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bug#35250: Add simplest Advice example possible first
From: |
Lars Ingebrigtsen |
Subject: |
bug#35250: Add simplest Advice example possible first |
Date: |
Tue, 09 Jul 2019 17:06:52 +0200 |
User-agent: |
Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux) |
積丹尼 Dan Jacobson <jidanni@jidanni.org> writes:
> (info "(elisp) Advising Functions") starts out with a complicated
> example.
>
> For example, in order to trace the calls to the process filter of a
> process PROC, you could use:
>
> (defun my-tracing-function (proc string)
> (message "Proc %S received %S" proc string))
>
> (add-function :before (process-filter PROC) #'my-tracing-function)
>
> Please first add the simplest example possible:
>
> Function A prints "a".
>
> Make it print "ab" from now on.
I've never used nadvice before, and I have to say that I found that
section a bit confusing, too, because my immediate response was (like
Dan's) to say "well, it should be trivial to make an advice that just
modifies the output", so I thought "well, :around has to be it".
But it isn't, :around is the super-flexible one:
(defun my-foo (x)
(* x 2))
(defun my-advice (old-fun x)
(+ (funcall old-fun x) 1))
(advice-add 'my-foo :around #'my-advice)
(my-foo 3)
=> 7
Instead :filter-return is it, and it's the one mentioned last. So I'm
adding this to the manual as an example first:
(defun my-double (x)
(* x 2))
(defun my-increase (x)
(+ x 1))
(advice-add 'my-double :filter-return #'my-increase)
(my-double 3)
7
--
(domestic pets only, the antidote for overdose, milk.)
bloggy blog: http://lars.ingebrigtsen.no
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