I came across an interesting paper "Codata and comonads in Haskell" by Richard Kieburtz, and some other work by Alberto Pardo on comonads. Kieburtz proposes an OI comonad, as an alternative to (or something alongside?) the IO monad. Just for reasons of symmetry, it's natural to wonder if they aren't as pervasive in functional programs as monads. Frustratingly, I can't seem to grasp the intuition. I'm aware that the state-monad s->(s,a) has a dual state-in-context comonad (s,s->a). How does the side-effect manifest itself? The (ineffective) hints the authors give revolve around the idea that side-effects "derive" from the context of a program. Any other hints? Are comonads especially appropriate for programming systems that react to changes in their environment? Is an object of OI type something like a program to react to events? I can't make the slightest sense of Kieburtz's OI co-monad, with commands like coGetChar :: OI Handle -> Char. Peter Hancock