./2). If successful,
write a reference to the head of the list into h and a
reference to the tail of the list into t. This reference to h
may be used for subsequent calls to this function. Suppose we want to
return a list of atoms from a char **. We could use the
example described by
PL_cons_list(),
followed by a call to PL_unify(),
or we can use the code below. If the predicate argument is unbound, the
difference is minimal (the code based on PL_cons_list()
is probably slightly faster). If the argument is bound, the code below
may fail before reaching the end of the word list, but even if the
unification succeeds, this code avoids a duplicate (garbage) list and a
deep unification.
Note that PL_unify_list()
is not used with env but with
tail, which is a copy of env. PL_copy_term_ref()
creates a copy term_t holding the same Prolog term, i.e., not
a copy of the Prolog term. The only thing that is allowed to be done
with an argument to a foreign predicate (such as env) is
unification; for anything that might over-write the term, you must use a
copy created by PL_copy_term_ref().
The name PL_unify_list()
is slightly misleading - it unifies the first argumment (l
but
overwrites the second (h) and third (t)
arguments.
foreign_t
pl_get_environ(term_t env)
{ term_t tail = PL_copy_term_ref(env);
term_t item = PL_new_term_ref();
extern char **environ;
for(const char **e = environ; *e; e++)
{ if ( !PL_unify_list(tail, item, tail) ||
!PL_unify_atom_chars(item, *e) )
PL_fail;
}
return PL_unify_nil(tail);
}
In this example, item is initialized outside the loop.
This allocates a single new reference to a term, which is used as a
temporary inside the loop - there is no need to allocate a new reference
each time around the loop because the item term reference
can be reused and the call to PL_unify_list()
copies a reference to the new list cell's head into the the term
referenced by
item.