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* for a given emacs session: insert a subject with an increasing counter
@ 2021-08-30 16:01 Uwe Brauer
  2021-08-30 16:10 ` Eric S Fraga
  0 siblings, 1 reply; 26+ messages in thread
From: Uwe Brauer @ 2021-08-30 16:01 UTC (permalink / raw)
  To: ding


Hi

Is the following possible for a given emacs session a internal counter
is created that is deleted after closing emacs and which would allow to
insert to each message in the subject line a counter of the form

Subject: message-of <2021-08-30 lun>: 1
Subject: message-of <2021-08-30 lun>: 2

Of course such a function could be used only for certain emails (using
say some bbdb functionality).

This for example

(defvar my-subject-counter 0)
(defun my-insert-add-counter ()
  (interactive)
  (insert (int-to-string (+ my-subject-counter 1)))) 

Does not work since the counter is not saved when increased.

Does anybody know about such a functionality?

Regards

Uwe Brauer



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-30 16:01 for a given emacs session: insert a subject with an increasing counter Uwe Brauer
@ 2021-08-30 16:10 ` Eric S Fraga
  2021-08-30 18:46   ` Eric Abrahamsen
  2021-08-31  6:50   ` Uwe Brauer
  0 siblings, 2 replies; 26+ messages in thread
From: Eric S Fraga @ 2021-08-30 16:10 UTC (permalink / raw)
  To: ding

On Monday, 30 Aug 2021 at 18:01, Uwe Brauer wrote:
> Does not work since the counter is not saved when increased.

Why not save it then?  I.e. something like this:

(defun my-insert-add-counter ()
  (interactive)
  (setq my-subject-counter (+ my-subject-counter 1))
  (insert (int-to-string my-subject-counter))) 

[untested]

HTH,
eric

PS by the way, there is an elisp function called 1+ which you might like
to use ;-))
-- 
Eric S Fraga via Emacs 28.0.50 & org 9.4.6 on Debian 11.0



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-30 16:10 ` Eric S Fraga
@ 2021-08-30 18:46   ` Eric Abrahamsen
  2021-08-31  6:50   ` Uwe Brauer
  1 sibling, 0 replies; 26+ messages in thread
From: Eric Abrahamsen @ 2021-08-30 18:46 UTC (permalink / raw)
  To: ding

Eric S Fraga <e.fraga@ucl.ac.uk> writes:

> On Monday, 30 Aug 2021 at 18:01, Uwe Brauer wrote:
>> Does not work since the counter is not saved when increased.
>
> Why not save it then?  I.e. something like this:
>
> (defun my-insert-add-counter ()
>   (interactive)
>   (setq my-subject-counter (+ my-subject-counter 1))
>   (insert (int-to-string my-subject-counter))) 
>
> [untested]
>
> HTH,
> eric
>
> PS by the way, there is an elisp function called 1+ which you might like
> to use ;-))

And if you really want to write the absolute minimum code, you can use `cl-incf'!



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-30 16:10 ` Eric S Fraga
  2021-08-30 18:46   ` Eric Abrahamsen
@ 2021-08-31  6:50   ` Uwe Brauer
  2021-08-31  6:58     ` Adam Sjøgren
  2021-08-31  7:44     ` Emanuel Berg
  1 sibling, 2 replies; 26+ messages in thread
From: Uwe Brauer @ 2021-08-31  6:50 UTC (permalink / raw)
  To: ding

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>>> "ESF" == Eric S Fraga <e.fraga@ucl.ac.uk> writes:

> On Monday, 30 Aug 2021 at 18:01, Uwe Brauer wrote:
>> Does not work since the counter is not saved when increased.

> Why not save it then?  I.e. something like this:

> (defun my-insert-add-counter ()
>   (interactive)
>   (setq my-subject-counter (+ my-subject-counter 1))
>   (insert (int-to-string my-subject-counter))) 

Thanks very much, recursion, sigh, I should have thought about it.

So I cooked up that 

(defun my-reset-subject-counter () 
"Reset the counter to ZERO!"
  (interactive)
  (setq my-subject-counter 0)
  (message "Now my-subject counter has been reset to ZERO!"))

