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Re: Defadvice use

From: Barry Margolin
Subject: Re: Defadvice use
Date: Tue, 19 Apr 2005 01:07:49 -0400
User-agent: MT-NewsWatcher/3.4 (PPC Mac OS X)

In article <address@hidden>,
 "rgb" <address@hidden> wrote:

> Stefan Monnier wrote:
> > > Doesn't (let (bookmark-bmenu-other-window) ...) have limited life.
> >
> >   (let (bookmark-bmenu-other-window) ...)
> >
> > only affects the bookmark-bmenu-other-window *variable*, not the
> > bookmark-bmenu-other-window function.
> >
> If that isn't a bug waiting to happen I don't know what is.
> You appear to be asserting that it's documented to work that
> way but I'd counter with 2 arguments.
> 1.  The `feature' is only _implied_, not documented.  There is
>     no mention in any Elisp manual section that covers plists
>     or symbol function cells about this behavior.

The Elisp manual specifically says that 'let' binds variables.  That's 
all it does.  Does it have to explicitly say all the things it *doesn't* 
do, like binding functions and property lists?  Function and variable 
bindings of symbols are totally independent.

If you want to bind a function, you can (require 'cl) and then use 

> 2.  The `feature' invalidates much of the documented behavior of
>     symbols unless you know how the symbol was created.
> Look at this:
> (defun test1 ()
>   (let (a01)
>     (put 'a01 'hold "this")
>     (symbol-plist 'a01)))
> (put 'a01 'hold "that") ; insure symbol already exists
> (symbol-plist 'a01)
>  => (hold "that")       ; value is there as expected
> (test1)
>  => (hold "this")
> (symbol-plist 'a01)
>  => (hold "this")       ; external value unexpectedly changed
> (unintern "a01")        ; insure it doesn't exist
> (symbol-plist 'a01)
>  => nil                 ; no error.
> (test1)
>  => (hold "this")
> (symbol-plist 'a01)    ; Given the external value should change
>  => nil                ; it now unexpectedly doesn't

You're looking at the plist of a different symbol.  test1 is still 
referencing the symbol that you uninterned.  When you then type 'a01 
later, you intern a new symbol, which is not the one whose plist is 
modified when you call test1.

> It's obviously impossible to easily create any function or
> macro that manipulates function or property cells unless you
> can guarantee none of the symbols are ever created by let.
> Since it appears to be by design, I'd be terribly curious to
> see any archive of discussions concerning why this is
> appropriate behavior.

Barry Margolin, address@hidden
Arlington, MA
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