Re: questioning let

[Tim X]

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

> David Kastrup wrote:
>> Andreas Roehler <andreas.roehler@online.de> writes:
>> 
>>> Hi,
>>>
>>> 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 compiler cannot know that this function will always be called in
such a way. All it knows is that at its time of definition, it is
setting an argument it doesn't know anything about. 
>
The point to note is that in a defun argument list, that argument list
is essentially an implicit let. The 'arg' referenced inside your
original arg-extern is not the same as the arg being referenced in the
first function. this is why I suggest you change the argument name in
the arg-extern function as this will make that distinction obvious. To
get the value set in arg-extern returned to its caller, you would do
something like

(setq arg (arg-extern arg))

Tim



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