Kaveh Shahbazian wrote:
Thanks All This is about my tries to understand monads and handling state - as you perfectly know - is one of them. I have understood a little about monads but that knowledge does not satidfy me. Again Thankyou
There are many tutorials available from the wiki at http://www.haskell.org/haskellwiki/Books_and_tutorials#Using_Monads and http://www.haskell.org/haskellwiki/Monad Another way is to look at the source code for the State monad and StateT monad transformer, then you can see that the mysterious monad is nothing other than a normal data or newtype declaration together with an instance declaration ie: -- from State.hs newtype State s a = S (s -> (a,s)) instance Monad (State s) where return a = S (\s -> (a, s)) S m >>= k = S (\s -> let (a, s1) = m s S n = k a in n s1) So if you want to understand what's going on when you write: do x <- q p a first step is to remove the syntactic sugar to get: q >>= (\x -> p) and then replace the >>= with it's definition for the monad you're using. For example with the State monad, (q) must be some expression which evaluates to something of the form S fq where fq is a function with type s -> (a,s), and similarly, (\x -> p) must have type a ->S ( s -> (a,s)). If we choose names for these values which describe the types we have: q = S s_as p = a_S_s_as so q >>= (\x -> p) === S s_as >>= a_S_s_as === S (\s0 -> let (a1, s1) = s_as s0 S s_a2s2 = a_S_s_as a1 in s_a2s2 s1) If we use State.runState s0 (q >>= (\x -> p)) to execute this composite action, from the source we see that: runState :: s -> State s a -> (a,s) runState s (S m) = m s so runState s0 (q >>= (\x -> p)) === runState s0 (S (\s0 -> let ... in s_a2s2 s1)) === (\s0 -> let ... in s_a2s2 s1) s0 === s_a2s2 s1 === a2s2 -- ie (a2, s2) Anyway I hope I haven't made things more complicated! ;-) The best thing is to just try and work through some examples yourself with pencil and paper and read lots of tutorials until things start clicking into place. Regards, Brian. -- Logic empowers us and Love gives us purpose. Yet still phantoms restless for eras long past, congealed in the present in unthought forms, strive mightily unseen to destroy us. http://www.metamilk.com