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

[Michael Heerdegen]

Yuri Khan <address@hidden> writes:

> A left fold which forces the intermediate result before recurring has
> good space complexity, but can only be lazy in the values of the
> elements, not their number.

AFAIK we only have left fold in seq.el (a right fold wouldn't make sense
for infinite streams anyway).  Every calculation is turned into a loop
where possible, since Emacs Lisp sucks at recursion.

If we had `seq-take-until', one could use that to stop folding at an
absorbing element, like in

#+begin_src emacs-lisp
(seq-reduce
 #'*
 (seq-take-until
  (lambda (x) (not (zerop x)))
  (stream-range -3)) ;-3, -2, -1, 0, 1, 2,...
 1)

==> 0
#+end_src

Would it make sense to add an `seq-take-until' for convenience?


> https://wiki.haskell.org/Foldr_Foldl_Foldl'

Note for readers from the future: the quote at the end is part of the
url.


Regards,

Michael.