12.4.4.7 An example: defining write/1 in C

Figure 6 shows a simplified definition of write/1 to illustrate the described functions. This simplified version does not deal with operators. It is called display/1, because it mimics closely the behaviour of this Edinburgh predicate.

foreign_t
pl_display(term_t t)
{ functor_t functor;
  int arity, len, n;
  char *s;

  switch( PL_term_type(t) )
  { case PL_VARIABLE:
    case PL_ATOM:
    case PL_INTEGER:
    case PL_FLOAT:
      PL_get_chars(t, &s, CVT_ALL);
      if (! Sfprintf(Scurrent_output, "%s", s) )
        PL_fail;
      break;
    case PL_STRING:
      if ( !PL_get_string_chars(t, &s, &len) &&
           !Sfprintf(Scurrent_output, "\"%s\"", s) )
        PL_fail;
      break;
    case PL_TERM:
    { term_t a = PL_new_term_ref();

      if ( !PL_get_name_arity(t, &name, &arity) &&
           !Sfprintf(Scurrent_output, "%s(", PL_atom_chars(name)) )
        PL_fail
      for(n=1; n<=arity; n++)
      { if ( ! PL_get_arg(n, t, a) )
          PL_fail;
        if ( n > 1 )
          if ( ! Sfprintf(Scurrent_output, ", ") )
            PL_fail;
        if ( !pl_display(a) )
          PL_fail;
      }
      if ( !Sfprintf(Scurrent_output, ")") )
        PL_fail;
      break;
    default:
      PL_fail;                          /* should not happen */
    }
  }

  PL_succeed;
}