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From: Eric Abrahamsen <eric@ericabrahamsen.net>
To: ding@gnus.org
Subject: Re: for a given emacs session: insert a subject with an increasing counter
Date: Tue, 07 Sep 2021 12:02:35 -0700	[thread overview]
Message-ID: <87fsuggqes.fsf@ericabrahamsen.net> (raw)
In-Reply-To: <87ilzcnwc3.fsf@zoho.eu> (Emanuel Berg's message of "Tue, 07 Sep 2021 19:12:44 +0200")

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


  reply	other threads:[~2021-09-07 19:02 UTC|newest]

Thread overview: 26+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-08-30 16:01 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 [this message]
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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