Arrow example

Arr.hs

{-# LANGUAGE Arrows #-}
module Main where

import Control.Arrow
import Control.Monad
import qualified Control.Category as Cat
import Data.List
import Data.Maybe
import System.Random

newtype Circuit a b = Circuit { unCircuit :: a -> (Circuit a b, b) }

instance Cat.Category Circuit where
    id = Circuit $ \a -> (Cat.id, a)
    (.) = dot
        where
          (Circuit cir2) `dot` (Circuit cir1) = Circuit $ \a ->
              let (cir1', b) = cir1 a
                  (cir2', c) = cir2 b
              in (cir2' `dot` cir1', c)

instance Arrow Circuit where
    arr f = Circuit $ \a -> (arr f, f a)
    first (Circuit cir) = Circuit $ \(b, d) ->
        let (cir', c) = cir b
        in (first cir', (c, d))

runCircuit _ [] = []
runCircuit cir (x:xs) =
    let (cir', x') = unCircuit cir x
    in x' : runCircuit cir' xs

runCircuit' cir inputs =
    snd $ mapAccumL (\cir x -> unCircuit cir x) cir inputs

accum :: acc -> (a -> acc -> (b, acc)) -> Circuit a b