Subject: Re: bug#34708: alist-get has unclear documentation

From: Michael Heerdegen <[email-masked]>
Date: Fri, 15 Mar 2019 16:54:51 +0100

Michael Heerdegen <[email-masked]> 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 <[email-masked]>
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)
--
2.20.1

Michael.