Re: alist-get in Emacs 24?

[Marcin Borkowski]

On 2015-10-02, at 09:34, Michael Heerdegen <michael_heerdegen@web.de> wrote:

> Marcin Borkowski <mbork@mbork.pl> writes:
>
>> I will probably just write functions like set-alist-element and
>> inc-alist-element (or maybe I'll name them better) so that the library
>> works in Emacs 24, but is there a better way?
>
> There was assoc.el in prior Emacsen (and it's still lying around in
> lisp/obsolete).  `aput' is its setter for alists.  Works great, but is
> obsolete.
>
> A different approach is to use `assoc', `delq' and `push', i.e., do it
> by hand.

OK, so I did it by hand (though without delq or push).  I am not
extremely happy with my solution, though, since in the setter function
I have to pass the alist variable name as a symbol.  (This is needed
because I might want to use it to introduce a new key-value pair into
the list.)  Does anyone have any suggestion for enhancing my solution?

--8<---------------cut here---------------start------------->8---
(require 'cl)

(defun my-alist-get (key alist)
  "Return the value associated to KEY in ALIST.
This function is needed for Emacsen older than v25."
  (cdr (assoc key alist)))

(defun my-set-alist-value (key alist value)
  "Set the value corresponding to KEY in ALIST to VALUE.
Note: ALIST should be a symbol.  This is morally equivalent to
`(setf (alist-get key (symbol-value alist)) value)',
but works in older Emacsen."
  (let ((pair (assoc key (symbol-value alist))))
    (if pair
        (setcdr pair value)
      (set alist (acons key value (symbol-value alist))))))

(defun my-inc-alist-value (key alist increment)
  "Increment the value corresponding to KEY in ALIST by INCREMENT.
Throw an error if KEY is not in ALIST."
  (let ((pair (assoc key alist)))
    (if pair
        (incf (cdr pair) increment)
      (error "Nothing to increment"))))
--8<---------------cut here---------------end--------------->8---

Best,

-- 
Marcin Borkowski
http://octd.wmi.amu.edu.pl/en/Marcin_Borkowski
Faculty of Mathematics and Computer Science
Adam Mickiewicz University