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

[Michael Heerdegen]

Damien Cassou <damien@cassou.me> writes:

> here is a patch adding seq-positions to seq.el.

+@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}.  If the optional argument
+@var{testfn} is non-@code{nil}, it is a function of two arguments to
+use instead of the default @code{equal}.

We do not need to limit this to equivalence relations.  A TESTFUN of, say,
(apply-partially #'<= 10) could be similarly useful.

+;;;###autoload
+(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."
+  (let ((result '())
+        (index 0))
+    (seq-doseq (e sequence)
+      (when (funcall (or testfn #'equal) e elt)
+        (push index result))
+      (setq index (1+ index)))
+    (nreverse result)))

Could this maybe (simpler) call `seq-do-indexed'?


TIA,

Michael.