<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=UTF-8" http-equiv="Content-Type"> </head> <body bgcolor="#ffffff" text="#000000"> Ah thanks for the link, Matthew. Initially it seemed to me that R5RS requires interaction-environment to be defined such that (eval (quote <expr>) (interaction-environment)) is always equivalent to just <expr>. However, the explanation in the PLT documentation convinced me this may not be a good requirement or even be impossible in general. Bas Matthew Flatt wrote: <blockquote cite="mid:20081107134227.77B936500AF@mail-svr1.cs.utah.edu" type="cite"> <pre wrap="">At Fri, 07 Nov 2008 09:14:09 +0100, Bas Steunebrink wrote: </pre> <blockquote type="cite"> <pre wrap="">I was a bit puzzled when I tried the following code in DrScheme: (define x 0) (let ((x 1)) (eval '(set! x 2) (interaction-environment)) x) x When running this in R5RS mode, it outputs 1 and then 2, while I would expect 2 and then 0. About interaction-environment, R5RS states that "The intent is that this procedure will return the environment in which the implementation would evaluate expressions dynamically typed by the user". </pre> </blockquote> <pre wrap=""> Although R5RS leaves the details to implementations, I believe the intent behind the R5RS description is that `interaction-environment' mimics a top-level read-eval-print loop. (All R5RS implementations that I know implement it that way.) </pre> <blockquote type="cite"> <pre wrap="">So I expected the set! expression to work on the local x variable, while PLT appears to take the top-level one. What do you think? </pre> </blockquote> <pre wrap=""> See also <a class="moz-txt-link-freetext" href="http://docs.plt-scheme.org/guide/eval.html#(part._.Local_.Scopes)">http://docs.plt-scheme.org/guide/eval.html#(part._.Local_.Scopes)</a> That explanation is specific to PLT Scheme, and it relies on a default second argument to `eval' that is essentially `(interaction-environment)'. But `eval' in a typical Scheme behaves the way for the same reason. Matthew </pre> </blockquote> <pre class="moz-signature" cols="72">-- Bas Steunebrink Intelligent Systems Group Utrecht University P.O. Box 80.089 3508TB Utrecht The Netherlands Tel: +31 (30) 253 2993</pre> </body> </html>