Your stage of Haskell Evolution?

Why is this poll closed after one vote?
Moreover, why is the result posted on the front page?

It seems that there are spam trackbacks, is there something we can do about that?

I'm at the Monad stage.

Its not listed in the Evolution, so here is my little contribution. I'm afraid I can't get the formatting to come out, so you'll have to infer it.


module FactorialMonad (factorial, Factorial) where


import Control.Monad.State


-- Declare the Factorial monad as a State holding two integers.
-- The first is the counter, the second is the accumulator. With
-- each tick the accumulator is multiplied by the counter and the
-- counter is incremented.
type Factorial = State (Integer, Integer)


-- Declare the initial state and the state modifier

factorialZero :: (Integer, Integer)
factorialZero = (0,1)

-- A 'tick' is a state computation that moves to the next factorial
-- and returns it.
tick :: Factorial Integer
tick =
do
modify next
gets snd
where
next (c, a) = (c+1, a * (c+1))

-- 'value' is a state computation that returns the current factorial.
value :: Factorial Integer
value = gets snd


-- The computation for 'factorial n' is therefore a sequence of n ticks.
factorial :: Int -> Integer
factorial n = evalState factorial' factorialZero
where
computation = sequence_ $ replicate n tick
factorial' = do
computation
value