Hello Andrew, Friday, June 24, 2005, 12:35:16 AM, you wrote: AW> Assuming I have a list of shapes somewhere in my program. The list would AW> be heterogeneous, or at least contain data constructed using various AW> different type constructors. The list would also be changing all the AW> time, for example in a drawing program. So, using the suggestion of a AW> list of drawing functions, would this not mean I would have to alter the AW> list of drawing functions each time I alter the list of shapes? of course, we suggest this solution only for cases when all you need from shapes - to draw them. as i write in previous letter, one time i changed a list of regular expressions to list of functions that check matches with these regular expressions because it was the only operation i need if you require several operations, you can pack them in tuple or structure (really, tuple is a structure without field names) if you need fields/interfaces inheritance, then this method will not work. but in 90% of cases it's enough. in C++, creating classes is the most useful method of structuring program, so it is used for any problem - from very simple to most complex. when you translate such thing to Haskell, it's better to see which concrete problem are solved with help of classes, and not to emulate the classes itself (because in this case you will write many unneeded code) AW> It does AW> not seem an extensible solution. When it comes to adding new types or AW> new functionality for existing types would it not be easy to miss AW> updating a part of the program source that needed updating? adding new "class" needs adding one "constructor" function. it's hard to skip that part :) adding new function to interface need to change interface structure definition or adding new element to tuple. then type-checker of Haskell compiler will force you to add new function to all places where this structure/tuple are created. you just never worked with such strong type-checking as in Haskell AW> The example I have found the simplest is the one given by Lennart, AW> however this to me seems equivalent to the way people use switch AW> statements in C to mimic dynamic binding. only because it's C-like :) you just can't believe that Haskell program can be 3-10 times smaller while keeping the same functionality :))) -- Best regards, Bulat mailto:bulatz@HotPOP.com