Why must facts be grouped together in Prolog?

Question

Say I am listing facts:

letter(a).
letter(b).
letter(c).
...
letter(z).
vowel(a).
consonant(b).
consonant(c).
consonant(d).
vowel(e).
consonant(f).
...
consonant(z).

If I declare the rules in 'alphabetical' order, I get the following warnings in the console:

Warning: /Users/…/prolog-example.pl:31:
  Clauses of vowel/1 are not together in the source-file
Warning: /Users/…/prolog-example.pl:32:
  Clauses of consonant/1 are not together in the source-file
Warning: /Users/…/prolog-example.pl:35:
  Clauses of vowel/1 are not together in the source-file
Warning: /Users/…/prolog-example.pl:36:
  Clauses of consonant/1 are not together in the source-file
Warning: /Users/…/prolog-example.pl:51:
  Clauses of vowel/1 are not together in the source-file

But if I do the following:

letter(a).
letter(b).
letter(c).
...
letter(z).
consonant(b).
consonant(c).
consonant(d).
...
consonant(z).
vowel(a).
vowel(e).
vowel(i).
vowel(o).
vowel(u).
vowel(y).

I dont get the warnings. Are the warnings just warnings or are they actual errors?

You have two vowel(a). facts. – JoErNanO Oct 10 '14 at 00:00 — JoErNanO, Oct 10 '14 at 00:00

Paulo Moura · Accepted Answer · 2014-10-10T01:11:42.957

When a predicate definition is discontiguous, the predicate should be declared as such using the standard discontiguous/1 directive before its clauses. In your case:

:- discontiguous([
    letter/1,
    vowel/,
    consonant/1
]).

If you have discontiguous predicates without corresponding discontiguous/1 directives, the consequences depend on the used Prolog system. For example, SWI-Prolog and YAP will print warnings but accept all clauses. GNU Prolog will ignore clauses. ECLiPSe will report a compilation error. In the case the Prolog system doesn't throw an error, a warning is usually still printed as a predicate may be detected as discontiguous due to e.g. a simple typo in a clause head.

Patrick J. S. · Answer 2 · 2014-10-10T01:31:57.337

2

They are just warnings on some systems. This is to prevent you from accidentally adding clauses to a predicate when you wanted to write a new one. You can get rid of those in SWI (the messages look like the ones you get from SWI, and I haven't used other dialects too much).

You can either use style_check/1, or the discontiugous directive.

:- discontiguous vowel/1,consonant/1,letter/1.
% alternative:
:- style_check(-discontiguous).

edited Oct 10 '14 at 01:31

answered Oct 10 '14 at 00:55

Patrick J. S.

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They are "just warnings" in **some** Prolog systems. – Paulo Moura Oct 10 '14 at 01:12
@PauloMoura: thanks, i only have SWI now, and it's a long time since i used anything else, edited it in. – Patrick J. S. Oct 10 '14 at 01:32

score 2 · Answer 3 · answered Oct 10 '14 at 09:09

The standard ISO/IEC 13211-1:1995 reads on this:

7.4.3 Clauses

...

All the clauses for a user-defined procedure P shall be
consecutive read-terms of a single Prolog text unless there
is a directive discontiguous(UP) ~~directive~~ indicating P in that Prolog text.

So the standards demands (»shall«) that all clauses are consecutive by default. A programmer or program that now relies on the clauses being added nevertheless does not rely on standard behavior.

Why must facts be grouped together in Prolog?

3 Answers3

7.4.3 Clauses