(defun my-new-insert-subject-counter ()
  "Insert a string of the form [`counter/Total'] "
  (interactive)
  (save-excursion
    (setq my-subject-counter (+ my-subject-counter 1))
    (let ((total (read-string "Total number: " )))
    (message-carefully-insert-headers (list (cons 'Subject (concat "[" (int-to-string my-subject-counter) "/" total "]")))))))

Which is very useful for my workflow, thanks again.

Uwe 

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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31  6:50   ` Uwe Brauer
@ 2021-08-31  6:58     ` Adam Sjøgren
  2021-08-31 15:42       ` Uwe Brauer
  2021-08-31  7:44     ` Emanuel Berg
  1 sibling, 1 reply; 26+ messages in thread
From: Adam Sjøgren @ 2021-08-31  6:58 UTC (permalink / raw)
  To: ding

Uwe writes:

>> (defun my-insert-add-counter ()
>>   (interactive)
>>   (setq my-subject-counter (+ my-subject-counter 1))
>>   (insert (int-to-string my-subject-counter))) 
>
> Thanks very much, recursion, sigh, I should have thought about it.

Where is the recursion? It's a global variable, right?


  Best regards,

    Adam

-- 
 "Tears in waves                                            Adam Sjøgren
  Lights on fire"                                      asjo@koldfront.dk



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31  6:50   ` Uwe Brauer
  2021-08-31  6:58     ` Adam Sjøgren
@ 2021-08-31  7:44     ` Emanuel Berg
  2021-08-31  7:56       ` Emanuel Berg
  2021-08-31 16:01       ` Uwe Brauer
  1 sibling, 2 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-08-31  7:44 UTC (permalink / raw)
  To: ding

Uwe Brauer wrote:

> Thanks very much, recursion, sigh, I should have thought
> about it.

Indeed not recursion, a global/special/dynamic variable, so

(defvar my-subject-counter)
(setq   my-subject-counter 0)

> (setq my-subject-counter 0)
> (message "Now my-subject counter has been reset to ZERO!"))

(message "... %s" my-subject-counter)

> (setq my-subject-counter (+ my-subject-counter 1))

(require 'cl-lib)

(defun ...
  (cl-incf my-subject-counter) ... )

> (read-string "Total number: " )

Hm ... don't you want to automate that as well?

> (concat "[" (int-to-string my-subject-counter) "/" total "]")

(format "[%s/%s]" my-subject-counter total)

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31  7:44     ` Emanuel Berg
@ 2021-08-31  7:56       ` Emanuel Berg
  2021-08-31 16:01       ` Uwe Brauer
  1 sibling, 0 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-08-31  7:56 UTC (permalink / raw)
  To: ding

>> (read-string "Total number: " )
>
> Hm ... don't you want to automate that as well?

If you don't,

(defun my-new-insert-subject-counter (total)
  (interactive "nTotal number: ") [...]

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31  6:58     ` Adam Sjøgren
@ 2021-08-31 15:42       ` Uwe Brauer
  2021-08-31 15:54         ` Adam Sjøgren
  2021-09-01  3:55         ` for a given emacs session: insert a subject with an increasing counter Bodertz
  0 siblings, 2 replies; 26+ messages in thread
From: Uwe Brauer @ 2021-08-31 15:42 UTC (permalink / raw)
  To: ding

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>>> "AS" == Adam Sjøgren <asjo@koldfront.dk> writes:

> Uwe writes:
>>> (defun my-insert-add-counter ()
>>> (interactive)
>>> (setq my-subject-counter (+ my-subject-counter 1))
>>> (insert (int-to-string my-subject-counter))) 
>> 
>> Thanks very much, recursion, sigh, I should have thought about it.

> Where is the recursion? It's a global variable, right?

Well the defined quantity my-subject-counter contains in its definition my-subject-counter

(setq my-subject-counter 1)

Is not recursive, but 
(setq my-subject-counter (+ my-subject-counter 1))

Looks a bit recursive to me.

