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[racket] internal definitions in r6rs

From: Jos Koot (jos.koot at telefonica.net)
Date: Sun Sep 19 04:45:37 EDT 2010		 
Thanks Matthew,
Jos 

> -----Original Message-----
> From: Matthew Flatt [mailto:mflatt at cs.utah.edu] 
> Sent: 19 September 2010 03:54
> To: Jos Koot
> Cc: 'PLT-Scheme Mailing List'
> Subject: Re: [racket] internal definitions in r6rs
> 
> This is a bug in the R6RS binding for `define'. Your program 
> works as it should if you use
> 
>  (define a
>    (lambda ()
>      (define (b) "who cares?")
>      (define c (b))
>      c))
> 
> or
> 
>  (define (a)
>    (let ()
>      (define (b) "who cares?")
>      (define c (b))
>      c))
> 
> because the problem is specific to `define' with the function 
> shorthand.
> 
> I've pushed a repair.
> 
> At Sat, 18 Sep 2010 12:52:26 +0200, "Jos Koot" 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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