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bug#34852: 26.1; seq-intersection ignores nil as element

From: Stefan Monnier
Subject: bug#34852: 26.1; seq-intersection ignores nil as element
Date: Thu, 14 Mar 2019 22:40:27 -0400
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/27.0.50 (gnu/linux)

>> > (seq-contains nil '(nil t foo) (lambda (x) t))  ->  nil
>> > It returns *nil* if testfn fails, or *nil* (ELT) if it succeds.
>> Just because it sometimes does doesn't mean it always does:
>>     (seq-contains '(0 3 4) 1 #'<)
>> returns 3
> Maybe it's too late here, but why is that a counterexample for
>> If ELT=nil, seq-contains currently returns nil anyway;"

Duh, sorry, I lost sight of the goal along the way.

A short real counter example is

    (seq-contains '(1) nil #'list)

but it's admittedly contrived.  A more real counter example is the one
alluded to by Nicolas earlier:

    (seq-contains '((t 1) (nil 2)) nil (lambda (x y) (equal x (car y))))

> The first element is returned for that the test succeeds, what's special
> about it?  < is not symmetrical, though, so it's a weird kind of
> equality test.

The test doesn't need to be an equality test.
IOW seq-contains is very close to seq-find.


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