12.4.17.3 Database

int PL_assert(term_t t, module_t m, int flags)

Provides direct access to asserta/1 and assertz/1 by asserting t into the database in the module m. Defined flags are:

PL_ASSERTZ

Add the new clause as last. Calls assertz/1. This macros is defined as 0 and thus the default.

PL_ASSERTA

Add the new clause as first. Calls asserta/1.

PL_CREATE_THREAD_LOCAL

If the predicate is not defined, create it as thread-local. See thread_local/1.

PL_CREATE_INCREMENTAL

If the predicate is not defined, create it as incremental see table/1 and section 7.7.

On success this function returns TRUE. On failure FALSE is returned and an exception is left in the environment that describes the reason of failure. See PL_exception().

This predicate bypasses creating a Prolog callback environment and is faster than setting up a call to assertz/1. It may be used together with PL_chars_to_term(), but the typical use case will create a number of clauses for the same predicate. The fastest way to achieve this is by creating a term that represents the invariable structure of the desired clauses using variables for the variable sub terms. Now we can loop over the data, binding the variables, asserting the term and undoing the bindings. Below is an example loading words from a file that contains a word per line.

#include <SWI-Prolog.h>
#include <stdio.h>
#include <string.h>

#define MAXWLEN 256

static foreign_t
load_words(term_t name)
{ char *fn;

  if ( PL_get_file_name(name, &fn, PL_FILE_READ) )
  { FILE *fd = fopen(fn, "r");
    char word[MAXWLEN];
    module_t m = PL_new_module(PL_new_atom("words"));
    term_t cl = PL_new_term_ref();
    term_t w  = PL_new_term_ref();
    fid_t fid;

    if ( !PL_unify_term(cl, PL_FUNCTOR_CHARS, "word", 1, PL_TERM, w) )
      return FALSE;

    if ( (fid = PL_open_foreign_frame()) )
    { while(fgets(word, sizeof word, fd))
      { size_t len;

        if ( (len=strlen(word)) )
        { word[len-1] = '\0';
          if ( !PL_unify_chars(w, PL_ATOM|REP_MB, (size_t)-1, word) ||
               !PL_assert(cl, m, 0) )
            return FALSE;
          PL_rewind_foreign_frame(fid);
        }
      }

      PL_close_foreign_frame(fid);
    }

    fclose(fd);
    return TRUE;
  }

  return FALSE;
}

install_t
install(void)
{ PL_register_foreign("load_words", 1, load_words, 0);
}