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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31 15:42       ` Uwe Brauer
@ 2021-08-31 15:54         ` Adam Sjøgren
  2021-09-02  8:15           ` Uwe Brauer
  2021-09-04 15:30           ` [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter) Uwe Brauer
  2021-09-01  3:55         ` for a given emacs session: insert a subject with an increasing counter Bodertz
  1 sibling, 2 replies; 26+ messages in thread
From: Adam Sjøgren @ 2021-08-31 15:54 UTC (permalink / raw)
  To: ding

Uwe writes:

> Well the defined quantity my-subject-counter contains in its
> definition my-subject-counter

Ah, yes, but setq doesn't define a quantity - it is much less abstract:
it assigns a value to a variable.

> (setq my-subject-counter 1)
>
> Is not recursive, but 
> (setq my-subject-counter (+ my-subject-counter 1))
>
> Looks a bit recursive to me.

Thanks for explaining, I was genuinely puzzled; you are using a
different definition of recursion than most people do, when it comes to
programming - where it is usually, implicitly about functions.

Unless '+' has started to call itself ;-)


  Best regards,

    Adam

-- 
 "I went for the fireengines                                Adam Sjøgren
  But they were all upside down"                       asjo@koldfront.dk



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31  7:44     ` Emanuel Berg
  2021-08-31  7:56       ` Emanuel Berg
@ 2021-08-31 16:01       ` Uwe Brauer
  2021-08-31 16:29         ` Eric Abrahamsen
  1 sibling, 1 reply; 26+ messages in thread
From: Uwe Brauer @ 2021-08-31 16:01 UTC (permalink / raw)
  To: ding

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>>> "EB" == Emanuel Berg <moasenwood@zoho.eu> writes:

> Uwe Brauer wrote:

Thanks for your advice:

I have now

(defun my-reset-subject-counter () 
  "Reset the counter to ZERO!"
  (interactive)
  (setq my-subject-counter 0)
  (message "Counter value is NOW: %s" my-subject-counter))

(defun my-new-insert-subject-counter (total)
  "Insert a string of the form [`counter/Total'] "
  (interactive "nTotal number: ")
  (save-excursion
    (setq my-subject-counter (+ my-subject-counter 1))
    (message-carefully-insert-headers (list (cons 'Subject (format "[%s/%s]" my-subject-counter total))))))

But this part I don't understand


> (require 'cl-lib)

> (defun ...
>   (cl-incf my-subject-counter) ... )

What do you suggest?

Uwe 

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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31 16:01       ` Uwe Brauer
@ 2021-08-31 16:29         ` Eric Abrahamsen
  2021-08-31 19:06           ` Uwe Brauer
  0 siblings, 1 reply; 26+ messages in thread
From: Eric Abrahamsen @ 2021-08-31 16:29 UTC (permalink / raw)
  To: ding

Uwe Brauer <oub@mat.ucm.es> writes:

>>>> "EB" == Emanuel Berg <moasenwood@zoho.eu> writes:
>
>> Uwe Brauer wrote:
>
> Thanks for your advice:
>
> I have now
>
> (defun my-reset-subject-counter () 
>   "Reset the counter to ZERO!"
>   (interactive)
>   (setq my-subject-counter 0)
>   (message "Counter value is NOW: %s" my-subject-counter))
>
> (defun my-new-insert-subject-counter (total)
>   "Insert a string of the form [`counter/Total'] "
>   (interactive "nTotal number: ")
>   (save-excursion
>     (setq my-subject-counter (+ my-subject-counter 1))
>     (message-carefully-insert-headers (list (cons 'Subject (format "[%s/%s]" my-subject-counter total))))))
>
> But this part I don't understand
>
>
>> (require 'cl-lib)
>
>> (defun ...
>>   (cl-incf my-subject-counter) ... )

The line above "increments" an integer variable "in place", in other
words that line is the exact equivalent of:

(setq my-subject-counter (+ my-subject-counter 1))

In effect, the `cl-incf' "contains" the `setq' inside it.


^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31 16:29         ` Eric Abrahamsen
@ 2021-08-31 19:06           ` Uwe Brauer
  0 siblings, 0 replies; 26+ messages in thread
From: Uwe Brauer @ 2021-08-31 19:06 UTC (permalink / raw)
  To: ding

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> Uwe Brauer <oub@mat.ucm.es> writes:

> The line above "increments" an integer variable "in place", in other
> words that line is the exact equivalent of:

> (setq my-subject-counter (+ my-subject-counter 1))

> In effect, the `cl-incf' "contains" the `setq' inside it.

