Monads somewhat explained w/ dense example. Read more: http://readlists.com/0bca8296 . Kudos to: bartoszmilewski.com .

gistfile1.hs

import Control.Monad

data StackCalc a = StackCalc { runStackCalc :: [Int] -> ([Int], a) }

instance Monad StackCalc where
  return x = StackCalc aux
    where aux stack = (stack, x)

  f >>= e = StackCalc aux
    where aux stack = let (newstack, newval) = runStackCalc f stack
                      in runStackCalc (e newval) newstack

cPush :: Int -> StackCalc ()
cPush x = StackCalc aux
  where aux stack = ((x:stack), ())

cPop :: StackCalc (Maybe Int)
cPop = StackCalc aux
  where aux (s:stack) = (stack, Just s)
        aux stack = (stack, Nothing)

cPeek :: StackCalc (Maybe Int)
cPeek = StackCalc aux
  where aux ss@(s:stack) = (ss, Just s)
        aux stack = (stack, Nothing)

cAdd :: StackCalc ()
cAdd = StackCalc aux
  where aux (x:y:stack) = ((xy:stack), ())
          where xy = x+y
        aux stack = (stack, ())


cFibNext :: StackCalc Int
cFibNext = StackCalc aux
  where aux (x:y:stack) = (xy:x:stack, xy)
          where xy = x+y
        aux stack = (stack, 0)


test = do
  cPush 0
  cPush 1
  cFibNext
  cFibNext
  cFibNext
  cFibNext
  cFibNext
  cFibNext
  cFibNext
  cFibNext
  cPeek
  cPeek
  cPeek
  cPeek
  cPeek
  cPeek
  cPop