* Generator Examples
@ 2016-09-22 0:47 raman
2016-09-25 0:16 ` Michael Heerdegen
0 siblings, 1 reply; 7+ messages in thread
From: raman @ 2016-09-22 0:47 UTC (permalink / raw)
To: emacs-devel
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I'm attaching a short texinfo file containing a couple of examples of
using generators in Emacs Lisp.
The texinfo manual contains one example which I found confusing in
parts:
1. The example has a nested call to iter-yield -- not sure why
2. I still dont quite understand the purpose of the optional argument
to iter-next, an example that leverages it would be good to have.
[-- Attachment #2: Generator Examples in texinfo --]
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\input texinfo @c -*- texinfo -*-
@c %**start of header
@setfilename ./generators.info
@settitle Generators In Emacs Lisp
@documentencoding UTF-8
@documentlanguage en
@c %**end of header
@finalout
@titlepage
@title Generators In Emacs Lisp
@author raman
@end titlepage
@ifnottex
@node Top
@top Generators In Emacs Lisp
@end ifnottex
@menu
* Define a Fibonacci generator::
* Iterate Over A List:: Iterate Over A List.
@end menu
@node Define a Fibonacci generator
@chapter Define a Fibonacci generator
@lisp
(iter-defun fibonacci-iter ()
"Return a Fibonacci sequence generator."
(let ((a 1)
(b 1))
(while t
(iter-yield a)
(cl-psetq a b b (+ a b )))))
@end lisp
Instantiate a Fibonacci generator and use it to collect the first 10
numbers in the sequence.
@lisp
(setq f (fibonacci-iter))
;;; The first 10 Fibonacci numbers:
(cl-loop for i from 1 to 10 collect (iter-next f))
@end lisp
@node Iterate Over A List
@chapter Iterate Over A List:
A generator that takes a list and returns an iterator over that list:
@lisp
(iter-defun list-iter (l)
"Return an iterator that iterates over list `l'."
(let ((local(copy-sequence l)))
(while local (iter-yield (pop local)))))
@end lisp
Test the above by iterating over a list. This example is mostly for
illustration demonstrates the use of @code{cl-loop} clause @code{iter-by}.
@lisp
;;; Create a list iterator:
(setq l-iter (list-iter '(a b c d e)))
;;; Loop through the iterator, collecting values:
(cl-loop for e iter-by l-iter collect e)
@end lisp
@bye
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--
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-22 0:47 Generator Examples raman
@ 2016-09-25 0:16 ` Michael Heerdegen
2016-09-25 0:29 ` raman
2016-09-25 0:32 ` raman
0 siblings, 2 replies; 7+ messages in thread
From: Michael Heerdegen @ 2016-09-25 0:16 UTC (permalink / raw)
To: raman; +Cc: emacs-devel
Hey raman,
> I'm attaching a short texinfo file containing a couple of examples of
> using generators in Emacs Lisp.
Great. This would be an improvement for the manual indeed.
> The texinfo manual contains one example which I found confusing in
> parts:
Yeah, the current doc is more for people already familiar with the
concept.
> 1. The example has a nested call to iter-yield -- not sure why
Just to demonstrate how it behaves, I think. It's probably not very
common/useful in practise.
> 2. I still dont quite understand the purpose of the optional argument
> to iter-next, an example that leverages it would be good to have.
I think it's more useful if you think of generators as coroutines. A
simple real life example would be: you have implemented a counter as a
generator (generating 1, 2, ...), and when you restart it, you want to
add a certain increment to its count so far before continuing.
> @lisp
> (iter-defun fibonacci-iter ()
> "Return a Fibonacci sequence generator."
> (let ((a 1)
> (b 1))
> (while t
> (iter-yield a)
> (cl-psetq a b b (+ a b )))))
> @end lisp
Looks good (but could we get rid of the a bit uncommon `cl-psetq'). I
remember that the suggested convention was to call the iter-defun a
generator, and the return value an iterator (in this regard, the
docstring would be wrong).
FWIW, some time ago a had posted a similar example, implementing a
generator of the prime numbers:
(iter-defun cross-through-multiples-of (n)
"Repeat indefinitely: Return `t' N-1 times, then return `nil' once."
(let ((i (1- n)))
(while t
(if (zerop i) (progn (setq i (1- n)) (iter-yield nil))
(iter-yield t)
(cl-decf i)))))
(iter-defun make-prime-gen ()
"Return a generator of the prime numbers."
(let ((n 2) (sieve '()))
(while t
(when (cl-every #'identity (mapcar #'iter-next sieve))
;; a new prime!
(push (cross-through-multiples-of n) sieve)
(iter-yield n))
(cl-incf n))))
> A generator that takes a list and returns an iterator over that list:
>
> @lisp
> (iter-defun list-iter (l)
> "Return an iterator that iterates over list `l'."
> (let ((local(copy-sequence l)))
> (while local (iter-yield (pop local)))))
> @end lisp
Isn't the `copy-sequence' redundant (popping the local variable doesn't
alter the original list)?
> @lisp
> ;;; Create a list iterator:
>
> (setq l-iter (list-iter '(a b c d e)))
> ;;; Loop through the iterator, collecting values:
> (cl-loop for e iter-by l-iter collect e)
> @end lisp
>
> @bye
To improve the doc even further, I think it would be good if we could as
well:
- Underline the difference between generators and its "instances"
(iterators), and that multiple iterators of the same "type" have
independent inner states.
- Have an example that doesn't create lists at the end, to underline
that it is a different concept, and/or
- Have a real life example (something like tree traversal, e.g. a
generator of directory files or so) that is an improvement compared to
using list.
