I am about to rewrite my Z animation tool (JAZA) in a style that makes more intensive use of state monads. However, my experiments with a simplified lambda-calculus example shows that (with GHC 5.00) the state monad is dramatically less efficient than the simple identity monad: 4 TIMES SLOWER, and 7 TIMES MORE MEMORY! Is this normal? Acceptable? Am I doing something wrong? Can anyone suggest ways of reducing these overheads? (I am very keen to use state-monads if possible, because it allows my 'eval' code to be generic over the monad that is used, which allows me to reuse the code with other similar monads. In fact, I am using it to simulate the 'visitor' design pattern from OO langs.) Hugs gives slightly smaller differences (3 times more reductions and 3.5 times more cells), but I had hoped that GHC would be able to optimize most of the state monad overhead away (especially when the monad uses newtype)? My code and speed measurements are attached. Mark.