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[racket] internal definitions in r6rs

From: Deren Dohoda (deren.dohoda at gmail.com)
Date: Sat Sep 18 15:35:21 EDT 2010		 
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
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