Hello – I’m proposing to restructure this< https://en.wikibooks.org/wiki/Haskell/Understanding_monads/State>;, and I have a draft here< https://en.wikibooks.org/wiki/Davjam2:Example/StateMonad>;. I’ve given my main reasons for the restructuring at the top of the draft page.
I’d very much like feedback before updating the real page, especially if people don’t like the new one much.
+1 for the new version by David. In addition to the content already present, I'd like to see examples (and exercises! these are great!) of State used with other common monadic combinators. You covered sequence, but what about mapM a.k.a. traverse, what about foldM? Can filterM be put to good use with state monads? You could introduce another type TurnstileInput = Push | Coin and combine pushS, coinS into a single function of type TurnstileInput -> State TurnstileState TurnstileOutput which can be used in mapM. Your exercises of different people can then be implemented as mapM of a single function (namely the FSM) over finite sequences of TurnstileInputs. As a side-note, Data.Traversable.mapAccumR is implemented using a StateR that passes the state backwards. Only an Applicative instance is given in Data.Functor.Utils. I wonder whether this is a lawful monad or not. The comment says so, but I don't see how. Here is a departure from the otherwise very concrete and down-to-earth explanation of State, but a fun exercise nevertheless: Implement return and (>>=) for type State s a = s -> (s,a) using the following two functions and the Functor instance of ((->) s). iso :: ((s,a) -> b) -> a -> s -> b iso uncurried = curry (uncurried . Data.Tuple.swap) iso' :: (a -> s -> b) -> (s,a) -> b -- inverse of iso iso' curried = uncurry (flip curried) Olaf