bug#8711: 24.0.50; binding _ to unused values with lexical-binding

[Helmut Eller]

* Stefan Monnier [2011-05-23 19:29] writes:

> I think the same problem happens with dotimes/dolist using the old
> definition: the loop vars `key' and `value' are let-bound outside the
> loop and then setq'd at each loop iteration (it's this setq that causes
> them to be "not left unused").  This was OK for the dynamic scoping case
> because let-binding is significantly more costly than setq, but it is
> not right for the lexical scoping case where the cost of let is not
> higher than `setq' and where the semantic is actually then wrong:
> e.g. if you "collect (lambda () value)" the current code ends up
> returning a list of functions that all return the last `value', rather
> than a list of functions that each return one of the `values' in
> the alist.

BTW, is there a good reason why we can't disallow setq on closed over
variables in Elisp?  That kind of mutability seems like a bad idea in a
concurrent world.  Closures are a new feature so we could make a
conscious decision not to introduce the problematic mutable variables.

Helmut