Path: utzoo!attcan!uunet!mcvax!ukc!its63b!aiva!jeff
From: jeff@aiva.ed.ac.uk (Jeff Dalton)
Newsgroups: comp.lang.lisp
Subject: Re: What's the value of lexical scoping?
Keywords: Scope, Binding, Locals, Break, Future of LISP
Message-ID: <452@aiva.ed.ac.uk>
Date: 9 Jun 88 18:41:27 GMT
References: <24508@ucbvax.BERKELEY.EDU> <515@dcl-csvax.comp.lancs.ac.uk>
Reply-To: jeff@uk.ac.ed.aiva (Jeff Dalton)
Organization: Dept. of AI, Univ. of Edinburgh, UK
Lines: 90

In article <515@dcl-csvax.comp.lancs.ac.uk> simon@comp.lancs.ac.uk (Simon Brooke) writes:
>In article <24508@ucbvax.BERKELEY.EDU> mkent@dewey.soe.berkeley.edu (Marty Kent) writes:
>>I've been wondering lately why it is that "modern" lisps like Common Lisp
>>and Scheme are committed to lexical scoping.  
>
>Good! somebody else prepared to stand up and say Common LISP is a mess. If
>you share this opinion, read the end of this posting even if you skip the
>middle... it is important.

I will respond to this, because I have talked with Simon before on
Uk.lisp, and because I am familiar with the various standardization
efforts mentioned in his message.  Nothing I say has any official
standing of course.

For one thing, it is significant that Marty Kent mentioned Scheme as
well as Common Lisp.  He did not say Common Lisp is a mess, nor did
he mention any problem with Common Lisp other than the effects of
its use of lexical scope on debugging.  I therefore think it a bit
unfair to enlist him in the anti-Common Lisp cause just yet.

This exchange is a sign of a general problem faced by the Lisp
community, namely that we are trying to standardize Lisp at a
time when our conception of the language is changing.  One aspect
of this change is the move towards lexical scoping.

In addition, there is a problem of "uneven development": some
people have gone further than others or in a different direction.
I do not mean to imply that those who have gone further are right.
Nonetheless, it may be difficult to explain "modern" Lisp without
retracing a lot of history.

It may even be that the only way to understand is to try out the
other point of view and see what it's like.

My own thinking on these matters was informed in part by the
first Scheme papers by Steele and Sussman starting in 1975,
by Stoy's book on Denotational Semantics (see especially his
comments on 1st-class functions), and by two papers in the
Conference Record of the 1980 Lisp Conference:

     Kent M. Pitman. Special Forms in Lisp. Pages 179-187.
     [An argument that macros are better than fexprs]

     Steven S. Muchnick and Uwe F. Pleban.  A Semantic
     Comparison of Lisp and Scheme.  Pages 56-64.
     [In part a reconstruction of Lisp development, as is
     Steele and Sussman's "The Art of the Interpreter".]

     [For other references, see the R*RS Scheme reports.]

Scheme-like ways of thinking were not immediately convincing, but
experience with Scheme showed that such an approach had virtues
despite seeming to impose a number of restrictions.  For one thing,
the lexical resolution of the differences between interpreter and
compiler semantics (many Lisps have partial lexical scoping in
compiled code but only dynamic scoping in interpreted) began to
seem better overall than the dynamic resolution (have compiled
code use dynamic scoping too), as did the lexical form of
functional values.

These ideas eventually became strong enough to influence Common Lisp:
  -  Common Lisp has lexical scoping with indefinite extent.
  -  Common Lisp does not support user-defined special forms.
  -  Common Lisp uses the same rules of scope and extent
     for both interpreted and compiled code.

It is important to note that for many these are all good things.
In particular, it is likely that all three will be true of any
Lisp standard, whether developed by x3j13 or ISO's WG-16.  They
are also true of the suggestions made by the EuLisp committee.
Indeed, one of the key questions now is how much further in the
Scheme direction the standard should move.

However, to others these things are all bad things and represent 
capitulation to the forces of Pascal, or something of that sort.
In addition, some, and I think Simon is among them, find Common
Lisp a confusing mixture.  They prefer both Scheme and "dynamic
Lisp" to Common Lisp, but still prefer dynamic Lisp to Scheme.
Such a position is not simply a complaint that Common Lisp has
lexical scoping; it is more complex.  If they would like a standard
that improves on Common Lisp in some respects, it may be possible
to achieve one; but I do not think the Lisp community has the
resources or the inclination to build a standard for dynamic Lisp.

This seems enough for one message; I will respond further in the
next.

Jeff Dalton,                      JANET: J.Dalton@uk.ac.ed             
AI Applications Institute,        ARPA:  J.Dalton%uk.ac.ed@nss.cs.ucl.ac.uk
Edinburgh University.             UUCP:  ...!ukc!ed.ac.uk!J.Dalton