Ok, thanks

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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31 15:42       ` Uwe Brauer
  2021-08-31 15:54         ` Adam Sjøgren
@ 2021-09-01  3:55         ` Bodertz
  2021-09-04 15:25           ` Uwe Brauer
  1 sibling, 1 reply; 26+ messages in thread
From: Bodertz @ 2021-09-01  3:55 UTC (permalink / raw)
  To: ding

Just for your information, if you don't want a global variable, you can
use let:

(let ((counter 0))
  (defun my-insert-add-counter ()
    (cl-incf counter)
    (insert (int-to-string counter))))



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-08-31 15:54         ` Adam Sjøgren
@ 2021-09-02  8:15           ` Uwe Brauer
  2021-09-04 15:30           ` [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter) Uwe Brauer
  1 sibling, 0 replies; 26+ messages in thread
From: Uwe Brauer @ 2021-09-02  8:15 UTC (permalink / raw)
  To: ding

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>>> "AS" == Adam Sjøgren <asjo@koldfront.dk> writes:

> Uwe writes:
>> Well the defined quantity my-subject-counter contains in its
>> definition my-subject-counter

>> (setq my-subject-counter 1)
>> 
>> Is not recursive, but 
>> (setq my-subject-counter (+ my-subject-counter 1))
> Thanks for explaining, I was genuinely puzzled; you are using a
> different definition of recursion than most people do, when it comes to
> programming - where it is usually, implicitly about functions.

Well, it is a bit like  the definition of GNU

GNU = GNU is Not Unix

So the defined object GNU is part the object that defines it. That
qualifies as recursion, for  me,  as does


      GNU                =   GNU               is Not Unix
(setq my-subject-counter (+ my-subject-counter 1))

;-) 

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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-01  3:55         ` for a given emacs session: insert a subject with an increasing counter Bodertz
@ 2021-09-04 15:25           ` Uwe Brauer
  2021-09-07 17:12             ` Emanuel Berg
  0 siblings, 1 reply; 26+ messages in thread
From: Uwe Brauer @ 2021-09-04 15:25 UTC (permalink / raw)
  To: ding

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>>> "B" == Bodertz  <bodertz@gmail.com> writes:

> Just for your information, if you don't want a global variable, you can
> use let:

> (let ((counter 0))
>   (defun my-insert-add-counter ()
>     (cl-incf counter)
>     (insert (int-to-string counter))))

Right, but it feels a bit counterintuitive: to define a function in a let. Anyhow a question of taste I presume.


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^ permalink raw reply	[flat|nested] 26+ messages in thread

* [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter)
  2021-08-31 15:54         ` Adam Sjøgren
  2021-09-02  8:15           ` Uwe Brauer
@ 2021-09-04 15:30           ` Uwe Brauer
  2021-09-07 20:04             ` Emanuel Berg
  1 sibling, 1 reply; 26+ messages in thread
From: Uwe Brauer @ 2021-09-04 15:30 UTC (permalink / raw)
  To: ding

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> Uwe writes:

> Ah, yes, but setq doesn't define a quantity - it is much less abstract:
> it assigns a value to a variable.


> Thanks for explaining, I was genuinely puzzled; you are using a
> different definition of recursion than most people do, when it comes to
> programming - where it is usually, implicitly about functions.

I realised that there was a misunderstanding. I thought you and others
suggest just to use 

(defun my-new-insert-subject-counter-old ()
  "Insert a string of the form [`counter/Total'] "
  (interactive)
  (save-excursion
    (setq my-subject-counter (+ my-subject-counter 1))
    (let ((total (read-string "Total number: " )))
    (message-carefully-insert-headers (list (cons 'Subject (format "[%s/%s]" my-subject-counter total)))))))

Without 
(defvar my-subject-counter 0) (or something like this.)

As a matter of fact I deleted the defvar definition from my code and it
still worked[1] but which puzzled me.

So I thought 
    (setq my-subject-counter (+ my-subject-counter 1))

defines and sets the variable in a recursive way, but it does not. You
need a defvar (or a surrounding let). 

