Re: [PATCH] Gnu Elpa: stream.el: Add some more basic stream operations

[Michael Heerdegen <michael_heerdegen@web.de>]

Nicolas Petton <nicolas@petton.fr> writes:

> > And this is how that looks.  WDYT?
> >
> > (defun stream-divide-with-get-rest-fun (stream get-rest-fun)
>
> Is this function public on purpose?
>
> > (defun stream-partition-with-get-rest-fun (stream get-rest-fun)
>
> Same question :)

Yes, it's public on purpose.  They have ugly names (maybe we find better
ones?), but they are useful per se as the more general tool.  I think
they are also quite nice to use, because the creation of streams is
factored out, and you can pass the essence of the calculation as a
function argument.

And `stream-divide' and `stream-partition' no doubt don't cover all use
cases one can think of.  Looking at two subsequent elements at the same
time as in `stream-divide' gives you flexibility, but the first part of
`stream-divide' will always have at least one element.  This will not
fit in some cases.

Here is another use case where I think `stream-divide-with-get-rest-fun'
suits better: I want to have the last 3 Fibonacci number less than 1000.
This is how I would do it:

#+begin_src emacs-lisp
;; -*- lexical-binding: t -*-
(let ((count 3) fibs)
  (setq fibs (stream-make (cons 1 (stream-scan #'+ 1 fibs))))
  (seq-into-sequence
   (seq-take
    (cadr
     (stream-divide-with-get-rest-fun
      fibs
      (lambda (stream)
        (let ((rest stream))
          (dotimes (_ count) (stream-pop stream))
          (while (< (stream-pop stream) 1000)
            (stream-pop rest))
          rest))))
    count)))
#+end_src

You could also do the same with `stream-divide', but it would rather
look more complicated.


Michael.