ChrisPenner

After listening to the latest Magic Read-along episode "You should watch this" (which you should go listen to now) I got caught up thinking about Brian's idea of an Endomorphism version of Kleisli composition for use with Redux, it's actually a very similar model to what I'm using in my event framework for event listeners so I figured I'd try to formalize the pattern and recognize some of the concepts involved. IIRC Brian described the idea of a Redux-reducer, which is usually of type s -> Action -> s, it takes a state and an action and returns a new state. He then re-arranged the arguments to Action -> s -> s. He then recognized this as Action -> Endo s (an Endo-morphism is just any function from one type to itself: a -> a). He would take his list of reducers and partially apply them with the Action, yielding a list of type Endo s where s

cfchou

import Data.Char
import Control.Monad
import Control.Monad.Trans.State.Lazy (StateT(..), runStateT)
import Control.Monad.Trans.Reader
import Control.Monad.State.Class
import Control.Monad.Trans.Class

type Pos = (Int, Int)         -- (line, column) 
type PString = (Pos, String)