Re: What's missing in ELisp that makes people want to use cl-lib?

[Dmitry Gutov]

On 15/11/2023 03:07, João Távora wrote:
> On Wed, Nov 15, 2023 at 12:46 AM Dmitry Gutov <dmitry@gutov.dev> wrote:
>
> > Then it must have a seq-do implementation that yields :secret-voodoo one
> > or several times.
>
> Not really.  Only if I use my seq in contexts where I need that
> generic, and being second argument to seq-difference doesn't count.
> So why should I bother implementing that when my code is working
> fine today?

Commentary at the top of seq.el says:

;; seq.el can be extended to support new type of sequences.  Here are
;; the generic functions that must be implemented by new seq types:
;; - `seq-elt'
;; - `seq-length'
;; - `seq-do'
;; - `seqp'
;; - `seq-subseq'
;; - `seq-into-sequence'
;; - `seq-copy'
;; - `seq-into'

> But say I did that seq-do, then what is the seq-contains-p generic good
> for then?  Why some many redundant generics that the user learns the
> hard way have to be consistent with each other?

So any new type gets a lot of functions implemented very easily, and 
could add specialized implementations for performance.

> > And then the seq-contains-pred implementation will do
> > right by it (the default one, not the specialization for lists).
> >
> > > > This is not documented,
> > >
> > > You don't say? :-)  i think we should get started on documenting all
> > > these ad-hoc rules (sorry but in this case they are quite literally
> > > ad-hoc).  Better than nothing, I guess, since as to actually enforcing
> > > them, I think that train has left the station a long time ago and is
> > > now in a voodoo swamp in monkey island.
> >
> > Not sure "enforcing them" is the right phrase: I wonder if there's ever
> > been a change proposed to seq.el which would break them.
>
> I think it's pretty easy to do, especially if these optimziations
> want going to go all the way to cl-lib's easier-to-get optimal
> performance.  What about this?  Is this toy implementation of
> compressed space-saving lists legal?
>
> (cl-defmethod seq-do (function (l (head :m6-sparse)))
>    (mapc (lambda (e)
>            (if (eq e :double-o-seven)
>                (progn (funcall function 0)
>                       (funcall function 0)
>                       (funcall function 7))
>              (funcall function e)))
>          (cdr l)))
>
> (cl-defmethod seq-reverse ((l (head :m6-sparse)))
>    (let (res)
>      (seq-do (lambda (e) (push e res)) l)
>      res))
>
> (seq-difference '(:m6-sparse 1 2 :double-o-seven 4) '(7)) ;; => (1 2 0 0 4)
>
> Again, not implementing all the generics, just the ones I need
> to get my desired effect, which in this case is passing a
> custom sequence type as a first argument to seq-difference.

Very clever. I'm not thinking too well this time of night, but the first 
part of the reply in this email probably answers the question (there is 
a certain set of generics that must be implemeneted).