Re: (byte-compile '(append '(1 2) '(3 4)))

[Philip Kaludercic]

"Basil L. Contovounesios" <basil@contovou.net> writes:

> Philip Kaludercic [2024-03-16 12:46 +0000] wrote:
>
>> I am not sure what the danger is in the case of constant, quoted lists,
>> but I am not familiar with the byte compiler either.  It seems like this
>>
>> diff --git a/lisp/emacs-lisp/byte-opt.el b/lisp/emacs-lisp/byte-opt.el
>> index f75be3f71ad..e6f18590705 100644
>> --- a/lisp/emacs-lisp/byte-opt.el
>> +++ b/lisp/emacs-lisp/byte-opt.el
>> @@ -1599,6 +1599,12 @@ byte-optimize-append
>>                           (cdr args))
>>                     (cdr newargs)))
>>  
>> +            ;; (append '(C1...) ... '(C2...)) -> (append C1... ... C2...)
>> +            ((cl-loop for arg in args
>> +                      always (and (eq (car arg) 'quote)
>> +                                  (proper-list-p (cdr arg))))
>> +             `',(mapcan #'cadr args))
>> +
>>              ;; non-terminal arg
>>              ((cdr args)
>>               (cond
>>
>>
>> would do the trick, and the byte-code is even better:
>>
>> byte code:
>>   args: nil
>> 0    constant  (1 2 3 4)
>> 1    return    
>
> Is this correct?  According to its docstring, append's last argument
> must be eq to the tail of its return value.

This little test indicates that it would still be eq:

((macro . (lambda (arg)
            `(eq (cddadr (byte-optimize-append '(append '(1 2) ,arg)))
                 ,arg)))
 '(3 4)) ;=> t

since the mapcan or rather nconc makes the same promise.

-- 
        Philip Kaludercic on peregrine