(Forwarding to the list, sorry Albert) On Sunday, 29 December 2013 08:32:06, Benjamin Edwards < edwards.benj@gmail.com> wrote: I would also recommend looking at use / zoom from the lens package. I find them indispensable for when you want to use a compound state. If you just have two parts you can write all your state calculations on the separate parts then user StateT m (a,b) r and zoom _1 / zoom _2 to access them in the same do block.