Re: questioning let

[David Kastrup <dak@gnu.org>]

Andreas Röhler <andreas.roehler@easy-emacs.de> writes:

> David Kastrup wrote:
>> Andreas Roehler <andreas.roehler@online.de> writes:
>> 
>>> behaviour of the example code below puzzles me. Would
>>> expect setting of arg by external function, but inside
>>> `let', recognised. But remains `1'.
>>>
>>> (defun arg-setting ()
>>>   (interactive)
>>>   (let ((arg 1))
>>>     (message "%s" arg)
>>>     (arg-extern arg)
>>>     (message "%s" arg)))
>>>
>>> (defun arg-extern (arg)
>>>   (setq arg (1- arg)))
>>>
>>> Any help?
>> 
>> The argument binding in arg-extern is the innermost one and consequently
>> the only affected one.  If you make the function argument-less, it will
>> likely work as expected by you, affecting the binding in arg-setting.
>
> That works, thanks a lot!
> However, stored in some eil.el, get a compiler warning than:
>
>
> In arg-extern:
> eil.el:9:9:Warning: reference to free variable `arg'
> eil.el:9:17:Warning: assignment to free variable `arg'
>
> Would think a useless warning, as the compiler should know being inside a let (?)

The warning is completely accurate since arg-extern can be called from
outside arg-setting, in which case it will assign to a global variable
called "arg".

Whether or not some let-binding might be effective at the point of
calling arg-extern is unknown to the compiler.

In a Lisp variant with lexical binding (like Common Lisp or Scheme),
arg-extern has no way to fiddle with the let-binding of arg-setting: it
is completely inaccessible by name outside of arg-setting itself.

-- 
David Kastrup