GNU bug report logs - #34852
26.1; seq-intersection ignores nil as element

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From: Michael Heerdegen <michael_heerdegen <at> web.de>
To: "Basil L. Contovounesios" <contovob <at> tcd.ie>
Cc: Nicolas Petton <nicolas <at> petton.fr>, 34852 <at> debbugs.gnu.org, Stefan Monnier <monnier <at> iro.umontreal.ca>, "Miguel V. S. Frasson" <mvsfrasson <at> gmail.com>
Subject: bug#34852: 26.1; seq-intersection ignores nil as element
Date: Thu, 14 Mar 2019 14:09:15 +0100

[Message part 1 (text/plain, inline)]

"Basil L. Contovounesios" <contovob <at> tcd.ie> writes:

> One solution is to leave seq-contains as it is, and switch to using
> seq-position (or some new predicate) as a predicate instead.  Another is
> to make seq-contains return a boolean instead of the needle found, which
> would be a backward-incompatible change similar to that for
> map-contains-key.  I attach a patch for each of these respective
> solutions; WDYT?

We also have `seq-some' which can be used as a contain predicate,
e.g. to fix this bug:

[0001-Fix-seq-intersection-with-nil.patch (text/x-diff, inline)]

From c53a80c29e696ab64d4279ca6f495c8e0e1b16b4 Mon Sep 17 00:00:00 2001
From: Michael Heerdegen <michael_heerdegen <at> web.de>
Date: Thu, 14 Mar 2019 13:55:41 +0100
Subject: [PATCH] Fix seq-intersection with nil

---
 lisp/emacs-lisp/seq.el | 13 +++++++------
 1 file changed, 7 insertions(+), 6 deletions(-)

diff --git a/lisp/emacs-lisp/seq.el b/lisp/emacs-lisp/seq.el
index 4a811d7895..5718343a8f 100644
--- a/lisp/emacs-lisp/seq.el
+++ b/lisp/emacs-lisp/seq.el
@@ -409,12 +409,13 @@ seq-sort-by
 (cl-defgeneric seq-intersection (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in both SEQUENCE1 and SEQUENCE2.
 Equality is defined by TESTFN if non-nil or by `equal' if nil."
-  (seq-reduce (lambda (acc elt)
-                (if (seq-contains sequence2 elt testfn)
-                    (cons elt acc)
-                  acc))
-              (seq-reverse sequence1)
-              '()))
+  (let ((testfn (or testfn #'equal)))
+    (seq-reduce (lambda (acc elt)
+                  (if (seq-some (apply-partially testfn elt) sequence2)
+                      (cons elt acc)
+                    acc))
+                (seq-reverse sequence1)
+                '())))

 (cl-defgeneric seq-difference (sequence1 sequence2 &optional testfn)
   "Return a list of the elements that appear in SEQUENCE1 but not in SEQUENCE2.
--
2.20.1

[Message part 3 (text/plain, inline)]

(probably needed in more places as you did in one of your patches)

So I think we don't necessarily need something new or a backward
incompatible change.


Michael.

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