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Re: Change a symbol's plist member with (let ...)?


From: Thorsten Jolitz
Subject: Re: Change a symbol's plist member with (let ...)?
Date: Sun, 16 Mar 2014 17:01:02 +0100
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/24.3 (gnu/linux)

Nicolas Richard <address@hidden> writes:

> Thorsten Jolitz <address@hidden> writes:
>>> (letf (((get major-mode 'derived-mode-parent) 'special-mode))
>>>            (derived-mode-p 'text-mode))
>>> returns nil.
>>
>> I have to get more involved with this cl stuff it seems ... 
>
> This one is rather easy to understand. 'letf' does a temporary 'setf'
> and then puts the previous value back. So what is 'setf' ?
>
> 'setf' is a generic way to *change* a value if you know how to *get*
> that value.
>
> Example : if foo is a cons cell, you know how to get its car
> : (car foo). Well, now (setf (car foo) 'bar) will change the car of foo
> to 'bar. IOW, setf arranges so that the next invocations of (car foo)
> will now return 'bar instead of its previous value. And it knows
> internally how to do it : it uses 'setcar' :
> (macroexpand '(setf (car foo) 'special-mode))
> => (let* ((v foo)) (setcar v (quote special-mode)))
> (the surrounding let*-binding isn't needed in this case, is needed in
> other cases, but let's ignore it.)
>
> So now :
> (setf (get major-mode 'derived-mode-parent) 'special-mode)
> simply arranges to change whatever is required so that (get major-mode
> 'derived-mode-parent) will return 'special-mode. Internally, it knows
> what to do :
> (macroexpand '(setf (get major-mode 'derived-mode-parent) 'special-mode))
> gives :
> (let* ((v major-mode)) (put v (quote derived-mode-parent) (quote 
> special-mode)))
>
> And now if you want to do that temporarily, you use letf : letf will
> arrange to revert the change upon exiting the letf form.
> (macroexpand'(letf (((get major-mode 'derived-mode-parent) 'special-mode))
>            (derived-mode-p 'text-mode)))
> will give (I added comments) :
> (let* ((v major-mode)
>        (old (get v (quote derived-mode-parent)))) ; save old value
>   (unwind-protect
>       (progn
>         (put v ; set new value
>              (quote derived-mode-parent)
>              (quote special-mode))
>         (derived-mode-p ; run the body
>          (quote text-mode)))
>     (put v ; set old value back.
>          (quote derived-mode-parent)
>          old)))
>
> Simple, yet powerful.

Interesting, thanks for the explanations!
Amazing how intelligent this macro is ...

I do have those Paul Graham CL books, and with these CL idioms becoming
more standard in Elisp, I should take a second look at them I guess.

-- 
cheers,
Thorsten




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