Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!accuvax.nwu.edu!krulwich
From: krulwich@ils.nwu.edu (Bruce Krulwich)
Newsgroups: comp.lang.scheme
Subject: Re: Non-local variables
Message-ID: <7478@accuvax.nwu.edu>
Date: 9 May 90 17:40:10 GMT
References: <9005081621.AA13048@mtecv2.mty.itesm.mx> <1990May8.205719.2014@sun.soe.clarkson.edu>
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Reply-To: krulwich@ils.nwu.edu (Bruce Krulwich)
Organization: Institute for the Learning Sciences, Northwestern University, Evanston, IL 60201
Lines: 38
In-reply-to: jk0@image.soe.clarkson.edu (Jason Coughlin)

In article <1990May8.205719.2014@sun.soe.clarkson.edu>, jk0@image (Jason
Coughlin) writes:
>	Hi.  I've got a short little example which creates a non-local
>variable to implement a simple little counter.  In my implementation of
>Scheme, I create it like this:
>
>(let ([a 0])
>   (define counter
>      (lambda ()
>	   (set! a (+ a 1))
>	   a
>      )
>   )
>)
>
>  PC-Scheme barfs on this complaining about an illegal letrec syntax.

I think that the problem is that DEFINE's that are not at the top-level are
considered by Scheme (but not T or CL) to be local definitions like
LETREC's.  Probably the code you give above is transformed into a LET
containing a body-less LETREC, which is illegal.

This is an issue that has been discussed before.  Basically, the treatment
of internal DEFINE's as local definitions makes it impossible to have
static locally-scoped variables that span across procedures.  (Having
static variables for one procedure can be achieved as you showed later in
your message.)  Having internal DEFINE's result in local definitions
doesn't add any power, because they're identical in meaning to LETREC, but
to some people this seems more semantically correct than simply having all
DEFINE's result in top-level definitions.  Of course, another issue for
many people is that changing the meaning would make old code incorrect.


Bruce Krulwich
Institute for the Learning Sciences
krulwich@ils.nwu.edu