((( [logo]Racket )))Need Help?
AboutDownloadDocumentationPLaneTCommunityLearning

[racket] internal definitions in r6rs

From: Robby Findler (robby at eecs.northwestern.edu)
Date: Sat Sep 18 19:02:56 EDT 2010		 
It has been a while but I believe the r6 formal model covers this point.

Robby

On Saturday, September 18, 2010, 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
>
>
>
Posted on the users mailing list.