On Friday, January 09, 2004 2:48 AM, Ashley Yakeley [SMTP:ashley@semantic.org] wrote:
Do you have an example of use of Y for letrec where a program would violate R5RS?
Sure, take a look at my implementation of Ben Rudiak-Gould's implementation of Alan Bawden's implementation of boxes. In 4.2.2 of R5RS, it says, re letrec: Semantics: The <variable>s are bound to fresh locations holding undefined values, the <init>s are evaluated in the resulting environment (in some unspecified order), each <variable> is assigned to the result of the corresponding <init>, the <body> is evaluated in the resulting environment, and the value(s) of the last expression in <body> is(are) returned. Each binding of a <variable> has the entire `letrec' expression as its region, making it possible to define mutually recursive procedures. The result of the corresponding <init> is *assigned to* each <variable> (anyone know why the wording is backward above?), and that is after the <init>s are evaluated, which is after the <variable>s are bound. There was a discussion on comp.lang.scheme a couple of years ago about this. http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=fdcf3 554852a3cad&seekm=3AC66F16%40MailAndNews.com#link1 http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=a47e0 3e456b2dc2a&seekm=200102220358.TAA77339%40adric.cs.nps.navy.mil#link1 http://groups.google.com/groups?hl=en&lr=&ie=UTF-8&oe=UTF-8&th=58a68 6525be78d16&rnum=1
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Kevin S. Millikin