Here is a very rough draft of an idea I've floated often, but not with much specification. Take this as "ideas" with little firm commitment to details from me. PRs, or issues, or whatever, can go to https://github.com/DavidMertz/peps/blob/master/pep-9999.rst as well as mentioning them in this thread.

PEP: 9999
Title: Generalized deferred computation
Author: David Mertz <dmertz@gnosis.cx>
Discussions-To: https://mail.python.org/archives/list/python-ideas@python.org/thread/
Status: Draft
Type: Standards Track
Content-Type: text/x-rst
Created: 21-Jun-2022
Python-Version: 3.12
Post-History:

Abstract
========

This PEP proposes introducing the soft keyword ``later`` to express the concept
of deferred computation. When an expression is preceded by the keyword, the
expression is not evaluated but rather creates a "thunk" or "deferred object."
Reference to the deferred object later in program flow causes the expression to
be executed at that point, and for both the value and type of the object to
become the result of the evaluated expression.

Motivation
==========

"Lazy" or "deferred" evaluation is useful paradigm for expressing relationships
among potentially expensive operations prior their actual computation. Many
functional programming languages, such as Haskell, build laziness into the
heart of their language. Within the Python ecosystem, the popular scientific
library `dask-delayed <dask-delayed>`_ provides a framework for lazy evaluation
that is very similar to that proposed in this PEP.

.. _dask-delayed:
https://docs.dask.org/en/stable/delayed.html

Examples of Use
===============

While the use of deferred computation is principally useful when computations
are likely to be expensive, the simple examples shown do not necessarily use
such expecially spendy computations. Most of these are directly inspired by
examples used in the documentation of dask-delayed.

In dask-delayed, ``Delayed`` objects are create by functions, and operations
create a *directed acyclic graph* rather than perform actual computations. For
example::

>>> import dask
>>> @dask.delayed
... def later(x):
... return x
...
>>> output = []
>>> data = [23, 45, 62]
>>> for x in data:
... x = later(x)
... a = x * 3
... b = 2**x
... c = a + b
... output.append(c)
...
>>> total = sum(output)
>>> total
Delayed('add-8f4018dbf2d3c1d8e6349c3e0509d1a0')
>>> total.compute()
4611721202807865734
>>> total.visualize()

.. figure:: pep-9999-dag.png
:align: center
:width: 50%
:class: invert-in-dark-mode

Figure 1. Dask DAG created from simple operations.

Under this PEP, the soft keyword ``later`` would work in a similar manner to
this dask.delayed code. But rather than requiring calling ``.compute()`` on a
``Delayed`` object to arrive at the result of a computation, every reference to
a binding would perform the "compute" *unless* it was itself a deferred
expression. So the equivalent code under this PEP would be::

>>> output = []
>>> data = [23, 45, 62]
>>> for later x in data:
... a = later (x * 3)
... b = later (2**x)
... c = later (a + b)
... output.append(later c)
...
>>> total = later sum(output)
>>> type(total) # type() does not un-thunk
<class 'DeferredObject'>
>>> if value_needed:
... print(total) # Actual computation occurs here
4611721202807865734

In the example, we assume that the built-in function `type()` is special in not
counting as a reference to the binding for purpose of realizing a computation.
Alternately, some new special function like `isdeferred()` might be used to
check for ``Deferred`` objects.

In general, however, every regular reference to a bound object will force a
computation and re-binding on a ``Deferred``. This includes access to simple
names, but also similarly to instance attributes, index positions in lists or
tuples, or any other means by which an object may be referenced.

Rejected Spellings
==================

A number of alternate spellings for creating a ``Deferred`` object are
possible. This PEP-author has little preference among them. The words
``defer`` or ``delay``, or their past participles ``deferred`` and ``delayed``
are commonly used in discussions of lazy evaluation. All of these would work
equally well as the suggested soft keyword ``later``. The keyword ``lazy`` is
not completely implausible, but does not seem to read as well.

No punctuation is immediately obvious for this purpose, although surrounding
expressions with backticks is somewhat suggestive of quoting in Lisp, and
perhaps slightly reminiscent of the ancient use of backtick for shell commands
in Python 1.x. E.g.::

might_use = `math.gcd(a, math.factorial(b))`

Relationship to PEP-0671
========================

The concept of "late-bound function argument defaults" is introduced in
:pep:`671`. Under that proposal, a special syntactic marker would be permitted
in function signatures with default arguments to allow the expressions
indicated as defaults to be evaluated at call time rather than at runtime. In
current Python, we might write a toy function such as::

def func(items=[], n=None):
if n is None:
n = len(items)
items.append("Hello")
print(n)

func([1, 2, 3]) # prints: 3

Using the :pep:`671` approach this could be simplified somewhat as::

def func(items=[], n=>len(items)):
# late-bound defaults act as if bound here
items.append("Hello")
print(n)

func([1, 2, 3]) # prints: 3

Under the current PEP, evaluation of a ``Deferred`` object only occurs upon
reference. That is, for the current toy function, the evaluation would not
occur until the ``print(n)`` line.::

def func(items=[], n=later len(items)):
items.append("Hello")
print(n)

func([1, 2, 3]) # prints: 4

To completely replicate the behavior of PEP-0671, an extra line at the start of
the function body would be required::

def func(items=[], n=later len(items)):
n = n # Evaluate the Deferred and re-bind the name n
items.append("Hello")
print(n)

func([1, 2, 3]) # prints: 3

References
==========

https://github.com/DavidMertz/peps/blob/master/pep-9999.rst

Copyright
=========

This document is placed in the public domain or under the
CC0-1.0-Universal license, whichever is more permissive.

Keeping medicines from the bloodstreams of the sick; food
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