state-monad

Documentation for the parsec package states that u argument is used to carry some user state through monadic computation. But the same functionality can be achieved by basing ParsecT monad transformer on State monad. So if my parser is not stateful, i don't need u altogether, but have to set it to () with parsec. What's rationale for adding non-optional state support to ParsecT ? Because a parser of type ParsecT s () (State st) a behaves differently from a parser of type Parsec s st Identity a when it comes to backtracking: User state resets when parsec tries an alternative after a failing

Trying to learn to write applications with Gtk2Hs I'm getting difficulties bridging the gap between the event driven Gtk2HS and the persistent state of my model. So to simplify, lets say that I have this simple application module Main where import Graphics.UI.Gtk import Control.Monad.State main = do initGUI window <- windowNew button <- buttonNew set button [buttonLabel := "Press me"] containerAdd window button -- Events onDestroy window mainQuit onClicked button (putStrLn ---PUT MEANINGFUL CODE HERE---) widgetShowAll window mainGUI and the state of my application is how many times the button

I'm working on implementing the UCT algorithm in Haskell, which requires a fair amount of data juggling. Without getting into too much detail, it's a simulation algorithm where, at each "step," a leaf node in the search tree is selected based on some statistical properties, a new child node is constructed at that leaf, and the stats corresponding to the new leaf and all of its ancestors are updated. Given all that juggling, I'm not really sharp enough to figure out how to make the whole search tree a nice immutable data structure à la Okasaki . Instead, I've been playing around with the ST

I haven't seen many examples of the scalaz state monad. There is this example but it is hard to understand and there is only one other question on stack overflow it seems. I'm going to post a few examples I've played with but I would welcome additional ones. Also if somebody can provide example on why init , modify , put and gets are used for that would be great. Edit: here is an awesome 2 hours presentation on the state monad. I assume, scalaz 7.0.x and the following imports (look at answer history for scalaz 6.x ): import scalaz._ import Scalaz._ The state type is defined as State[S, A]