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
