"""
This module provides the Scan Op.

Scanning is a general form of recurrence, which can be used for looping.
The idea is that you *scan* a function along some input sequence, producing
an output at each time-step that can be seen (but not modified) by the
function at the next time-step. (Technically, the function can see the
previous K  time-steps of your outputs and L time steps (from past and
future) of your inputs.

So for example, ``sum()`` could be computed by scanning the ``z+x_i``
function over a list, given an initial state of ``z=0``.

Special cases:

* A *reduce* operation can be performed by using only the last
  output of a ``scan``.
* A *map* operation can be performed by applying a function that
  ignores previous steps of the outputs.

Often a for-loop or while-loop can be expressed as a ``scan()`` operation,
and ``scan`` is the closest that aesara comes to looping. The advantages
of using ``scan`` over `for` loops in python (amongs other) are:

* it allows the number of iterations to be part of the symbolic graph
* it allows computing gradients through the for loop
* there exist a bunch of optimizations that help re-write your loop
such that less memory is used and that it runs faster
* it ensures that data is not copied from host to gpu and gpu to
host at each step

The Scan Op should typically be used by calling any of the following
functions: ``scan()``, ``map()``, ``reduce()``, ``foldl()``,
``foldr()``.

"""


__docformat__ = "restructedtext en"
__authors__ = (
    "Razvan Pascanu "
    "Frederic Bastien "
    "James Bergstra "
    "Pascal Lamblin "
    "Arnaud Bergeron "
    "PyMC Developers "
)
__copyright__ = "(c) 2010, Universite de Montreal"

from aesara import configdefaults


configdefaults.add_scan_configvars()

from aesara.scan import opt
from aesara.scan.basic import scan
from aesara.scan.checkpoints import scan_checkpoints
from aesara.scan.utils import until
from aesara.scan.views import foldl, foldr, map, reduce
