In the following thread, I reported a problem I was having with programs diverging when they use infinite lists with StateT: http://www.haskell.org//pipermail/haskell-cafe/2005-November/012253.html Roberto Zunino and Wolfgang Jeltsch pointed out that this is a bug in Control.Monad.State: (>>=) is lazy in State, but strict in StateT. The source of the problem is that the pattern match in the definition of (>>=) is inside a let expression in State - so it is lazy - but inside a do expression inside StateT - so it strict. Roberto suggests a one-byte patch to fix this problem: add '~' to make the pattern match in StateT irrefutable. (Note that this has already been done for the MonadFix instance of StateT.) The same fix should be applied to evalStateT and execStateT. They are also stricter than their non-transformer counterparts, for the same reason. -Yitz