Hi Sergey Further... The writer monad is accompanied by a function 'tell' that is called explicitly to produce output. 'tell' is the 'non-proper morphism' of the writer monad (well the standard Monad Transformer Library - MTL version has a couple of other functions but that's an implementation detail). It characterizes the writer monad in the same way that 'ask' characterises the reader monad and the combination of get & put characterise the state monad. In Haskell, you have to represent a Monad with a data type or newtype - non-proper morphisms need to 'unpeel' the constructor to access the internals of the monad. Here is the code from MTL for Writers tell: instance (Monoid w) => MonadWriter w (Writer w) where tell w = Writer ((), w) ... (listen and pass - the other non-proper morphisms). Non-proper morphisms contrast with general monadic functions that are usable with any monad. Take liftM as an example - liftM needs only (>>=) and 'return' it doesn't have to look inside a monad's constructor: -- | Promote a function to a monad. liftM :: (Monad m) => (a1 -> r) -> m a1 -> m r liftM f m1 = do { x1 <- m1; return (f x1) } -- aka. liftM f m1 = m1 >>= \x1 -> return (f x1) You would want to add a 'tell' function to your module. You're not obliged to call it 'tell' of course, the MonadLib library, which is an alternative to the standard MTL library, calls the function 'put'. Best wishes Stephen