[Haskell-cafe] Detecting Cycles in Datastructures

In a pure language, is it possible to detect cycles in recursive data structures? For example, is it possible to determine that "cyclic" has a loop? ... data Expr = Constant Int | Addition Expr Expr cyclic :: Expr cyclic = Addition (Constant 1) cyclic Or phased differently, is it possible to make "Expr" an instance of "Eq" such that cyclic == cyclic is smart enough to avoid a recursive decent? -Tom