bug#57548: Add new function `seq-positions'

[Eli Zaretskii]

> From: Damien Cassou <damien@cassou.me>
> Date: Fri, 02 Sep 2022 20:43:24 +0200
> 
> +@defun seq-positions sequence elt &optional testfn
> +  This function returns a list of the positions of the elements in
> +@var{sequence} that are equal to @var{elt}.

We use "index", not "position".  In any case, the documentation should
explain what you mean by that, and it should definitely say that the
index/position are zero-based.

> +@example
> +@group
> +(seq-positions '(a b c a d) 'a)
> +@result{} (0 3)
> +@end group
> +@group
> +(seq-position '(a b c a d) 'z)
> +@result{} nil

seq-position or seq-positions?

> +(cl-defgeneric seq-positions (sequence elt &optional testfn)
> +  "Return a list of the positions of ELT in SEQ.
> +Equality is defined by TESTFN if non-nil or by `equal' if nil."

Our style is to say

  Equality is defined by the function TESTFN, which defaults to `equal'.

> --- a/test/lisp/emacs-lisp/seq-tests.el
> +++ b/test/lisp/emacs-lisp/seq-tests.el
> @@ -482,6 +482,11 @@ test-seq-position
>      (should (= (seq-position seq 'a #'eq) 0))
>      (should (null (seq-position seq (make-symbol "a") #'eq)))))
>  
> +(ert-deftest test-seq-positions ()
> +  (with-test-sequences (seq '(1 2 3 1 4))
> +    (should (equal '(0 3) (seq-positions seq 1)))
> +    (should (seq-empty-p (seq-positions seq 9)))))

Should we test more than just one type of sequences?

Thanks.