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PureScriptAdvent Calendar 2017

Day 12
@legokichi

Functor, Applicative, Monad, Comonad, MonadTrans, Profunctor, Strong, Arrow, Choice, ArrowChoice, Costrong, ArrowLoop

Last updated at Posted at 2018-02-06

Functor, Applicative, Monad, Comonad

monad.jpg

Kleisli 圏の射 >=>

kleisli.jpg

MonadTrans

trans.jpg

Profunctor, Strong, Arrow

arrow.jpg

Arrow, ArrowLoop

arrow2.jpg

Profunctor, Strong, Arrow, Choice, ArrowChoice, Costrong, ArrowLoop

PureScript による Strong, Costrong, Choice のデモコード
PureScript は正格評価のため遅延評価が必要な Costrong (->) の loop == unfirst を実装していない
個々では型だけ合わせて擬似的に再現したが unfirst, unsecond は本来の意図通りのフィードバックループとしては機能しない

-- purescript 0.13.3

import Control.Applicative (pure)
import Control.Bind ((>>=), discard, bind)
import Control.Semigroupoid ((>>>))
import Data.Eq (class Eq, (==))
import Data.Either (Either(Left, Right))
import Data.Function (($), (#))
import Data.Profunctor (class Profunctor dimap, arr)
import Data.Profunctor.Choice ((+++), (|||))
import Data.Profunctor.Costrong (class Costrong, unfirst unsecond)
import Data.Profunctor.Strong ((&&&), (***), first, second)
import Data.Semigroup ((<>))
import Data.Show (class Show, show)
import Data.Tuple (Tuple(Tuple))
import Data.Unit (Unit, unit)
import Effect (Effect)
import Effect.Console (log)

main :: Effect Unit
main = do
  log "Arrow Demo"
  log $ ("arrow: " <> _) $ show $ A # a2bc >>> bc2cc >>> cc2aa >>> (second a2b)
  log $ ("choice: " <> _) $ show $ Left A # (a2b +++ a2c) >>> (b2c +++ c2a) >>> (c2a +++ a2b) >>> (a2c ||| b2c)
  log $ ("loop: " <> _) $ show $ let Function' a2a = unfirst (Function' ooo) in a2a $ a2a A
  log $ ("loop2: " <> _) $ show $ let Function' f = unfirst (Function' iii) in f true
  pure unit
  where
  ooo :: (Tuple A (Maybe Int)) -> (Tuple A (Maybe Int))
  ooo = case _ of
    Tuple a (Just c) ->
      Tuple a $ Just $ spy "ooo" $ c + 1
    Tuple a _ -> Tuple a $ Just $ spy "ooo" 0

  iii :: (Tuple Boolean (Unit -> Int)) -> (Tuple Boolean (Unit -> Int))
  iii (Tuple reset lazy) = case reset of
    true -> Tuple false \_ -> spy "iii" 0
    false -> Tuple false $ \_ -> spy "iii" $ 1 + lazy unit

  a2b :: A -> B
  a2b A = B

  a2c:: A -> C
  a2c A = C

  b2c :: B -> C
  b2c B = C

  c2a :: C -> A
  c2a C = A

  a2bc :: A -> (Tuple B C)
  a2bc = (a2b &&& a2c)

  bc2cc :: (Tuple B C) -> (Tuple C C)
  bc2cc = first b2c

  cc2aa :: (Tuple C A) -> (Tuple A A)
  cc2aa = (c2a *** c2a)

--- Costrong (->) === ArrowLoop のために必要
--- PureScript は正格評価のため遅延評価が必要な Costrong (->) を実装できない!!! 

newtype Function' a b = Function' (a -> b)

instance profunctorFn' :: Profunctor Function' where
  dimap a2b c2d (Function' b2c) = Function' (a2b >>> b2c >>> c2d)

instance costrongFn' :: Costrong Function' where
  unfirst :: forall a b c. (Function' (Tuple a c) (Tuple b c)) -> Function' a b
  unfirst (Function' f) =
    Function' \a ->
      let
        (Tuple b c) = f (Tuple a c)
      in
        b
    where
    c = undefined -- 正格評価の PureScript ではフィードバック値 c の初期値を遅延評価で算出できない!!!
  unsecond :: forall a b c. (Function' (Tuple a b) (Tuple a c)) -> Function' b c
  unsecond (Function' f) =
    Function' \b ->
      let
        (Tuple a c) = f (Tuple a b)
      in
        c
    where
    a = undefined -- 正格評価の PureScript ではフィードバック値 a の初期値を遅延評価で算出できない!!!

data A = A
derive instance eqA :: Eq A
instance showA :: Show A where
  show A = "A"

data B = B
derive instance eqB :: Eq B
instance showB :: Show B where
  show B = "B"

data C = C
derive instance eqC :: Eq C
instance showC :: Show C where
  show C = "C"

data D = D
derive instance eqD :: Eq D
instance showD :: Show D where
  show D = "D"

Kleisli Triple

todo...

referencce

reference2

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