True when Value is returned by the Python Iterator. Python iterators
may be used to implement non-deterministic foreign predicates. The
implementation uses these steps:
- Evaluate Iterator as py_call/2 evaluates its first argument,
except the
Obj:Attr = Value construct is not accepted.
- Call
__iter__ on the result to get the iterator itself.
- Get the
__next__ function of the iterator.
- Loop over the return values of the next function. If
the Python return value unifies with Value, succeed with
a choicepoint. Abort on Python or unification exceptions.
- Re-satisfaction continues at (4).
The example below uses the built-in iterator range():
?- py_iter(range(1,3), X).
X = 1 ;
X = 2.
Note that the implementation performs a look ahead, i.e., after
successful unification it calls `__next__()` again. On failure the
Prolog predicate succeeds deterministically. On success, the next
candidate is stored.
Note that a Python generator is a Python iterator. Therefore,
given the Python generator expression below, we can use
py_iter(squares(1,5),X) to generate the squares on backtracking.
def squares(start, stop):
for i in range(start, stop):
yield i * i
- Arguments:
-
- Compatibility
- - PIP. The same remarks as for py_call/2 apply.
- bug
- - Iterator may not depend on janus.
query(), i.e., it is not
possible to iterate over a Python iterator that under the hoods
relies on a Prolog non-deterministic predicate.
