A Control.Arrow in base package introduces an arrow type, and ghc have good support for arrow notation. Many things, avaible in monads, are avaible in arrows as well. There is an arrows package, that introduces some arrow classes : state, reader, writer and so on. However, it does not introduce systematic lifting of arrow classes operations. Arrows are generalisation of monads. There are libraries, that introduces systematic lifting of operations from monad classes. So, the quesions are: what operations should be in arrow transformer class? Captain Obvious says, it should look at least like: class ArrowTrans t where lift :: ( Arrow a, Arrow (t a) ) => a b c -> t a b c what laws arrow transformers must obey? C.O. says, at least lift a b c >>> lift a c d == lift ( a b c >> a c d ) how to perform lifting of actions for classes like this: class Arrow a => ArrowError a e| a-> e where raise :: a e () handle :: a e c -> a b c -> a b c class Arrow a => ArrowReader a e | a -> e where look :: a () e local :: a b c -> a (b,e) c The answers lies somewhere in category theory, C.O. says. Of course, such answer is not satisfactory.