Well, yes, you can add a rectangle quite easily: type Shape = | Circle of float | Square of float | Rectangle of float * float The

Well, yes, you can add a rectangle quite easily: type Shape = | Circle of float | Square of float | Rectangle of float * float The thought about adding new shapes (mentioned several times in this thread) is just misplaced. If you're releasing library code people should not be extending your built-in types, that's bad form -- consumers of a library should interact with it through interfaces only.