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To: loup.vaillant@gmail.com, caml-list@inria.fr
Subject: Re: How must we teach lexical scope?
Reply-To: oleg@pobox.com
Errors-To: oleg@okmij.org
From: oleg@pobox.com
Message-Id: <20070329034546.2AF4DAD35@Adric.metnet.fnmoc.navy.mil>
Date: Wed, 28 Mar 2007 20:45:46 -0700 (PDT)
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X-Spam: no; 0.00; oleg:01 metaocaml:01 endline:01 'x':01 lexical:01 lexical:01 wrote:01 variables:02 binding:02 string:02 scope:03 scope:03 languages:03 optimization:03 optimization:03 


Loup Vaillant wrote:
> Personally, I think environments are about the implementation of
> lexical scope, not  its specifications.

I agree. A language system that eagerly performs substitutions
inherent in the beta-rule needs no environments. An environment, which
is a list of pending substitutions, is merely an optimization. A quite
good optimization though. This is discussed in an article

	http://okmij.org/ftp/Scheme/misc.html#closure-misconceptions

Regarding free variables: some of the multi-staged languages,
including MetaOCaml, can literally manipulate code with `free
variables'. For example,

      let bar u = print_endline "here"; .<.~u + 1>. in
      .<fun x -> .~(bar .<x+3>.)>.;;

Evaluating this code prints the string "here" and returns a code
value, which can be printed as .<fun x_1 -> ((x_1 + 3) + 1)>.
We note that the function bar received a piece of code .<x+3>.
containing a free variable 'x' and incorporated that piece of code 
in the larger code. All while the binding for "x", "fun x -> ..." will
be evaluated only in the future.