Re: [Caml-list] destructive local opens

[Oleg <oleg@okmij.org>]

[Since this discussion still continues, I think I will comment on
Gabriel's comment after all.]

 "It is remarkable that programming languages have avoided introducing
  explicit weakening (the popping of a symbol out of scope) for now, and
  I think it is a property that should be preserved. We're not yet ready
  to go there."

Actually there have been proposals to go into that direction, about 30
years ago.

	http://research.microsoft.com/lampson/39-Pebble/39-PebbleAbstract.htm

Please look at p 287 of the paper: ``Pebble also has an anti-LET,
which impoverishes the environment instead of enriching it.'' This is
exactly the issue under discussion in this thread.

[It is amazingly that the paper tackles the issues we are still
struggling with today, including dependent types. It reads
surprisingly modern...]

There is an independent motivation for restricting the environment:
syntactic conditional independence. If two stochastic expressions
share no variables, the are independent and conditionally
independent. Alas, this syntactic test -- which David McAllister likes
very much [see his presentation at the workshop on probabilistic
programming at NIPS 2008] -- has limited applicability since many
expressions are in the scope of many let-bindings (even though a
particular expression depends on a fraction of all the bindings in
scope). It would be great to have a construct to remove bindings from
scope, to strengthen so to speak.