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[racket] internal definitions in r6rs

From: Deren Dohoda (deren.dohoda at gmail.com)
Date: Sat Sep 18 15:33:36 EDT 2010		 
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
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