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[racket] internal definitions in r6rs

From: Jos Koot (jos.koot at telefonica.net)
Date: Sat Sep 18 15:37:50 EDT 2010		 
Thanks anyway for taking the trouble to answer.
Jos 

> -----Original Message-----
> From: Deren Dohoda [mailto:deren.dohoda at gmail.com] 
> Sent: 18 September 2010 21:35
> To: Jos Koot
> Cc: PLT-Scheme Mailing List
> Subject: Re: [racket] internal definitions in r6rs
> 
> I see, please ignore my last response, as you were talking about
> letrec* and not letrec.
> 
> Deren
> 
> On Sat, Sep 18, 2010 at 3:33 PM, Deren Dohoda 
> <deren.dohoda at gmail.com> wrote:
> > Hi Jos,
> >
> > (meant to reply to all... sorry)
> >
> > (letrec (<bindings>) exps ...)
> > where <bindings> is
> > (<var> <init>) ...
> >
> > From page 36 on letrec:
> > "It must be possible to evaluate each <init> without assigning or 
> > referring to the value of the corresponding <variable> or the 
> > <variable> of any bindings that follow it in <bindings>."
> >
> > In short:
> > you can use the *name* but you can't use the *value*.
> >
> >> This I interpret as allowing an internal definition to 
> refer to the 
> >> value of a previously defined variable.
> > It does not. That would be the * forms, which is why your 
> last example works.
> >
> > Deren
> >
> > On Sat, Sep 18, 2010 at 6:52 AM, Jos Koot 
> <jos.koot at telefonica.net> wrote:
> >> Hi,
> >>
> >> Section 11.3 of R6RS states: "An expanded <body> (see chapter 10) 
> >> containing variable definitions can always be converted into an 
> >> equivalent letrec* expression." The semantics of letrec* includes 
> >> "each <variable> is assigned in left-to-right order". This I 
> >> interpret as allowing an internal definition to refer to 
> the value of a previously defined variable. However:
> >>
> >> #!r6rs
> >>
> >> (import
> >>  (rnrs base (6))
> >>  (rnrs io simple (6)))
> >>
> >> (define (a)
> >>   (define (b) "who cares?")
> >>   (define c (b))
> >>   c)
> >>
> >> (write (a))
> >>
> >> Produces:
> >>
> >> Welcome to DrRacket, version
> >> 
> 5.0.1.5--2010-09-13(5b54caebb066920e2585244a5ee444a3f121c966/a) [3m].
> >> Language: r6rs; memory limit: 2000 MB.
> >> . . procedure application: expected procedure, given: #<undefined> 
> >> (no
> >> arguments)
> >>
> >> The code expands as follows. The hot spots is marked with an arrow.
> >>
> >> (module anonymous-module r6rs
> >>   (#%plain-module-begin
> >>    (#%require r6rs/private/prelims)
> >>    (#%require (lib "rnrs/base-6.rkt"))
> >>    (#%require (for-meta #f (lib "rnrs/base-6.rkt")))
> >>    (#%require (lib "rnrs/io/simple-6.rkt"))
> >>    (#%require (for-meta #f (lib "rnrs/io/simple-6.rkt")))
> >>    (define-values (a)
> >>      (#%plain-lambda
> >>       ()
> >>       (let-values (((b) undefined) ((c) undefined))
> >>         (let-values ; <====== prohibits reference to value of 
> >> previously defined var
> >>             (((newtemp)
> >>               (#%plain-lambda () (letrec-values () (let-values () 
> >> '"who
> >> cares?"))))
> >>              ((newtemp:19) (#%app b))) ; <======
> >>           (set! b newtemp)
> >>           (set! c newtemp:19)
> >>           (let-values:20 () (let-values:21 () c))))))
> >>    (#%app write (#%app a))))
> >> Is this a bug or do I misunderstand R6RS?
> >> The following works correct:
> >>
> >> #!r6rs
> >>
> >> (import
> >>  (rnrs base (6))
> >>  (rnrs io simple (6)))
> >>
> >> (define (a)
> >>   (letrec*
> >>    ((b (lambda () "who cares?"))
> >>     (c (b)))
> >>    c))
> >>
> >> (write (a))
> >>
> >> Thanks, Jos
> >> _________________________________________________
> >>  For list-related administrative tasks:
> >>  http://lists.racket-lang.org/listinfo/users
> >>
> >
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