Cogs and Levers A blog full of technical stuff

A Quick Lap with the State Monad

16 Mar 2014

Introduction

The State monad gives functionality of both the Reader monad and Writer monad in one. When using the State monad you’re able to read the state at any time and then set it back again, providing read/write access.

Key Points

  • The function get is used to read the current state
  • The function put is used to set the state
  • runState is used to manage execution of functions that run in the State monad
  • Operations in the State monad can use >>= to be chained together
  • Functions in the State monad are decorated with State s v. Where s is the type of the state and v is the return type from the function<

An Example

import Control.Monad.State

-- | Starts a value off.
-- This function doesn't perform any calculation at all, it just prepares an
-- initial value to start in the calculation pipeline
--
start :: Int -> State [String] Int
start x = do
  put ["Starting with " ++ show x]
  return x

-- | Halve a value
-- Any value passed into this function gets halved
--
half :: Int -> State [String] Int
half x = do
  s <- get
  let ns = s ++ ["Halving " ++ show x]
  put ns
  return (x `div` 2)

-- | Squares a value
-- Any value passed into this function gets squared
--
sqr :: Int -> State [String] Int
sqr x = do
  s <- get
  let ns = s ++ ["Squaring " ++ show x]
  put ns
  return (x * x)

main :: IO ()
main = do
  let c = runState $ start 10 >>= half >>= sqr >>= half
  let work = c [""]
  let ans  = fst $ work
  let log  = snd $ work

  putStrLn $ "Answer: " ++ show ans
  putStrLn ""
  putStrLn " ==== Log ==== "

  mapM_ putStrLn log

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