I've got one of those algorithms which "threatens to march off the right edge" (in the words of Goerzen et al). I need something like a State or Maybe monad, but this is inside the IO monad. So I presume I need StateT or MaybeT. However, I'm still (sdlowly) learning about monads from first principles. I thought I might present my code and get some pointers... maybe someone could actually show me how to rewrite it, which would be a neat way to see MaybeT and StateT in action. I'm hoping to get anything from a one-line response to a rewrite of my code. Anything will help.
Perhaps this is useful: http://www.haskell.org/haskellwiki/Equational_reasoning_examples#Coding_styl... I can't quite tell--is that example in the IO monad? Part of my difficulty is that I'm inside IO. I know how to do this with Maybe, except that I have to combine Maybe and IO (use MaybeT?)
It was in the GHC.Conc.STM monad, so yes, it used a MaybeT and Control.Monad.Trans.MonadTrans's lift (btw, the MonadTrans docs only point to [1], but [2] might also be of interest, if rather more compact/terse). Claus [1] http://web.cecs.pdx.edu/~mpj/pubs/springschool.html [2] http://web.cecs.pdx.edu/~mpj/pubs/modinterp.html