Re: lexical-let detail semantics

[Daniel Kraft <d@domob.eu>]

Hi Pascal,

thanks for your nice answers!

Pascal J. Bourguignon wrote:
> You can avoid the problem by putting the dynaminc bindings outside of
> reach of lexical-let:
> 
> (defun call-dyna ()
>   (let ((a 3)) (print (dyna))))
> 
> (lexical-let ((a 2))
>         (call-dyna))
> prints: 3
> --> 3
> 
> 
> Notice also that in languages that have both special variables and
> lexical variables, it is found worthwhile to keep them in separate
> name spaces.  In ISO-LISP, this is done with the (dynamic var) form
> for special variables.  In Common Lisp it's done with the *earmuff*
> convention.

That's true of course (and good advice), but unfortunatly I'm working on 
another implementation of elisp which should include lexical-let 
natively (instead of being a user) and thus I'm forced to care about 
such special cases, too.

> You may report the bug to the maintainers, but I'm not sure it's
> worthwhile.  If you want a real language, perhaps you could use
> emacs-cl?  http://www.lisp.se/emacs-cl/

Hm...  My main point was whether this is expected behaviour I should 
mimic in my implementation, but I take your response that you would 
prefer to have my implementation behave differently (like what I 
described as my expectation)?

Do you think this would lead to compatibility problems with existing code?

>> 2) Closures:
>>
>> I'm happy that lexical-let works well to build closures (and in fact
>> it seems that this is the main intention for lexical-let at all);
>> however this code does not work as expected:
>>
>> (setq a 1)
>> (lexical-let ((a 2))
>>   ((lambda () (print a))))
>>   => 1
>>
>> I don't know why, but it seems that calling a closure directly fails,
>> while storing it and calling it later succeeds (as in the examples at
>> http://www.delorie.com/gnu/docs/emacs/cl_21.html for instance).  Is
>> this a bug or again something expected?  If the latter, what's the
>> exact rationale and semantics then?
> 
> I guess you have a bug in your version.  Mine works ok.
> Again, the macroexpansion explains what lexical-let does in this case:

Yes, that's right; I checked with a current development version of emacs 
and there it works as expected.

Yours,
Daniel