My OS is ubuntu. I've used synaptic to install: hat 2.05 rerolled-5ubuntu1. The installed files include several in /usr/include/hat-2.05, including Control/Monad/State.hx. I would like to use hat to trace what happens with the code: plus n x shown here: http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona... However, I've tried several variations of the Makefile: {--cut here-- MAIN=tickSimple INCS= GHC.FLAGS=-i$(INCS) GHC.FLAGS=-XMultiParamTypeClasses -XFunctionalDependencies -XFlexibleInstances GHC.PKGS= -package Hat.Control.Monad.State GHC.PKGS= GHC.OPTS=$(GHC.FLAGS) $(GHC.PKGS) hat: hmake -hat $(GHC.OPTS) $(MAIN) hat-trans $(GHC.OPTS) $(MAIN).hs ghc -package hat $(GHC.OPTS) -c Hat/$(MAIN).o Hat/$(MAIN).hs ghc -package hat $(GHC.OPTS) -o Hat/$(MAIN) Hat/$(MAIN).o {--cut here-- On the source file: {--tickSimple.hs-- module Main where -- newtype State -- class MonadState -- instance MonadState (State s) -- cp'ed from: -- http://www.haskell.org/all_about_monads/html/statemonad.html#definition newtype State s a = State { runState :: (s -> (a,s)) } instance Monad (State s) where return a = State $ \s -> (a,s) (State x) >>= f = State $ \s -> let (v,s') = x s in runState (f v) s' class MonadState m s | m -> s where get :: m s put :: s -> m () instance MonadState (State s) s where get = State $ \s -> (s,s) put s = State $ \_ -> ((),s) -- execState: -- just guesses based on: -- http://hackage.haskell.org/packages/archive/mtl/latest/doc/html/Control-Mona... execState :: State s a -> s -> a execState s = snd(runState(s)) tick :: State Int Int tick = do n <- get put (n+1) return n plusOne :: Int -> Int plusOne n = execState tick n plus :: Int -> Int -> Int plus n x = execState (sequence $ replicate n tick) x main = do print (plusOne 2) print (plus 2 3) }--tickSimple.hs-- However, I always get some error. I have tried just using: import Control.Monad.State But the compiler then says it can't find the library. What should I do to debug this code with hat? TIA -regards, Larry