Here's a second, cleaner attempt that separates the `cl--loop-handle-accum`
function into two functions, one to deal with lists and one to deal w/
non-lists.

The tail-tracking optimizing is also applied to `append(ing)` and
`nconc(ing)`.

On Sun, Apr 8, 2018 at 4:29 PM, Tianxiang Xiong <tianxiang.xiong@gmail.com>
wrote:

> One thing I don't understand is the common
>
> (push `(progn (setq ...) t) cl--loop-body)
>
> pattern found in the code. I'm not sure why the `(progn ... t)` is
> necessary. If anyone could explain that I'd add it as a comment.
>
> On Sun, Apr 8, 2018 at 2:13 PM, Stefan Monnier <monnier@iro.umontreal.ca>
> wrote:
>
>> > Avoid O(n^2) nconc-ing by keeping track of tail of collection.
>>
>> I took a quick look at your patch, and it looks pretty good.
>> See comments below.
>>
>>
>>         Stefan
>>
>>
>> >       ((memq word '(collect collecting))
>> > -      (let ((what (pop cl--loop-args))
>> > -         (var (cl--loop-handle-accum nil 'nreverse)))
>> > -     (if (eq var cl--loop-accum-var)
>> > -         (push `(progn (push ,what ,var) t) cl--loop-body)
>> > -       (push `(progn
>> > -                   (setq ,var (nconc ,var (list ,what)))
>> > -                   t)
>> > -                cl--loop-body))))
>> > +      (let ((what (pop cl--loop-args)))
>> > +        (cl-multiple-value-bind (var var-tail)
>>
>> `cl-multiple-value-bind` is the "destructor" corresponding to the
>> `cl-values` "constructor".   Since your code doesn't use `cl-values` it
>> should not use `cl-multiple-value-bind` either (you probably meant to
>> use cl-destructuring-bind instead).
>>
>> > +            (cl--loop-handle-accum nil 'nreverse)
>> > +          (if (eq var cl--loop-accum-var)
>> > +              (push `(progn (push ,what ,var) t) cl--loop-body)
>> > +            (push `(progn
>> > +                     (if (null ,var-tail)
>> > +                         (setq ,var (list ,what) ,var-tail (last ,var))
>> > +                       (setq ,var-tail (setcdr ,var-tail (list
>> ,what))))
>> > +                     t)
>> > +                  cl--loop-body)))))
>>
>> The cl-loop macro's code lacks comments.  Could you take advantage of
>> "being there" to try and add comments?  E.g. in the above code I see
>> that depending on (eq var cl--loop-accum-var) we end up accumulating in
>> the from or in the back.  Could you add a comments explaining why and
>> mentioning where we correct this discrepancy?
>>
>> > +      (let ((what (pop cl--loop-args)))
>> > +     (cl-destructuring-bind (var) (cl--loop-handle-accum nil 'nreverse)
>> > +          (push `(progn
>> > +                   (setq ,var
>> > +                         ,(if (eq var cl--loop-accum-var)
>> > +                              `(nconc
>> > +                                (,(if (memq word '(nconc nconcing))
>> > +                                      #'nreverse #'reverse)
>> > +                                 ,what)
>> > +                                ,var)
>> > +                            `(,(if (memq word '(nconc nconcing))
>> > +                                   #'nconc #'append)
>> > +                              ,var ,what)))
>> > +                   t)
>> > +                cl--loop-body))))
>>
>> In the `nconc` case (when (eq var cl--loop-accum-var) is nil) we could
>> also use the `var-tail` to speed up the `nconc`.
>>
>> Also, to avoid the N² behavior for the `append` case, maybe we
>> could/should make it use `copy-sequence`, i.e.
>>
>>     `(nconc ,var-tail (copy-sequence ,what))
>>
>> > -(defun cl--loop-handle-accum (def &optional func) ; uses loop-*
>> > -  (if (eq (car cl--loop-args) 'into)
>> > +(defun cl--loop-handle-accum (def &optional func type) ; uses loop-*
>> > +  (setq type (or type 'list))
>>
>> Please add a docstring explaining whatever you managed to understand of
>> this code, and describing also what this new arg `type` does.
>>
>>
>>
>