Is your intent is to call main function? How do you compile this module, is it part of a bigger project, or just a single-file program? If single-file -- rename main to something else, then create a main function which has a type "main :: IO ()", and put something like:

main :: IO ()

main = print (mainRoutine 10 5)

It'll call mainRoutine with 10 and 5 args and print its result. You can then run it with "runhaskell MyProg.hs"

On Sun, Dec 13, 2015 at 7:14 PM, Imants Cekusins <imantc@gmail.com> wrote:

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)
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