Take a look at the following: data Writer o v = Writer [o] v instance Monad (Writer o) where return v = Writer [] v (Writer os v) >>= f = let (Writer os' v') = f v in Writer (os ++ os') v' write :: o -> Writer o () write o = Writer [o] () writes :: [o] -> Writer o () writes os = Writer os () By this code, I define a new monad where I can "write" stuff into a big list. (Although I notice this is going to get slower and slower as the list grows...) For example, test = do write 1 write 2 write 3 yields "Writer [1,2,3] ()". Now, how do I implement the following? data Reader i v = Reader [i] v instance Monad (Reader i) where return = ??? (>>=) = ??? read :: Reader o o such that a Reader is created with an initial list, and the read function fetches 1 element out of that list. That is, the expression "x <- read" will take the head element of the list and put it into x, keeping the tail to be read later. (Oh yeah - and apparently that clashes with Prelude.read. Oh well!) I can't figure out how to implement this... The closest I managed was to make a Reader object also contain a function that tells (>>=) what to do to the Reader object you're binding against... But that seems to be horribly buggy. Any thoughs?