From ...
From: Erik Naggum <erik@naggum.no>
Subject: Re: clever flatten?
Date: 1996/02/10
Message-ID: <19960210T235219Z@arcana.naggum.no>#1/1
X-Deja-AN: 138839500
references: <Ul6vo1C00iUvE7tL4d@andrew.cmu.edu> <SMISHRA.96Feb9170920@gaia.cc.gatech.edu>
organization: Naggum Software; +47 2295 0313
newsgroups: comp.lang.lisp

[Sunil Mishra]

|   Why not write your own function? I don't really understand what the
|   constraints are, perhaps I should give an example of what might work,
|   and you tell me what's wrong with it. I'm also trying to keep memory
|   constraints in mind, and this will result in an ugly function.
|   
|   (defun flatten (tree &aux (output '(())) output-end)
|     (declare (special output-end))
|     (setq output-end output)
|     (flatten-loop tree)
|     (cdr output))
|   
|   (defun flatten-loop (tree)
|     (declare (special output-end))
|     (mapc #'(lambda (tree-el)
|   	    (cond ((listp tree-el)
|   		   (flatten-loop tree-el))
|   		  (t (push tree-el (cdr output-end))
|   		     (setq output-end (cdr output-end)))))
|   	tree))

using a lexical variable in a closure around a `labels' form, you could do
away with the expensive special variable and the separate function.

#<Erik 3032985139133767>
-- 
imagine if the buyers of the first PC's with PC-DOS didn't have to upgrade.