Actually, after reading more, letrec* also demands (again page 36, top of second column):
"It must be possible to evaluate each <init> without assigning or referring to the value of the corresponding <variable> or the <variable> of any of the bindings that follow it in <bindings>." letrec* is not like let* at all, really, which kind of crushes my mental model a bit. It seems, then, that your example at the end using letrec* should not work, as you have evaluated an <init> while referring to the value of <variable>. Deren On Sat, Sep 18, 2010 at 3:37 PM, Jos Koot <jos.k...@telefonica.net> wrote: > Thanks anyway for taking the trouble to answer. > Jos > >> -----Original Message----- >> From: Deren Dohoda [mailto:deren.doh...@gmail.com] >> Sent: 18 September 2010 21:35 >> To: Jos Koot >> Cc: PLT-Scheme Mailing List >> Subject: Re: [racket] internal definitions in r6rs >> >> 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.doh...@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.k...@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 >> >> _________________________________________________ >> >> For list-related administrative tasks: >> >> http://lists.racket-lang.org/listinfo/users >> >> >> > > > > _________________________________________________ For list-related administrative tasks: http://lists.racket-lang.org/listinfo/users
