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Re: Novice trouble with generator.el
From: |
Davin Pearson |
Subject: |
Re: Novice trouble with generator.el |
Date: |
Mon, 16 Nov 2015 19:52:44 -0800 (PST) |
User-agent: |
G2/1.0 |
On Friday, November 6, 2015 at 7:13:57 AM UTC+13, Michael Heerdegen wrote:
> Davin Pearson <davin dot pearson at gmail dom com> writes:
>
> > The file generator.el in emacs version 25.0.50 looks like a good
> > implementation of multi-threading, but the documentation is aimed at
> > experts in using the system. What I would like is a simple example of
> > how to go about this multi-threading.
>
> AFAIR Daniel planned to implement coroutines based on generator.el
> later.
>
> For now, we have the iterators defined there. I agree that the
> documentation is quite academical.
>
> Actually, `iter-defun' is quite easy to use and understand. It is much
> like a normal defun. If you don't call `iter-yield' or
> `iter-yield-from' in the body, it is exactly defun. Likewise
> `iter-lambda'.
>
> Let's leave `iter-yield-from' aside for a moment.
>
> Write your `iter-defun' like normal code, like a defun that would do all
> the work at once or return all elements in succession. To return an
> element and give control back to the caller, just use `iter-yield'. To
> reinvoke the generator (produce one more element), the caller should use
> `iter-next' on it. That's all. When the body of the `iter-defun'
> completes normally (i.e. terminates without calling `iter-yield') the
> generator has run out of elements and stops/ your coroutine has
> finished.
>
> Here are two real life examples. The first one is an implementation of
> the sieve of Eratosthenes (don't we use this at home all the time?):
>
> --8<---------------cut here---------------start------------->8---
> (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))))
> --8<---------------cut here---------------end--------------->8---
>
> Both generators never finish, that's why they look like an infloop.
>
> Test:
>
> (setq g (make-prime-gen))
>
> (iter-next g)
>
> ==> 2
>
> (iter-next g)
>
> ==> 3
>
> (setq h (make-prime-gen))
>
> (iter-next h)
>
> ==> 2
>
> (iter-next g)
>
> === 5
>
>
> Second example: a generator that returns all files in a directory in
> succession, recursively if you want:
>
> --8<---------------cut here---------------start------------->8---
> (iter-defun iterator-of-directory-files (directory &optional full match
> nosort recurse)
> "Return an iterator of files in DIRECTORY.
> The arguments FULL, MATCH and NOSORT are like in
> `directory-files'. Forth optional arg RECURSE non-nil means
> recurse on subdirectories. If RECURSE is a function, it should
> accept one argument, a directory file name, and return non-nil
> when we should recurse into that directory. Any other non-nil
> value means recurse into every readable subdirectory."
> (when (file-executable-p directory) ; i.e. DIRECTORY is readable
> (let ((files (directory-files directory full match nosort)) file)
> (while (setq file (pop files))
> (cond
> ((not (file-directory-p file))
> (iter-yield file))
> ((member (file-name-nondirectory (directory-file-name file)) '("."
> "..")))
> (t
> (iter-yield file)
> (when (or (and (functionp recurse)
> (funcall recurse file))
> recurse)
> (iter-yield-from (iterator-of-directory-files file full match
> nosort recurse)))))))))
> --8<---------------cut here---------------end--------------->8---
>
> `iter-yield-from' is much like `iter-yield', but instead of returning
> one element, it takes an iterator as argument and returns the elements
> it produces, one at a time and one after the other, until that generator
> runs out, and continues afterwards.
>
> The nomenclature of generator vs. iterator is a bit confusing in
> "generator.el". In that package, a generator is a function that, when
> called, returns an iterator object. An iterator is a function accepting
> zero arguments that produces elements.
>
> In the above examples, `cross-through-multiples-of', `make-prime-gen'
> and `iterator-of-directory-files' are generators, `g' and `h' are
> iterators.
>
>
> HTH,
>
> Michael.
When I try to execute
iter-defun iterator-of-directory-files (directory &optional full match nosort
recurse)
it gives me the following error:
Assertion failed: lexical-binding
What gives? Note that I am using Emacs 24.3.1.
Do I have to execute Emacs 25.x.y?