From: "Štěpán Němec" <stepnem@gmail.com>
To: emacs-devel@gnu.org
Subject: seq-union calling seq-reverse? (Emacs master 0cf0a2b98671: Add new sequence function 'seq-union')
Date: Fri, 17 Sep 2021 13:13:28 +0200 [thread overview]
Message-ID: <20210917131328+0200.462267-stepnem@gmail.com> (raw)
diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index f0dc283f57d5..b7dcde87f412 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -467,6 +467,17 @@ seq-partition
(setq sequence (seq-drop sequence n)))
(nreverse result))))
+(cl-defgeneric seq-union (sequence1 sequence2 &optional testfn)
+ "Return a list of all elements that appear in either SEQUENCE1 or SEQUENCE2.
+Equality is defined by TESTFN if non-nil or by `equal' if nil."
+ (let ((accum (lambda (acc elt)
+ (if (seq-contains-p acc elt testfn)
+ acc
+ (cons elt acc)))))
+ (seq-reverse
+ (seq-reduce accum sequence2
+ (seq-reduce accum sequence1 '())))))
+
`seq-reverse' seems gratuitous here WRT the purpose of `seq-union'.
Given that this is a set operation, it makes more sense not to waste
resources on element order.
(
For comparison, both Common Lisp (n)union [1] and SRFI-1 lset-union [2]
leave the element order unspecified, and e.g. in SBCL:
* (union '(a b c) '(f a d))
(C B F A D)
[1] http://www.lispworks.com/documentation/HyperSpec/Body/f_unionc.htm
[2] https://srfi.schemers.org/srfi-1/srfi-1.html
)
--
Štěpán
next reply other threads:[~2021-09-17 11:13 UTC|newest]
Thread overview: 3+ messages / expand[flat|nested] mbox.gz Atom feed top
2021-09-17 11:13 Štěpán Němec [this message]
2021-09-17 12:18 ` seq-union calling seq-reverse? (Emacs master 0cf0a2b98671: Add new sequence function 'seq-union') Stefan Kangas
2021-09-17 12:25 ` Stefan Monnier
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