Re: general lazy list facility for Emacs Lisp?

[Tassilo Horn <tassilo@member.fsf.org>]

Ted Zlatanov <tzz@lifelogs.com> writes:

Hi Ted,

> Yes, that's what I mean.  In Haskell such lists are simply part of the
> core language; Perl 5 can hack them together with list accessors; Perl
> 6 has them in the core too...  I think "lazy list" is a popular term
> for the general facility?  I'm surprised no one has needed them,
> whatever the name :)

Lately, I do some Clojure coding for fun, and I really enjoy lazy seqs
there.  However, I don't see a real use case in emacs.  So, pants off,
what are you planning to do?

> Slightly related: is there a general memoization package or standard
> approach to memoizing functions?

I think for "real" memoization, you need clojures.  However, I think you
can simulate memoization with a macro like that (tested only very
briefly).

--8<---------------cut here---------------start------------->8---
(defmacro memoize (name fun)
  (let ((map-name (gensym "memo-map"))
	(args-name (gensym))
	(val-name (gensym)))
    `(progn
       (defvar ,map-name (make-hash-table :test 'equal))
       (defun ,name (&rest ,args-name)
	 (let ((,val-name (gethash ,args-name ,map-name)))
	   (if ,val-name
	       ,val-name
	     (puthash ,args-name (apply (quote ,fun) ,args-name)
		      ,map-name)))))))

;; Create a memoized version of +
(memoize memo-+ +)
;; Call it
(memo-+ 1 1)
;; ==> 2
(memo-+)
;; ==> 2, not computed by looked up in the hash table
--8<---------------cut here---------------end--------------->8---

Bye,
Tassilo