Re: Code for cond*

[JD Smith <jdtsmith@gmail.com>]

> On Jan 24, 2024, at 10:55 AM, Stefan Monnier <monnier@iro.umontreal.ca> wrote:
> 
>> To explain this correctly, I’d need to understand where the mental
>> model breaks down.  For example, I was surprised that this
>> forward/reverse symmetry was not maintained:
>> 
>> (let ((list-var '(foo moo loo))
>>      (other 'boo))
>>  `(,@list-var . ,other)). ; -> (foo moo loo . boo)
>> 
>> (pcase '(foo moo loo . boo)
>>  (`(,@list-var . ,other)
>>   (format "Got %S: %S" list-var other))). ; -> nil, expected “Got (foo moo loo): boo"
> 
> Note that
> 
>    (let ((list-var '(foo moo))
>          (other '(loo . boo)))
>      `(,@list-var . ,other))
> 
> returns the same `(foo moo loo . boo)`, so there is not a unique way for
> a pattern like `(,@list-var . ,other) to match a given list.
> That's a large part of the reason why ,@ is not supported in Pcase's
> backquote patterns.

Not a unique way without a policy, but could it not take the largest possible list at that position, ala regexp greedy matching?

Any other forward/reverse asymmetries you are aware of (other than the use of meaningful sub-lists with symbols like pred/and/or/guard for pcase to interpret)?