Hi guys, the way `StateT` are implemented as `Applicative` have been buggling my mind for some time. https://hackage.haskell.org/package/transformers-0.5.2.0/docs/src/Control.Mo... https://hackage.haskell.org/package/transformers-0.5.2.0/docs/src/Control.Mo... instance (Functor m, Monad m) => Applicative (StateT s m) where pure a = StateT $ \ s -> return (a, s) StateT mf <*> StateT mx = StateT $ \ s -> do (f, s') <- mf s (x, s'') <- mx s' return (f x, s'') Using dependant monadic computations, this implementation cannot be expressed in term of applicative. This explains why we cannot have `instance (Applicative m) => Applicative (State s m)`. However using real monadic style computations for implementing `<*>` buggles my mind. Moreover `liftA2 (<*>)` can be used to generically compose applicative functors so why monads are needed? https://www.haskell.org/haskellwiki/Applicative_functor#Applicative_transfom... https://www.haskell.org/haskellwiki/Applicative_functor#Applicative_transfom... Any inputs would be greatly appreciated! Cheers, Laurent