maybeTransitionMatrix.hs

{-# LANGUAGE GeneralizedNewtypeDeriving #-}

import Control.Applicative
import Data.Maybe
import Data.Monoid

newtype X a = X { getX :: Maybe a } deriving (Eq, Ord, Read, Show, Functor, Applicative, Monad)

instance Monoid a => Monoid (X a) where
  mempty = X (Just mempty)
  X (Just x) `mappend` X (Just y) = X (Just (x `mappend` y))
  X (Nothing) `mappend` _ = X Nothing
  _ `mappend` X (Nothing) = X Nothing

g = X . Just . Sum

xs = map g [1..10]
ys = (map g [1..5]) ++ [X Nothing] ++ (map g [6..10])