Re: let-bind a varlist only known at run time

[Michael Heerdegen <michael_heerdegen@web.de>]

"Roland Winkler" <winkler@gnu.org> writes:

> > FWIW that's what `cl-progv' is doing, more or less.  The body is pasted
> > into a lambda constructed before calling eval so that it can be
> > compiled.  The resulting semantics can be a bit surprising, though; for
> > example
> > 
> > (let ((x 0))
> >   (cl-progv (list 'x) (list 1)
> >     (funcall (lambda () x))))
> > 
> > yields 0 with lexical-binding on, and
> > 
> > (cl-progv (list 'x) (list 1)
> >     (funcall (lambda () x)))
> > 
> > yields 1.
>
> Thanks, I thought about that:
>
> If var-list is the VARLIST for the let form, you should also use
> var-list to construct the alist that becomes the 2nd arg LEXICAL of
> eval.  I believe, then it should not matter whether you have lexical
> binding on or off (untested!)

I don't think it's that simple: BINDS is unevaluated: it's a list of the
form ((SYMBOL EXPRESSION) ...).  We would need to evaluate the bindings
twice (once to calculate the environment for `eval', and the second time
when evaluating the constructed form), which is unacceptable.

> So is this a bug in cl-progv?  (I checked: this macro ignores the
> 2nd arg LEXICAL of eval.)

I think it's a bug, it's now filed as #27674.


Regards,

Michael.