bug#34708: alist-get has unclear documentation

[Eric Abrahamsen]

Michael Heerdegen <michael_heerdegen@web.de> writes:

> Michael Heerdegen <michael_heerdegen@web.de> writes:
>
>> Actually, it's not even trivial at all.  Setting alist-get as place
>> could also build a new alist, leaving the one stored in the ALIST place
>> intact, and set place ALIST to the new list.  But it potentially
>> modifies the list stored in place ALIST.  That's not inevitable, so it
>> could be worth telling that.
>
> I tried that - see the draft at the end of the message.  And I tried to
> cover everything that has been noted here to some degree while still
> keeping the docstring fluent and at an acceptable length.
>
> I did not touch the manual, maybe someone else wants to give that a try?
>
> From 3b8ef1ee9d9a5001e2de930ec34e3bdd3d4f87ad Mon Sep 17 00:00:00 2001
> From: Michael Heerdegen <michael_heerdegen@web.de>
> Date: Tue, 12 Mar 2019 15:13:55 +0100
> Subject: [PATCH] WIP: Improve documentation of alist-get
>
> (Bug#34708)...
> ---
>  lisp/subr.el | 26 ++++++++++++++++++++++++--
>  1 file changed, 24 insertions(+), 2 deletions(-)
>
> diff --git a/lisp/subr.el b/lisp/subr.el
> index 4024c68e68..8ea8fb602c 100644
> --- a/lisp/subr.el
> +++ b/lisp/subr.el
> @@ -756,9 +756,31 @@ alist-get
>  If KEY is not found in ALIST, return DEFAULT.
>  Use TESTFN to lookup in the alist if non-nil.  Otherwise, use `assq'.
>
> -This is a generalized variable suitable for use with `setf'.
> +You can use `alist-get' in PLACE expressions.  This will modify
> +an existing association (more precisely, the first one if
> +multiple exist), or add a new element to the beginning of ALIST,
> +destructively modifying the list stored in ALIST.
> +
> +Example:
> +
> +   (setq foo '((a . 0)))
> +   (setf (alist-get 'a foo) 1
> +         (alist-get 'b foo) 2)
> +
> +   foo ==> ((b . 2) (a . 1))
> +
> +
>  When using it to set a value, optional argument REMOVE non-nil
> -means to remove KEY from ALIST if the new value is `eql' to DEFAULT."
> +means to remove KEY from ALIST if the new value is `eql' to
> +DEFAULT (more precisely the first found association will be
> +deleted from the alist).
> +
> +Example:
> +
> +  (setq foo '((a . 1) (b . 2)))
> +  (setf (alist-get 'b foo nil 'remove) nil)
> +
> +  foo ==> ((a . 1))"
>    (ignore remove) ;;Silence byte-compiler.
>    (let ((x (if (not testfn)
>                 (assq key alist)

I like it! Thanks for your work on this.