I only discovered my error misunderstanding when I restarted emacs and
the code did not work anymore

Sorry for the misunderstanding.


Footnotes:
[1]  because it was already defined


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^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-04 15:25           ` Uwe Brauer
@ 2021-09-07 17:12             ` Emanuel Berg
  2021-09-07 19:02               ` Eric Abrahamsen
  0 siblings, 1 reply; 26+ messages in thread
From: Emanuel Berg @ 2021-09-07 17:12 UTC (permalink / raw)
  To: ding

Uwe Brauer wrote:

>> Just for your information, if you don't want a global
>> variable, you can use let:
>>
>> (let ((counter 0))
>>   (defun my-insert-add-counter ()
>>     (cl-incf counter)
>>     (insert (int-to-string counter))))
>
> Right, but it feels a bit counterintuitive: to define
> a function in a let.

Very counterintuitive to the point I never thought about it
and wouldn't no matter how much Lisp I ever get to do...

> Anyhow a question of taste I presume.

I don't think they are equivalent, but I'm not the right
person to explain/provide examples to illustrate ... not in
this case at least.

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 17:12             ` Emanuel Berg
@ 2021-09-07 19:02               ` Eric Abrahamsen
  2021-09-07 19:25                 ` Emanuel Berg
  0 siblings, 1 reply; 26+ messages in thread
From: Eric Abrahamsen @ 2021-09-07 19:02 UTC (permalink / raw)
  To: ding

Emanuel Berg <moasenwood@zoho.eu> writes:

> Uwe Brauer wrote:
>
>>> Just for your information, if you don't want a global
>>> variable, you can use let:
>>>
>>> (let ((counter 0))
>>>   (defun my-insert-add-counter ()
>>>     (cl-incf counter)
>>>     (insert (int-to-string counter))))
>>
>> Right, but it feels a bit counterintuitive: to define
>> a function in a let.
>
> Very counterintuitive to the point I never thought about it
> and wouldn't no matter how much Lisp I ever get to do...

That's called a "closure", because the function "closes over" the
let-bound symbol. Once evaluated, _only_ the function body has access to
that symbol: it can treat it like a globally-defined variable, but no
one else can see it. When lexical-binding is non-nil, you're always
making closures:

(setq lexical-binding t)
(lambda (arg) (message "%S" arg)) -->
  (closure (t) (arg) (message "%S" arg))

The (t) is where the closed-over symbols and their current values would
be stored, if there were any. It's sort of like the function's own
private let-form.

Eric


^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 19:02               ` Eric Abrahamsen
@ 2021-09-07 19:25                 ` Emanuel Berg
  2021-09-07 20:09                   ` Emanuel Berg
  0 siblings, 1 reply; 26+ messages in thread
From: Emanuel Berg @ 2021-09-07 19:25 UTC (permalink / raw)
  To: ding

Eric Abrahamsen wrote:

>>> Right, but it feels a bit counterintuitive: to define
>>> a function in a let.
>>
>> Very counterintuitive to the point I never thought about it
>> and wouldn't no matter how much Lisp I ever get to do...
>
> That's called a "closure", because the function "closes
> over" the let-bound symbol. Once evaluated, _only_ the
> function body has access to that symbol: it can treat it
> like a globally-defined variable, but no one else can see
> it. When lexical-binding is non-nil, you're always making
> closures:
>
> (setq lexical-binding t)
> (lambda (arg) (message "%S" arg)) -->
>   (closure (t) (arg) (message "%S" arg))
>
> The (t) is where the closed-over symbols and their current
> values would be stored, if there were any. It's sort of like
> the function's own private let-form.

But ... isn't that what you get with lexical/static `let' _in_
the function?

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter)
  2021-09-04 15:30           ` [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter) Uwe Brauer
@ 2021-09-07 20:04             ` Emanuel Berg
  0 siblings, 0 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-09-07 20:04 UTC (permalink / raw)
  To: ding

Uwe Brauer wrote:

> I realised that there was a misunderstanding. I thought you
> and others suggest just to use

I didn't know of the closure thing, it is actually an
excellent for this task in particular!