Regards,
Michael.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-25 0:16 ` Michael Heerdegen
@ 2016-09-25 0:29 ` raman
2016-09-25 1:03 ` Michael Heerdegen
2016-09-25 0:32 ` raman
1 sibling, 1 reply; 7+ messages in thread
From: raman @ 2016-09-25 0:29 UTC (permalink / raw)
To: Michael Heerdegen; +Cc: emacs-devel
I'm familiar with generators as a concept across multiple languages --
but I still found the elisp docs confusing:-)
We explicitly say the interface is similar to that in languages like
Python -- which is why i coded up the Fibonacci example.
The cl-psetq is the right idiom here in that sense -- if you use plain
setq, you'd have to use a temporary variable?
Thanks for the coroutine example, again that would do well in the
manual -- there is a reference to coroutines either in the manual or in
code comments in generators.el that is presently "intriguing".
Finally, I still dont understand the nested call to iter-yield in the
manual and if it is correct, it would be good to have it better
explained there.
--
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-25 0:16 ` Michael Heerdegen
2016-09-25 0:29 ` raman
@ 2016-09-25 0:32 ` raman
2016-09-25 1:06 ` Michael Heerdegen
1 sibling, 1 reply; 7+ messages in thread
From: raman @ 2016-09-25 0:32 UTC (permalink / raw)
To: Michael Heerdegen; +Cc: emacs-devel
P.S. In my quick tests, the copy-sequence was necessary --
If it weren't generators would get inefficient quickly because you'd
have to do a deep copy of the variable you were snapshotting in the closure.
--
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-25 0:29 ` raman
@ 2016-09-25 1:03 ` Michael Heerdegen
2016-09-25 2:26 ` T.V Raman
0 siblings, 1 reply; 7+ messages in thread
From: Michael Heerdegen @ 2016-09-25 1:03 UTC (permalink / raw)
To: raman; +Cc: emacs-devel
raman <raman@google.com> writes:
> The cl-psetq is the right idiom here in that sense -- if you use plain
> setq, you'd have to use a temporary variable?
Sure, but I want to avoid to confront people with functions they
potentially don't know in these examples, so that they can concentrate
on the gist. A minor issue, sure.
> Thanks for the coroutine example, again that would do well in the
> manual -- there is a reference to coroutines either in the manual or
> in code comments in generators.el that is presently "intriguing".
AFAIK the original author wanted to add some code for working with
coroutines.
> Finally, I still dont understand the nested call to iter-yield in the
> manual and if it is correct, it would be good to have it better
> explained there.
I think it is correct (behaves exactly as described, and the behavior is
what I expect). What in particular don't you understand?
I think the thing that this part wants to demonstrate is that
`iter-yield' behaves normally wrt when it is evaluated (function
arguments before function calls etc.; so the inner `iter-yield' is
called first, and the outer not before the iterator has been restarted).
And the outer `iter-yield' yields a value that is computed from the
"return value" of the inner, which is the second argument of the
according `iter-next' call.
Michael.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-25 0:32 ` raman
@ 2016-09-25 1:06 ` Michael Heerdegen
0 siblings, 0 replies; 7+ messages in thread
From: Michael Heerdegen @ 2016-09-25 1:06 UTC (permalink / raw)
To: raman; +Cc: emacs-devel
raman <raman@google.com> writes:
> P.S. In my quick tests, the copy-sequence was necessary -- If it
> weren't generators would get inefficient quickly because you'd have to
> do a deep copy of the variable you were snapshotting in the closure.
I don't understand what you mean, but it's already late (an example,
maybe?)
Regards,
Michael.
^ permalink raw reply [flat|nested] 7+ messages in thread
* Re: Generator Examples
2016-09-25 1:03 ` Michael Heerdegen
@ 2016-09-25 2:26 ` T.V Raman
0 siblings, 0 replies; 7+ messages in thread
From: T.V Raman @ 2016-09-25 2:26 UTC (permalink / raw)
To: michael_heerdegen; +Cc: emacs-devel, raman
The bit you wrote in response to my "dont understand why nested
iter-yield" might do well in the manual. Also, it might do what it
claims, but why it does it is opaque -- and examples that do something
useful are usually easier to learn from.
Michael Heerdegen writes:
> raman <raman@google.com> writes:
>
> > The cl-psetq is the right idiom here in that sense -- if you use plain
> > setq, you'd have to use a temporary variable?
>
> Sure, but I want to avoid to confront people with functions they
> potentially don't know in these examples, so that they can concentrate
> on the gist. A minor issue, sure.
>
> > Thanks for the coroutine example, again that would do well in the
> > manual -- there is a reference to coroutines either in the manual or
> > in code comments in generators.el that is presently "intriguing".
>
> AFAIK the original author wanted to add some code for working with
> coroutines.
>
> > Finally, I still dont understand the nested call to iter-yield in the
> > manual and if it is correct, it would be good to have it better
> > explained there.
>
> I think it is correct (behaves exactly as described, and the behavior is
> what I expect). What in particular don't you understand?
>
> I think the thing that this part wants to demonstrate is that
> `iter-yield' behaves normally wrt when it is evaluated (function
> arguments before function calls etc.; so the inner `iter-yield' is
> called first, and the outer not before the iterator has been restarted).
> And the outer `iter-yield' yields a value that is computed from the
> "return value" of the inner, which is the second argument of the
> according `iter-next' call.
>
>
> Michael.
--
--
^ permalink raw reply [flat|nested] 7+ messages in thread
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2016-09-22 0:47 Generator Examples raman
2016-09-25 0:16 ` Michael Heerdegen
2016-09-25 0:29 ` raman
2016-09-25 1:03 ` Michael Heerdegen
2016-09-25 2:26 ` T.V Raman
2016-09-25 0:32 ` raman
2016-09-25 1:06 ` Michael Heerdegen
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