Hi all! How can we model in Haskell three hypothetic data structures A, B, C which obey following conditions: 1. A is anything 2. B is list of A's 3. C is list of A's or B's or another C's So, for example, it would be possible to write something like: C [A, B [A, A], A, C [A, B [A]]]. I wrote this code: data Super = A | B [A] | C[Super] Not surprisingly, Haskell refused to accept subexpression `B[A]`, because "Not in scope: type constructor or class `A'". How to solve this problem? More "real" example of A, B and C: data Shape = Point Int Int | Polyline [Point] | Shape [Shape] Interestingly, if I replace Polyline [Point] to Polyline [Shape] then definition will be accepted. But what if I want *polylines* consists of *points*, not any *shapes*? Thank you! -- Nikolay Artamonov