redux-like-state.hs

module Main where

import Control.Monad.State.Lazy

data AppState = AppState {
                  _appCount :: Int
                , _appActions :: [String]
               } deriving Show

data Action
  = Increment 
  | Decrement
  | Reset
  | SetTo Int deriving (Show)

updateState :: Action -> AppState -> AppState
updateState action state = state { _appCount = updateCount action (_appCount state), _appActions = show action : _appActions state }
  where
    updateCount :: Action -> Int -> Int
    updateCount Increment x = x + 1
    updateCount Decrement x = x - 1
    updateCount Reset _ = 0
    updateCount (SetTo i) _ = i

initialState :: AppState
initialState =  AppState 0 []

simpleStateExample :: (Int, AppState)
simpleStateExample = runState someStateComputation initialState 
  where
    someStateComputation :: State AppState Int
    someStateComputation = do
      modify (updateState Increment)
      modify (updateState Decrement)
      modify (updateState (SetTo 2))
      modify (updateState Reset)
      gets _appCount

main :: IO ()
main = do
  print simpleStateExample