haskellstate-monad

Convert State Int to Int

I am trying to implement a counter and I'd like to convert something of type State Int to Int, so I can add it to a variable. Can somebody help me develop an auxiliary function to perform this cast?

I have been stuck on this for a while now, because I am not very used to working with monad State.

Solution

  • Use execState to run the stateful action and obtain the final state:

    execState :: State s a -> s -> s

    For example, incByOne increments the counter by one:

    incByOne :: State Int ()
incByOne = modify (+1)

    Then, if you use the execState monad runner with the initial state of 7:

    main :: IO ()
main = do
  let initialCount = 7
  let count = execState incByOne initialCount
  print count

    It will print 8