Only I still don't know what total is supposed to express,
nevermind, take a look:

(require 'cl-lib)
(require 'message)

(let ((subject-counter 0))
  (defun insert-subject-counter (total)
    (interactive "nTotal: ")
    (save-mark-and-excursion
      (message-carefully-insert-headers
       (list (cons 'Subject
                   (format "[%s/%s]"
                           (cl-incf subject-counter)
                           total) ))))))

(defalias 'iscr #'insert-subject-counter)

> Without (defvar my-subject-counter 0) (or something
> like this.)
>
> As a matter of fact I deleted the defvar definition from my
> code and it still worked[1] but which puzzled me.

This is a tricky case. No, you don't need the `defvar' to make
globals, you can do it like this

(defun make-global ()
  (setq global 5) )
;; (make-global)
;; global ; 5

I don't recommend it but it is possible. So that isn't where
the shoe hurts but here

> So I thought 
> (setq my-subject-counter (+ my-subject-counter 1))
> defines and sets the variable in a recursive way, but it
> does not. You need a defvar (or a surrounding let).

`setq' doesn't get to do that as (+ my-subject-counter 1) uses
an "void-value variable symbol" in my-subject-counter, to
paraphrase the error message ... (I think you realized that
but I say/write it anyway.)

Also, check out the `1+' and, which is even better here,
`cl-incf' functions!

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 19:25                 ` Emanuel Berg
@ 2021-09-07 20:09                   ` Emanuel Berg
  2021-09-07 20:35                     ` Emanuel Berg
  2021-09-07 20:44                     ` Eric Abrahamsen
  0 siblings, 2 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-09-07 20:09 UTC (permalink / raw)
  To: ding

>> That's called a "closure", because the function "closes
>> over" the let-bound symbol. Once evaluated, _only_ the
>> function body has access to that symbol: it can treat it
>> like a globally-defined variable, but no one else can see
>> it. When lexical-binding is non-nil, you're always making
>> closures:
>>
>> (setq lexical-binding t)
>> (lambda (arg) (message "%S" arg)) -->
>>   (closure (t) (arg) (message "%S" arg))
>>
>> The (t) is where the closed-over symbols and their current
>> values would be stored, if there were any. It's sort of
>> like the function's own private let-form.
>
> But ... isn't that what you get with lexical/static `let'
> _in_ the function?

Ah, now I understand what you mean. With `let' inside, the
variable will reset, so it cannot count, but with `let'
outside (the closure), as you say "it can treat it like
a globally-defined variable".

"globally-defined", very good!

Oh, no!

I didn't know of this (never seen it). I have used global
variables to hold the "state" (be a memory between function
calls) but I've also done more involved solutions like
properties and even self-modifying code :O

And all the while, it was this easy ...

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 20:09                   ` Emanuel Berg
@ 2021-09-07 20:35                     ` Emanuel Berg
  2021-09-07 20:44                     ` Eric Abrahamsen
  1 sibling, 0 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-09-07 20:35 UTC (permalink / raw)
  To: ding

> Ah, now I understand what you mean. With `let' inside, the
> variable will reset, so it cannot count, but with `let'
> outside (the closure), as you say "it can treat it like
> a globally-defined variable".

But now, the byte-compiler will complain

  Warning: the function ‘insert-subject-counter’ is not known
  to be defined.

so now we need not `defvar' but "deffunc" to make it
explicitly global/dynamic :P

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 20:09                   ` Emanuel Berg
  2021-09-07 20:35                     ` Emanuel Berg
@ 2021-09-07 20:44                     ` Eric Abrahamsen
  2021-09-21  3:32                       ` Emanuel Berg
  1 sibling, 1 reply; 26+ messages in thread
From: Eric Abrahamsen @ 2021-09-07 20:44 UTC (permalink / raw)
  To: ding

Emanuel Berg <moasenwood@zoho.eu> writes:

>>> That's called a "closure", because the function "closes
>>> over" the let-bound symbol. Once evaluated, _only_ the
>>> function body has access to that symbol: it can treat it
>>> like a globally-defined variable, but no one else can see
>>> it. When lexical-binding is non-nil, you're always making
>>> closures:
>>>
>>> (setq lexical-binding t)
>>> (lambda (arg) (message "%S" arg)) -->
>>>   (closure (t) (arg) (message "%S" arg))
>>>
>>> The (t) is where the closed-over symbols and their current
>>> values would be stored, if there were any. It's sort of
>>> like the function's own private let-form.
>>
>> But ... isn't that what you get with lexical/static `let'
>> _in_ the function?
>
> Ah, now I understand what you mean. With `let' inside, the
> variable will reset, so it cannot count, but with `let'
> outside (the closure), as you say "it can treat it like
> a globally-defined variable".

