In the transformers library, there are two implementations for callCC for use with StateT (both lazy and strict) [1,2], the second of which is documented to not satisfy the laws of a monad transformer. [1]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-T... [2]: http://hackage.haskell.org/package/transformers-0.4.2.0/docs/Control-Monad-T... However, in mtl the MonadCont class [3] uses the second definition for the StateT instance. Is there a good reason for this, or just to match the behaviour of pre-transformers mtl [4] ? [3]: http://hackage.haskell.org/package/mtl-2.2.1/docs/Control-Monad-Cont-Class.h... [4]: http://hackage.haskell.org/package/mtl-1.1.1.1/docs/src/Control-Monad-State-... (I did raise an issue for this on mtl's Github issue tracker, but it hasn't had any responses for two months.) -- Ivan Lazar Miljenovic Ivan.Miljenovic@gmail.com http://IvanMiljenovic.wordpress.com