[Haskell-cafe] Detecting Cycles in Datastructures

[Tom Hawkins]

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