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

[Dmitry Gutov <dmitry@gutov.dev>]

On 14/11/2023 04:27, João Távora wrote:
> This is all interesting, until one ponders what happens if an existing
> seq.el user somewhere has:
> 
> (cl-defmethod seq-contains-p ((seq my-voodoo-seq)
>                                (elt (eql :secret-voodoo)) &optional _tesfn)
>    (invoke-voodoo-priests seq))
> 
> making use of seq.el's support for abstract polymorphic sequences.
> 
> With seq.el 2.24 a seq-difference operation would consider this user's
> method, with seq.el 2.24.dmitry (i.e. your fast seq-difference-3) it
> simply won't.  This user's code is clearly broken.
> 
> But was the user allowed to do that in the first place?  If not,
> why is seq-contains-p a public generic function?

Allow me to try to answer this again: special voodoo is probably not 
allowed. What the specialized methods are supposed to do is implement 
the same logic (described in the generic function's docstring) but in a 
more efficient manner.

So the author of the new data type (who works on the implementation of 
seq-do and specialized methods) is responsible for making sure the new 
definition is equivalent but faster.

This is not documented, but seems to hold for all the code in the 
package. If someone found a case where violating the above rule would be 
helpful for a certain practical purpose, we could discuss it.

Note that all the functions in seq.el are currently non-destructive or 
perhaps even "pure", so it's not like there could be something special 
done to the input data behind the scenes, aside from what's in the 
contract (data in + data out).