<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"> <HTML><HEAD> <META http-equiv=Content-Type content="text/html; charset=iso-8859-1"> <META content="MSHTML 6.00.6000.16587" name=GENERATOR> <STYLE></STYLE> </HEAD> <BODY bgColor=#ffffff> <DIV><FONT face=Arial size=2>Finally I decided to try contracts. I tried:</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>#| require one or two numbers<BR>   promise to return a number if two numbers are given<BR>   promise void if one number is given |#</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=2>#lang scheme</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=2>(provide/contract<BR> (example<BR>  (or/c<BR>   (-> number? number? number?)<BR>   (-> number? void?))))</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=2>(define (example a (b #f))<BR> (if b (+ a b) (void)))</FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>This compiles, but it wont run:</FONT></DIV> <DIV> </DIV> <DIV><FONT face=Arial size=2>Welcome to DrScheme, version 3.99.0.10-svn4feb2008 [3m].<BR>Language: Module custom.<BR>or/c: two arguments, (-> number? void?) and (-> number? number? number?), might both match #<procedure:example></FONT></DIV> <DIV><FONT face=Arial size=2></FONT> </DIV> <DIV><FONT face=Arial size=2>Why? What is wrong in my contract? May be I am wanting an impossible contract?</FONT></DIV> <DIV><FONT face=Arial size=2>or/c repeats exactly what I want, but does not accept it.</FONT></DIV> <DIV><FONT face=Arial size=2>I think I am missing something fundamental, but I can't find an answer in the docs.</FONT></DIV> <DIV><FONT face=Arial size=2>Thanks, </FONT><FONT face=Arial size=2>Jos Koot</FONT></DIV></BODY></HTML>