foo.hs

import Control.Monad.State (State, get, put, runState)

inc :: State Int ()
inc = fmap (+ 1) get >>= put
   -- get returns the state's int
   -- fmap applies (+ 1) on that int
   -- >>= applies the result to put which makes it the current state

reset :: State Int ()
reset = put 0
   -- put makes 0 the current state

getCounter :: State Int Int
getCounter = get
   -- get same as in inc, it get state

foobar :: Int
foobar = let
  -- here we got a "private" interface within foobar
  inc' = fmap (+ 1) get >>= put
  reset' = put 0
  getCounter' = get
  in (\(_, x) -> x) $ runState (do inc' -- here we got a sequence of actions
  {- the above is a lambda -}      reset' -- each replaces the current state
  {- which simply extracts -}      inc'
  {- the result from the -}        inc'
  {- struct (tuple) that gets -}   _ <- getCounter' -- here we bind the int to the var _
  {- returned -}                   inc') 0 -- this 0 is the default state

-- if you run foobar you will get 3