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Re: Declaring a local dynamic variable?

From: Andreas Röhler
Subject: Re: Declaring a local dynamic variable?
Date: Sun, 13 Oct 2013 10:12:42 +0200
User-agent: Mozilla/5.0 (X11; Linux i686; rv:24.0) Gecko/20100101 Thunderbird/24.0

Am 12.10.2013 23:25, schrieb Kai Grossjohann:
On Saturday, October 12, 2013 6:54:40 PM UTC+2, Andreas Röhler wrote:
Am 25.09.2013 14:26, schrieb Stefan Monnier:

So in essence Emacs doesn't really have local dynamic variables?

Dynamic scoping is inherently global,

Reads like a mistake for me.

I think it makes sense.  In a language that has lexical scoping, you can get 
the effect of dynamic scoping by changing the value of a global variable.

(defvar x 5)
bla bla
(let ((x 6))
     yadda yadda)
mumble mumble

The above code has the same effect as
- create global variable x, initialize it to 5
- execute bla bla
- change value of x to 6

No. Introduces a let-bound x, which is unrelated to global x

dynamical-scoped let-bound behave different than lexical-scoped let-bound vars, 
that's at stake.

If a global var of the same name exists or not is another issue.

Therefor consider it a misuse and design-flaw to use "defvar" signaling 
dynamic-scope in let-bound vars.

"defvar" initialised a global var. Never was related to stuff inside let. At 
least not at the Emacs Lisp symbol-level.

- execute yadda yadda
- change value of x back to 5
- execute mumble mumble

But dynamically-scoped let has the advantage that it remembers the old value for you, and 
it catches all ways that the "yadda yadda" might exit.

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