From: steve-humphreys@gmx.com
To: Jean Louis <bugs@gnu.support>
Cc: Help Gnu Emacs <help-gnu-emacs@gnu.org>
Subject: Re: Understanding the "let" construct and the setting of variables
Date: Thu, 17 Dec 2020 06:12:00 +0100 [thread overview]
Message-ID: <trinity-7b78191b-0c53-44c8-9232-76059a22ff63-1608181920893@3c-app-mailcom-bs11> (raw)
In-Reply-To: <X9rf64FEKYsB4Oa1@protected.rcdrun.com>
> How I understand it is that `setq' I can freely use on variables
> already defined with and within my `let' as then the variable
> will not become global.
>
> (defun my-fun ()
> (let ((my-var nil))
> (setq my-var 2)))
>
> (my-fun)
>
> my-var is not defined
Here the variable remains local. my-var does not become a global
variable because it is defined within the let. So one can use "set"
with variables defined in a "let" construct. I thought you could not
do that - call "setq" on a local variable defined in a "let" expression.
> (defun my-fun ()
> (let ((my-var nil)))
> (setq my-var 2))
>
> (my-fun)
>
> my-var is here defined as 2 and became global variable.
>
> And each time that variable is already defined with `defvar' one
> can then change it with setq.
And here "my-var" becames a global variable because my-var is
set using "setq" outside the "let" expression.
Thanks. Was not too difficult as I thought.
> Sent: Thursday, December 17, 2020 at 5:34 AM
> From: "Jean Louis" <bugs@gnu.support>
> To: steve-humphreys@gmx.com
> Cc: "Help Gnu Emacs" <help-gnu-emacs@gnu.org>
> Subject: Re: Understanding the "let" construct and the setting of variables
>
> > -*- lexical-binding: t; -*-
> * steve-humphreys@gmx.com <steve-humphreys@gmx.com> [2020-12-17 03:26]:
> > Let's introspect two questions.
> >
> > 1. In what simple circumstances would one use a "setq" in the body
> > of a let?
>
> Whenever I find myself in linear programming within a function and
> need to change variable I will use setq. Some global variables are
> rather set with setq:
>
> (set-buffer buffer)
> (setq header-line-format (concat buffer " ➜ Finish with `q' or `h'"))
> (cf-org-view-mode)
> (insert blob)
> (setq org-hierarchical-todo-statistics nil)
> (org-update-parent-todo-statistics)
> (goto-char 1)
>
> But I will often use it in construction of lists:
>
> (defun rcd-cgi-parse-query-string (query-string)
> "Parse QUERY-STRING that normally comes from the environment
> variable `QUERY_STRING'. Return PLIST."
> (let* ((query-string (url-unhex-string query-string))
> (parts (split-string query-string "&"))
> (length (length parts))
> (plist '()))
> (dolist (part parts plist)
> (let* ((data (split-string part "="))
> (prop (car data))
> (val (cadr data)))
> (setq plist (plist-put plist (intern prop) val))))))
>
>
> (defun iota (count &optional start step)
> "Return a list containing COUNT numbers, starting from START
> and adding STEP each time. The default START is 0, the default
> STEP is 1"
> (let* ((start (if start start 0))
> (step (if step step 1))
> (last (+ start count))
> (counter 0)
> (list '())
> (elt start))
> (while (< counter count)
> (push elt list)
> (setq elt (+ elt step))
> (setq counter (1+ counter)))
> (reverse list)))
>
> How I understand it is that `setq' I can freely use on variables
> already defined with and within my `let' as then the variable
> will not become global.
>
> (defun my-fun ()
> (let ((my-var nil))
> (setq my-var 2)))
>
> (my-fun)
>
> my-var is not defined
>
> (defun my-fun ()
> (let ((my-var nil)))
> (setq my-var 2))
>
> (my-fun)
>
> my-var is here defined as 2 and became global variable.
>
> And each time that variable is already defined with `defvar' one
> can then change it with setq.
>
> Jean
>
next prev parent reply other threads:[~2020-12-17 5:12 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-12-17 0:10 Understanding the "let" construct and the setting of variables steve-humphreys
2020-12-17 0:21 ` Joost Kremers
2020-12-17 2:08 ` steve-humphreys
2020-12-17 3:12 ` steve-humphreys
2020-12-17 8:01 ` Joost Kremers
2020-12-17 8:31 ` steve-humphreys
2020-12-17 8:50 ` Joost Kremers
2020-12-17 8:10 ` Joost Kremers
2020-12-17 8:43 ` steve-humphreys
2020-12-17 8:56 ` Joost Kremers
2020-12-18 20:48 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-18 20:46 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-18 21:07 ` Jean Louis
2020-12-18 22:31 ` tomas
2020-12-18 20:39 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-17 2:49 ` steve-humphreys
2020-12-17 7:58 ` Joost Kremers
2020-12-17 16:55 ` Drew Adams
2020-12-17 20:11 ` steve-humphreys
2020-12-17 21:57 ` Drew Adams
2020-12-17 22:35 ` Michael Heerdegen
2020-12-18 9:01 ` tomas
2020-12-18 9:16 ` Michael Heerdegen
2020-12-18 20:55 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-19 2:17 ` Michael Heerdegen
2020-12-19 2:52 ` Drew Adams
2020-12-19 5:15 ` Stefan Monnier
2020-12-18 20:33 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-17 0:25 ` steve-humphreys
2020-12-17 0:35 ` steve-humphreys
2020-12-17 1:05 ` Joost Kremers
2020-12-17 1:20 ` steve-humphreys
2020-12-18 20:58 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-17 4:34 ` Jean Louis
2020-12-17 5:12 ` steve-humphreys [this message]
2020-12-19 6:06 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-17 7:31 ` steve-humphreys
2020-12-19 5:55 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-19 6:49 ` Jean Louis
2020-12-20 5:19 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-18 17:14 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-18 17:48 ` tomas
2020-12-18 15:33 ` Emanuel Berg via Users list for the GNU Emacs text editor
2020-12-18 18:12 ` Jean Louis
2020-12-18 18:20 ` Drew Adams
2020-12-18 18:45 ` Jean Louis
2020-12-18 19:16 ` Drew Adams
2020-12-18 20:00 ` Jean Louis
2020-12-18 21:27 ` Christopher Dimech
2020-12-19 6:23 ` Emanuel Berg via Users list for the GNU Emacs text editor
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