How can I write this simple code using the state monad?

Question

I\'m a beginner at Haskell and I\'ve come across a situation where I would like to use the state monad. (Or at least, I think I that\'s what I\'d like to use.) There are a milli

Answer 1

First you need to convert your existing functions to return State Machine a values:

import Control.Monad.State.Lazy

data Machine = Register Int

addToState :: Int -> State Machine ()
addToState i = do
        (Register x) <- get
        put $ Register (x + i)

subtractFromState :: Int -> State Machine ()
subtractFromState i = do
        (Register x) <- get
        put $ Register (x - i)

getValue :: State Machine Int
getValue = do
        (Register i) <- get
        pure i

then you can combine them into a stateful computation:

program :: State Machine Int
program = do
  addToState 6
  subtractFromState 4
  getValue

finally you need can run this computation with evalState to get the final result and discard the state:

runProgram :: Int
runProgram = evalState program (Register 0)