<!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="mid200402160125.i1G1P981226880@pimout5-ext.prodigy.net" type="cite"> <pre wrap="">At Sat, 14 Feb 2004 08:39:02 -0500, Terrence Brannon wrote: </pre> <blockquote type="cite"> <pre wrap=""> For list-related administrative tasks: <a class="moz-txt-link-freetext" href="http://list.cs.brown.edu/mailman/listinfo/plt-scheme">http://list.cs.brown.edu/mailman/listinfo/plt-scheme</a> In this section, a data definition for a family tree is developed... However, there are no restrictions on eye color, yet I think eye color should, in the terminology of Pascal and MySQL, be an enumerated type with only certain things allowed in that field. How does one typically enforce such restrictions in Scheme? </pre> </blockquote> <pre wrap=""> In HtDP, data definitions are not program artifacts, per se. They live in comments.</pre> </blockquote> Comments do not enforce type safety. A comment in a program does not prevent me from putting something there that never belongs there, perhaps by a typo or some sort of intentional malice. <blockquote cite="mid200402160125.i1G1P981226880@pimout5-ext.prodigy.net" type="cite"> <pre wrap=""> FWIW, this is one of the clear advantages of Scheme over languages like Pascal -- types can correspond to subsets of other types. </pre> </blockquote> Now you really have me lost: how in the heck is something in a comment going to force the eye-color to only be one of a set of symbols? I am sure you are familar with the concept of an enumerated type. How does one restrict the value for eye color ? <blockquote cite="mid200402160125.i1G1P981226880@pimout5-ext.prodigy.net" type="cite"> <pre wrap="">Robby </pre> </blockquote> </body> </html>