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From: alain@kulcs.uucp (Alain Callebaut)
Newsgroups: comp.lang.prolog
Subject: Prolog/C Term passing
Message-ID: <1790@kulcs.kulcs.uucp>
Date: 17 Nov 89 12:01:11 GMT
Reply-To: alain@kulcs.UUCP ()
Organization: Katholieke Universiteit Leuven, Dept. Computer Science
Lines: 83


In article <2716@munnari.oz.au>, ok@mudla.cs.mu.OZ.AU (Richard O'Keefe) 
talks about passing Prolog terms between Prolog and C.  He works out an 
example in one system, and briefly refers to a feature of some other 
Prolog systems :
> Some Prolog systems expose their data structures directly to C code in
> order to make it easy for you to build Prolog terms in C.  ...
He argues that the system he treats in more detail, doesn't do that, 
leaving the implementors of that system free to improve their data 
structures any time they get a bright idea.  Instead, they provide a 
library package that can be used to access and construct terms. 

To illustrate that a hybrid program (mix of Prolog and C) does not have 
to be as complicated as it is with his system, let me describe how 
this all works in BIM_Prolog. 

The same example (a predicate, mainly written in C, that converts a 
floating point number to a list of characters representing its printed 
form), programmed in BIM_Prolog (and C) looks as follows. 


:- extern_predicate( float_to_chars(integer:r,real:i,bpterm) ) .
{ The arguments of float_to_chars/3 are :
   arg1 : integer return parameter indicating success or failure
   arg2 : real input parameter - the floating point number
   arg3 : a Prolog term - the list of characters }

float_to_chars( _Float , _Chars ) :-
   float_to_chars( 1 , _Float , _Chars ) . { 1 for success, otherwise fail }


#include <BIMprolog.h>	/* The BIM_Prolog external interface definitions */

int float_to_chars( flt , chars )
float flt;
Term chars;
{
   char buffer[28], c, *p;
   Atom nil;
   Term term_lst, term, term_char;

   nil = BIM_Prolog_string_to_atom( "nil" );
   sprintf( buffer , "%.17e" , flt );
   term_lst = BIM_Prolog_new_term();	/* Create a new term */
   term = term_lst;
   p = buffer;
   while ( c = *p++ )	/* Treat each char, further instantiating the term ... */
   {
      BIM_Prolog_unify_term_value( term , BPT_LIST );	/* to a list */
      BIM_Prolog_get_term_arg( term , 1 , &term_char );	/* with head */
      BIM_Prolog_unify_term_value( term_char , BPT_INTEGER , c ); /* the char */
      BIM_Prolog_get_term_arg( term , 2 , &term );	/* tail free by now */
   }
   BIM_Prolog_unify_term_value( term , BPT_ATOM , nil );/* tail is nil */

   return BIM_Prolog_unify_terms(chars,term_lst);
		/* Unify the new term with the argument term */

} /* float_to_chars */


As can be seen, with a cleanly designed, and highly functional external 
interface, it is possible to write clear hybrid programs.  As well the 
Prolog part as the C part is much simpler than in the other system. 
The Prolog predicate simply calls the external routine, and relies on 
unification of the return value to implement the normal Prolog unification 
of the result.  It is not necessary to call two extra predicates for 
converting and deallocating the externally constructed term, nor is it 
necessary to perform an explicit unification of the constructed term 
with the predicate's argument. 
The C routine can construct any Prolog term it wants, in a conventional 
procedural way.  It is not necessary to calculate how much space you 
will need and to allocate it in advance.  Just start creating a term and 
furhter instantiate it. 

I won't reply on the other topics that were illustrated (I don't have 
that much time).  Moreover, to my feeling, it is a violation of Netiquette 
to use the net for customer support. 

Alain Callebaut
K. U. Leuven
Dept. of Computer Science
Belgium