Simon, Regarding Justin Bailey's idea of a calculator -- here's a twist. There is some sample Haskell code of Conway's account of numbers as games floating around the internet (http://www.dcs.ed.ac.uk/home/pgh/conway.html, http://www.dcs.ed.ac.uk/home/pgh/Conway.lhs). i can't vouch for the code as i have not read it in anger. However, i've always thought it would be fun to do the standard calculator example, but with Conway games on the back end for doing the arithemetic. Some of the attractions: - you could have another set of buttons for displaying the games respresentation of the numbers - you could really emphasize the polymorphism of the basic operations - you could emphasize the use of laziness for calculations involving infinitary entities - you could explain Conway games (which are an intellectual treat for those who never seen them and just get better and better the more you return to them) Best wishes, --greg -- L.G. Meredith Managing Partner Biosimilarity LLC 505 N 72nd St Seattle, WA 98103 +1 206.650.3740 http://biosimilarity.blogspot.com