<!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN"> <html> <head> <meta content="text/html;charset=ISO-8859-1" http-equiv="Content-Type"> <title></title> </head> <body bgcolor="#ffffff" text="#000000"> Robby Findler wrote: <blockquote cite="mid20040216043251.A557B6BD40@laime.cs.uchicago.edu" type="cite"> <pre wrap=""> I am well aware of this. I answered the original poster's message in the context of How to Design Programs (a textbook for introductory programming, aka HtDP), which was clearly how it was asked. </pre> </blockquote> The original post did say this:  <pre wrap=""><quote> How does one typically enforce such restrictions in Scheme? </quote> So even though the launch pad for the discussion was a program in HtDP, I was using HtDP as the basis for a question about Scheme . In the future, I will be more explicit about what I am looking for and perhaps eliminate references to HtDP and just ask a general question. In fact, if it is a general question, perhaps it is best placed on comp.lang.scheme instead of here. The reason I asked is that I figured that a student of mine might ask me how and/or why anything is allowed in the eye-color slot and I wanted to be able to give them an authoritative answer, as well as put my mind to rest that Scheme is a good language to become religious about in my search for The One Perfect Language. </pre> </body> </html>