[racket] internal definitions in r6rs
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
> > _________________________________________________
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