From: Michael Heerdegen <michael_heerdegen@web.de>
Date: Tue, 12 Mar 2019 14:13:55 +0000 (+0100)
Subject: Improve documentation of 'alist-get' (Bug#34708)
X-Git-Tag: emacs-27.0.90~3315
X-Git-Url: http://git.eshelyaron.com/gitweb/?a=commitdiff_plain;h=6dfd28c3201719cc284183a74dc1cc1344410905;p=emacs.git

Improve documentation of 'alist-get' (Bug#34708)

* lisp/subr.el (alist-get): Enhance part of docstring explaining usage
in place expressions.
---

diff --git a/lisp/subr.el b/lisp/subr.el
index 6dc53cd7201..f1a1dddd81c 100644
--- a/lisp/subr.el
+++ b/lisp/subr.el
@@ -756,9 +756,31 @@ Elements of ALIST that are not conses are ignored."
 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)