That's right.

> "globally-defined", very good!
>
> Oh, no!
>
> I didn't know of this (never seen it). I have used global
> variables to hold the "state" (be a memory between function
> calls) but I've also done more involved solutions like
> properties and even self-modifying code :O
>
> And all the while, it was this easy ...

TBH, I've never actually used a closure in anger, probably just because
it doesn't occur to me. I'll bet if I went back and looked over the code
I've written in the past I could find some situations where I could have
used them, but... *shrug*


^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-07 20:44                     ` Eric Abrahamsen
@ 2021-09-21  3:32                       ` Emanuel Berg
  2021-09-21 19:12                         ` Bodertz
  0 siblings, 1 reply; 26+ messages in thread
From: Emanuel Berg @ 2021-09-21  3:32 UTC (permalink / raw)
  To: ding

Eric Abrahamsen wrote:

>> Oh, no!
>>
>> I didn't know of this (never seen it). I have used global
>> variables to hold the "state" (be a memory between function
>> calls) but I've also done more involved solutions like
>> properties and even self-modifying code :O
>>
>> And all the while, it was this easy ...
>
> TBH, I've never actually used a closure in anger, probably
> just because it doesn't occur to me. I'll bet if I went back
> and looked over the code I've written in the past I could
> find some situations where I could have used them, but...
> *shrug*

I think I heard that you shouldn't use, or that you should
avoid, global variables the first time when I was 12.
I understood it enough to stick by it and I guess my code was
good to me 5-6 days out of 7 at least.

However it would have been even better if it had been
formulized like this ...

  When the situation is <something>, while a global variable
  would solve it, <something else> is preferable ...

So, doing now what we were unable to do at age 12, we can
formulate one such rule:

  When the situation is like that, that a function needs
  a state or local memory, which cannot be reset from one
  function call to the other, use a closure!

It is easy to add rules ...

When the value has the nature of an user option, a global
variable is fine ...

But what about when two (or more) functions need access to the
same variable, how should that be done? Local functions
(`cl-labels'), parameters, evaluation - don't know, is there
a short answer? - or can one have several functions in a/one
closure `let'?

Aaanyway, I think the best way to approach this is to grep the
source for `defvar' (or baseline `setq'), prune the options,
and then ...

(Unbelievable that I never read this in a single computer book
of all the tons I've read. Unbelievable people who write
them. Or?)

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-21  3:32                       ` Emanuel Berg
@ 2021-09-21 19:12                         ` Bodertz
  2021-09-21 21:56                           ` Emanuel Berg
  0 siblings, 1 reply; 26+ messages in thread
From: Bodertz @ 2021-09-21 19:12 UTC (permalink / raw)
  To: ding

Emanuel Berg <moasenwood@zoho.eu> writes:

> or can one have several functions in a/one closure `let'?

You can.  A simple example:


    (let ((counter 0))
      (defun decrement-counter ()
        (cl-decf counter))

      (defun increment-counter ()
        (cl-incf counter))

      (defun reset-counter ()
        (setq counter 0)))


> (Unbelievable that I never read this in a single computer book
> of all the tons I've read. Unbelievable people who write
> them. Or?)

There is a book called 'Let Over Lambda' which is named after this
concept: the 'let' is over the 'lambda' (or 'defun', in this case).  I
haven't read it, but it's how I heard of this idea.



^ permalink raw reply	[flat|nested] 26+ messages in thread

* Re: for a given emacs session: insert a subject with an increasing counter
  2021-09-21 19:12                         ` Bodertz
@ 2021-09-21 21:56                           ` Emanuel Berg
  0 siblings, 0 replies; 26+ messages in thread
From: Emanuel Berg @ 2021-09-21 21:56 UTC (permalink / raw)
  To: ding

Bodertz wrote:

>> or can one have several functions in a/one closure `let'?
>
> You can.  A simple example:
>
>     (let ((counter 0))
>       (defun decrement-counter ()
>         (cl-decf counter))
>
>       (defun increment-counter ()
>         (cl-incf counter))
>
>       (defun reset-counter ()
>         (setq counter 0)))

Sweet!

