{- | This module provides a pure implementation of Reflex, which is intended to serve as a reference for the semantics of the Reflex class.  All implementations of Reflex should produce the same results as this implementation, although performance and laziness/strictness may differ.
-}
{-# LANGUAGE TypeFamilies, MultiParamTypeClasses, EmptyDataDecls #-}
module Reflex.Pure where

import Reflex.Class
import Data.Functor.Misc

import Control.Monad
import Data.MemoTrie
import qualified Data.Dependent.Map as DMap

data Pure t

-- | The Enum instance of t must be dense: for all x :: t, there must not exist any y :: t such that pred x < y < x
--   The HasTrie instance will be used exclusively to memoize functions of t, not for any of its other capabilities
instance (Enum t, HasTrie t, Ord t) => Reflex (Pure t) where
  newtype Behavior (Pure t) a = Behavior { unBehavior :: t -> a }
  newtype Event (Pure t) a = Event { unEvent :: t -> Maybe a }
  type PushM (Pure t) = (->) t
  type PullM (Pure t) = (->) t
  never = Event $ \_ -> Nothing
  constant x = Behavior $ \_ -> x
  push f e = Event $ memo $ \t -> unEvent e t >>= \o -> f o t
  pull = Behavior . memo
  merge events = Event $ memo $ \t ->
    let currentOccurrences = unwrapDMapMaybe (($ t) . unEvent) events
    in if DMap.null currentOccurrences
       then Nothing
       else Just currentOccurrences
  fan e = EventSelector $ \k -> Event $ \t -> unEvent e t >>= DMap.lookup k
  switch b = Event $ memo $ \t -> unEvent (unBehavior b t) t
  coincidence e = Event $ memo $ \t -> unEvent e t >>= \o -> unEvent o t

instance Ord t => MonadSample (Pure t) ((->) t) where
  sample = unBehavior

instance (Enum t, HasTrie t, Ord t) => MonadHold (Pure t) ((->) t) where
  hold initialValue e initialTime = Behavior f
    where f = memo $ \sampleTime ->
            if sampleTime <= initialTime -- Really, the sampleTime should never be prior to the initialTime, because that would mean the Behavior is being sampled before being created
            then initialValue
            else let lastTime = pred sampleTime
                 in case unEvent e lastTime of
                   Nothing -> f lastTime
                   Just x -> x