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Re: Any disadvantages of using put/get instead of defvar?


From: Sebastian Wiesner
Subject: Re: Any disadvantages of using put/get instead of defvar?
Date: Fri, 21 Feb 2014 10:56:55 +0100

Am 21.02.2014 10:47 schrieb "Andreas Röhler" <address@hidden
>:
>
> Am 21.02.2014 10:39, schrieb Tassilo Horn:
>
>> Oleh <address@hidden> writes:
>>
>>>>> The situation is that I have a function that uses one global variable.
>>>>> It's for sure that no other function will want this variable.  In an
>>>>> effort to have all code in one place I want to move from:
>>>>>
>>>>>      (defvar bar-foo 1)
>>>>>      (defun bar ()
>>>>>        ;; use bar-foo here
>>>>>        )
>>>>>
>>>>> to:
>>>>>
>>>>>      (defun bar ()
>>>>>        (let ((foo (or (get 'bar 'foo) 1)))
>>>>>          ;; use foo here
>>>>>          ))
>>>>>
>>>>> So the advantage is that I can move and rename the function without
>>>>> worry that the function/variable coupling will break, because now
>>>>> everything is inside one function.
>>>>
>>>>
>>>> You could also define the variable inside the function, i.e., that's a
>>>> buffer-local counter:
>>>>
>>>>    (defun counter ()
>>>>      (defvar counter-var 1)
>>>>      (setq-local counter-var (1+ counter-var)))
>>>>
>>>
>>> Thanks, Tassilo,
>>>
>>> But doesn't `defvar` introduce overhead this way?
>>
>>
>> Well, I've measured my counter above versus a version using symbol
>> properties as you suggest:
>>
>>    (defun bar ()
>>      (let ((foo (or (get 'bar 'foo) 1)))
>>        (put 'bar 'foo (1+ foo))))
>>
>> My counter is way faster although it uses defvar and setq-local, so that
>> overhead is still small compared to looking up/putting a symbol
>> property.
>>
>
> BTW in earlier times a "let" was used.
> IIUC the way to make a function-local value now is "defvar" inside?

Not if the value should persist across invocations of the function.


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