prolog.
In particular the specific format how built-in predicates are codified which includes a particular style of documentation but also the way how errors are signaled. See 8.1 The format of built-in predicate definitions of ISO/IEC 13211-1:1995. You find definitions in that style online in Cor.2 and the Prolog prologue.
A very good example of a library that follows the ISO error signaling conventions up to the letter (and yet is not standardized) is the implementation of library(clpfd) in SICStus and SWI. While both implementations are fundamentally different in their approach, they use the error conventions to their best advantage.
Back to ISO. This is ISO's format for built-in predicates:
xyz Name/Arity
In the beginning, there may be a short optional informal remark.
xyz1 Description
A declarative description is given which starts very often with the most general goal using descriptive variable names such that they can be referred to later on. Should the predicate's meaning be not declarative at all, it is either stated "is true" or some otherwise unnecessary operationalizing word like "unifies", "assembles" is used. Let me give an example:
8.5.4 copy_term/2
8.5.4.1 Description
copy_term(Term_1, Term_2) is true iff Term_2 unifies with a term T which is a renamed copy (7.1.6.2) of Term_1 .
So this unifies is a big red warning sign: Don't ever think this predicate is a relation, it can only be understood procedurally. And even more so it (implicitly) states that the definition is steadfast in the second argument.
Another example: sort/2 . Is this now a relation or not?
8.4.3 sort/2
8.4.3.1 Description
sort(List, Sorted) is true iff Sorted unifies with the sorted list of List (7.1.6.5).
So, again, no relation. Surprised? Look at 8.4.3.4 Examples:
8.4.3.4 Examples
...
sort([X, 1], [1, 1]).
Succeeds, unifying X with 1.
sort([1, 1], [1, 1]).
Fails.
If necessary, a separate procedural description is added, starting with "Procedurally,". It again does not cover any errors at all. This is one of the big advantages of the standard descriptions: Errors are all separated from "doing", which helps a programmer (= user of the built-in) catching errors more systematically. To be fair, it slightly increases the burden of the implementor who wants to optimize by hand and on a case-to-case basis. Such optimized code is often prone to subtle errors anyway.
xyz2 Template and modes
Here, a comprehensive, one or two line specification of the arguments' modes and types is given. The notation is very similar to other notations which finds its origin in the 1978 DECsystem-10 mode declarations.
8.5.2.2 Template and modes
arg(+integer, +compound_term, ?term)
There is, however, a big difference between ISO's approach and Covington et al.'s guideline which is of informal nature only and states how a programmer should use a predicate. ISO's approach describes how the built-in will behave - in particular which errors should be expected. (There are 4 errors following from above plus one extra error that cannot be seen from above spec, see below).
xyz3 Errors
All error conditions are given, each in its own subclause numbered alphabetically. The codex in 7.12 Errors :
When more than one error condition is satisfied, the error that is reported by the Prolog processor is implementation dependent.
That means, that each error condition must state all preconditions where it applies. All of them. The error conditions are not read like an if-then-elsif-then...
It also means that the codifier has to put extra effort for finding good error conditions. This is all to the advantage of the actual user-programmer but certainly a bit of a pain for the codifier and implementor.
Many error conditions directly follow from the spec given in xyz2 according to the NOTES in 8.1.3 Errors and according to 7.12.2 Error classification (summary). For the built-in predicate arg/3 , errors a, b, c, d follow from the spec. Only error e does not follow.
8.5.2.3 Errors
a) N is a variable
— instantiation_error .
b) Term is a variable
— instantiation_error .
c) N is neither a variable nor an integer
— type_error(integer, N) .
d) Term is neither a variable nor a compound term
— type_error(compound, Term) .
e) N is an integer less than zero
— domain_error(not_less_than_zero, N) .
xyz4 Examples
(Optional).
xyz5 Bootstrapped built-in predicates
(Optional). Defines other predicates that are so similar, they can be "bootstrapped".
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