[Haskell-beginners] basic State Monad

[Imants Cekusins]

This snippet increments integer n times using state monad.

How to call:
main 10 5




module BasicState where

import Control.Monad.State.Strict
import Debug.Trace

type St a = State a a


--  caller is not aware that main uses state
--  mai is a pure function
main :: Int -> Int -> Int
main start0 repeat0 =
        evalState (repeatN repeat0) start0


--  state-passing computation
repeatN :: Int -> St Int
repeatN n0              --  repeat n times
    | n0 < 1 = get      --  current state
    | otherwise = do
        withState pureStateModifier get     -- update state
        repeatN $ n0 - 1                    --  recurse


--  state unaware modifier function
pureStateModifier :: Int -> Int
pureStateModifier = (+ 1)