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

[Dmitry Gutov]

On 14/11/2023 14:14, João Távora wrote:
> > The way I understand this, is any new sequence type has to implement
> > seq-do. As soon as that happens, a lot of (probably all) sequence
> > functions in seq.el start working on that type.
> Right.  But seq-difference-3 doesn't call into seq-do anymore,
> at least not always and not in the same way.  So today,_before_
> your seq-difference "skips-the-seq-do" optimization, a given person's
> set difference operations would work just fine for their custom
> data type.  The person is happy and doesn't care about seq.el's
> performance.

seq-difference-3 calls seq-filter which calls seq-map, which in turn 
delegates to seq-do (or a specialized implementation).

There is a bug in my prototype patch -- the absence of the type check 
inside seq-contains-pred. I think it should be a specialized impl for 
seq-contains-pred anyway (for when sequence is a list). Since 
seq-contains-pred is only called once, that shouldn't have any 
measurable effect on performance, aside from very tiny lists.

> One day they upgrade seq.el which comes with your optimization to
> seq-difference, and that code is mysteriously broken.  Worse  for some
> equality predicates it is broken  in a given way, for other equality
> predicates it's a different error.
>
> Can't you see the fundamental problem?
>
> Right, but then if they fail the customization of the entry point
> generic like not calling the right generics at the right time, then
> _other_   customizations of_other_  generics won't be called, or will
> be called too much.  Just like in your seq-difference-3 optimization
> the user code breaks.
> > > cl-{n}set-difference  are standard stuff in the CL world.   There are even
> > > reference LOOP implementations (likely much more performant than ours,
> > > though possibly not compatible with some non-standard edge cases our
> > > cl-loop has of which I know a couple).
> > I do believe it's helpful to have it around.
> What is?

cl-lib.