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.