That example may be simple but it is exemplary nonetheless!

Did you ever consider writing a book on computing?

:)

>> (Unbelievable that I never read this in a single computer
>> book of all the tons I've read. Unbelievable people who
>> write them. Or?)
>
> There is a book called 'Let Over Lambda' which is named
> after this concept: the 'let' is over the 'lambda' (or
> 'defun', in this case). I haven't read it, but it's how
> I heard of this idea.

Interesting title, I'd love to read it ...

I did Lisp at the university ("Advanced Functional
Programming", 5 ECTS in 2013-02-01) - that course also
included Erlang and Haskell - Erlang because of
modularity/concurrency I guess, and Haskell because of
rigidity/modernity - again I guess.

I don't consider Lisp functional, or rather, it is functional
and everything else - or it can be, at least. It is
multi-paradigm but even that doesn't feel right because when
Lisp came (1958, USA; CL 1984, Elisp 1985) at that time there
were no programming paradigms.

Which BTW IMO are, at the best, thought models to enhance our
understanding, not a bunch of rules to follow.

Anyway, at that course there were some material, but I lost it
somewhere between the Second and Third Impact. Other than
that, I've read the Emacs 18 manual and three books on Lisp,
namely

@book{land-of-lisp,
  author     = {Conrad Barski},
  isbn       = 1593272812,
  publisher  = {No Starch},
  title      = {Land of Lisp},
  year       = {2010}
}

@book{lispcraft,
  author     = {Robert Wilensky},
  isbn       = 0393954420,
  publisher  = {Norton},
  title      = {LISPcraft},
  year       = {1984}
}

(the third one doesn't seem to be in the register, anyway it
was in Swedish so of less use even to "us" these days)

Conrad Barski's book is very good! (Searched, but didn't find
his e-mail; feel free to CC this to him.)

https://dataswamp.org/~incal/books/books.bib

-- 
underground experts united
https://dataswamp.org/~incal



^ permalink raw reply	[flat|nested] 26+ messages in thread

end of thread, other threads:[~2021-09-21 21:56 UTC | newest]

Thread overview: 26+ messages (download: mbox.gz / follow: Atom feed)
-- links below jump to the message on this page --
2021-08-30 16:01 for a given emacs session: insert a subject with an increasing counter Uwe Brauer
2021-08-30 16:10 ` Eric S Fraga
2021-08-30 18:46   ` Eric Abrahamsen
2021-08-31  6:50   ` Uwe Brauer
2021-08-31  6:58     ` Adam Sjøgren
2021-08-31 15:42       ` Uwe Brauer
2021-08-31 15:54         ` Adam Sjøgren
2021-09-02  8:15           ` Uwe Brauer
2021-09-04 15:30           ` [misunderstanding] (was: for a given emacs session: insert a subject with an increasing counter) Uwe Brauer
2021-09-07 20:04             ` Emanuel Berg
2021-09-01  3:55         ` for a given emacs session: insert a subject with an increasing counter Bodertz
2021-09-04 15:25           ` Uwe Brauer
2021-09-07 17:12             ` Emanuel Berg
2021-09-07 19:02               ` Eric Abrahamsen
2021-09-07 19:25                 ` Emanuel Berg
2021-09-07 20:09                   ` Emanuel Berg
2021-09-07 20:35                     ` Emanuel Berg
2021-09-07 20:44                     ` Eric Abrahamsen
2021-09-21  3:32                       ` Emanuel Berg
2021-09-21 19:12                         ` Bodertz
2021-09-21 21:56                           ` Emanuel Berg
2021-08-31  7:44     ` Emanuel Berg
2021-08-31  7:56       ` Emanuel Berg
2021-08-31 16:01       ` Uwe Brauer
2021-08-31 16:29         ` Eric Abrahamsen
2021-08-31 19:06           ` Uwe Brauer

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