prologportabilitydata-representationiso-prolog

The smallest `max_arity` of compound terms in ISO Prolog programs

Prolog systems aiming at conformity do not have to support compound terms with arbitrarily large arities. The Prolog flag max_arity reflects this.

According to ISO/IEC 13211-1:1995:

7.11.2.3 Flag: max_arity

Possible values: The default value only

Default value: implementation defined

Changeable: No

Description: The maximum arity allowed for any compound term, or unbounded when the processor has no limit for the number of arguments for a compound term.

So Prolog systems may or may not impose an upper limit on max_arity ... but what about the lower limit? Is, say, 5 okay?

NO! ISO/IEC 13211-1:1995/Cor.2:2012, Technical Corrigendum 2, defines call/2..8 as:

8.15.4 call/2..8
...
NOTE — A standard-conforming processor may implement call/N in one of the following ways because error condition d is implementation dependent (3.91).

  1. Implement only the seven built-in predicates call/2 up to call/8.
  2. Implement call/2..N up to any N that is within 8..max_arity (7.11.2.3). Produce existence errors for larger arities below max_arity.
  3. Implement call/9 and above only for certain execution modes.

All of these ways only imply Max_arity >= 8—but nothing more.

So my question has (at least) two sides:

  1. Prolog User:
    "What is the maximum arity I may use if I want to avoid vendor lock-in?"

  2. Prolog Implementor:
    "What is the smallest Max_arity I must support if I aim1 at ISO-Prolog conformity?"

Right now, I'm quite sure the answer is this:

Yes, Max_arity = 8 is okay.

But is it in fact so? Are there clues I am missing?

Footnotes:
1) I do.

Solution

  • There is a gap somehow. max_arity only covers the compound size. But there is no flag for predicate size. The two things might vary as SWI-Prolog shows. But SWI-Prolog names the limit wrongly max_arity, although its not its max_arity flag:

    /* SWI-Prolog 8.3.19 */

?- functor(_,f,10000).
true.

?- functor(F,f,10000), assertz(F).
ERROR: Cannot represent due to `max_arity' (limit is 1024, request = 10000)

?- current_prolog_flag(max_arity, X).
X = unbounded.

    The predicate size limit might be more relevant to call/n than the compound size. The ISO core standard definition is a little unsatisfactory.

    But its not thoroughly checked, but a fix on SWI-Prologs side is underway:

    ?- functor(F,f,10000), assertz(test:-F).
F = f(_ ...)

?- functor(F,f,10000), call(F,a).
%%% crash

    Edit 25.02.2021:
    The glitch in the error message is only explained when you look at the C-Code of SWI-Prolog, which has a C Constant MAXARITY with the meaning of predicate size. This Constant and the Error Message possibly predates the ISO core standard, which introduced another constant.