Hi Larry,
I approached Haskell very recently and I see in your posts many doubts
that I experienced myself, so perhaps I can be of some help by
providing you with the intuition that guided me, rather than with an
in depth explanation of how monadic state works.
On Sat, Nov 1, 2008 at 5:23 PM, Larry Evans
get = State (\s -> (s,s)) put newS = State (\oldS ->( (), newS ))
That still leaves me confused about:
put (n+1)
AFAICT, with the preceding |n <- get|, this means:
put (get+1)
pretty much as with IO, you shouldn't think of get as of a function (or value) representing the value of the state, nor should you think of put as of a function that modifies the state. You should think of get and put (n + 1) as of *actions* that, *when performed*, will read and write the state. In particular put (n + 1) is NOT equivalent to put (get + 1) because "get" here is not the state (of value Int), but it's an action that lets you access the state and n <- get is the way you have to say that you want to perform the get action and bind the value in the state to the name n. The thing that I found really weird in the beginning was that with State the state is never mentioned anywhere! But there is where the intuition "action that reads/writes the state" comes into the game. --luca