Re: Declaring a local dynamic variable?

[Andreas Röhler]

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.