{-# LANGUAGE GeneralizedNewtypeDeriving #-}


This module provides efficient higher-order discrete signals.  For a
non entirely trivial example, let's create a dynamic collection of
countdown timers, where each expired timer is removed from the
collection.  First of all, we'll need a simple tester function:

 sigtest gen = 'replicateM' 15 '=<<' 'start' gen

We can try it with a trivial example:

 \> sigtest $ 'stateful' 2 (+3)

Our first definition will be a signal representing a simple named

 countdown :: String -\> Int -\> SignalGen (Signal (String,Maybe Int))
 countdown name t = do
   let tick prev = do { t \<- prev ; 'guard' (t \> 0) ; 'return' (t-1) }
   timer \<- 'stateful' (Just t) tick
   'return' ((,) name '<$>' timer)

Let's see if it works:

 \> sigtest $ countdown \"foo\" 4
 [(\"foo\",Just 4),(\"foo\",Just 3),(\"foo\",Just 2),(\"foo\",Just 1),(\"foo\",Just 0),

Next, we will define a timer source that takes a list of timer names,
starting values and start times and creates a signal that delivers the
list of new timers at every point:

 timerSource :: [(String, Int, Int)] -\> SignalGen (Signal [Signal (String, Maybe Int)])
 timerSource ts = do
   let gen t = 'mapM' ('uncurry' countdown) newTimers
           where newTimers = [(n,v) | (n,v,st) \<- ts, st == t]
   cnt \<- 'stateful' 0 (+1)
   'generator' (gen '<$>' cnt)

Now we need to encapsulate the timer source signal in another signal
expression that takes care of maintaining the list of live timers.
Since working with dynamic collections is a recurring task, let's
define a generic combinator that maintains a dynamic list of signals
given a source and a test that tells from the output of each signal
whether it should be kept.  We can use @mdo@ expressions (a variant of
@do@ expressions allowing forward references) as syntactic sugar for
'mfix' to make life easier:

 collection :: Signal [Signal a] -\> (a -\> Bool) -\> SignalGen (Signal [a])
 collection source isAlive = mdo
   sig \<- 'delay' [] ('map' 'snd' '<$>' collWithVals')
   coll \<- 'memo' ('liftA2' (++) source sig)
   let collWithVals = 'zip' '<$>' ('sequence' '=<<' coll) '<*>' coll
   collWithVals' \<- 'memo' ('filter' (isAlive . 'fst') '<$>' collWithVals)
   'return' $ 'map' 'fst' '<$>' collWithVals'

We need recursion to define the @coll@ signal as a delayed version of
its continuation, which does not contain signals that need to be
removed in the current sample.  At every point of time the running
collection is concatenated with the source.  We define @collWithVals@,
which simply pairs up every signal with its current output.  The
output is obtained by extracting the current value of the signal
container and sampling each element with 'sequence'.  We can then
derive @collWithVals'@, which contains only the signals that must be
kept for the next round along with their output.  Both @coll@ and
@collWithVals'@ have to be memoised, because they are used more than
once (the program would work without that, but it would recalculate
both signals each time they are used).  By throwing out the respective
parts, we can get both the final output and the collection for the
next step (@coll'@).

Now we can easily finish the original task:

 timers :: [(String, Int, Int)] -\> SignalGen (Signal [(String, Int)])
 timers timerData = do
   src \<- timerSource timerData
   getOutput '<$>' collection src ('isJust' . 'snd')
     where getOutput = 'fmap' ('map' (\\(name,Just val) -> (name,val)))

As a test, we can start four timers: /a/ at t=0 with value 3, /b/ and
/c/ at t=1 with values 5 and 3, and /d/ at t=3 with value 4:

 \> sigtest $ timers [(\"a\",3,0),(\"b\",5,1),(\"c\",3,1),(\"d\",4,3)]

If the noise of the applicative lifting operators feels annoying, she
(<http://personal.cis.strath.ac.uk/~conor/pub/she/>) comes to the
save.  Among other features it provides idiom brackets, which can
substitute the explicit lifting.  For instance, it allows us to define
@collection@ this way:

 collection :: Stream [Stream a] -> (a -> Bool) -> StreamGen (Stream [a])
 collection source isAlive = mdo
   sig \<- 'delay' [] (|'map' ~'snd' collWithVals'|)
   coll \<- 'memo' (|source ++ sig|)
   collWithVals' \<- 'memo' (|'filter' ~(isAlive . 'fst') (|'zip' ('sequence' '=<<' coll) coll|)|)
   'return' (|'map' ~'fst' collWithVals'|)


module FRP.Elerea.Experimental.Simple
    ( Signal
    , SignalGen
    , start
    , external
    , delay
    , generator
    , memo
    , stateful
    , transfer
    ) where

import Control.Applicative
import Control.Monad
import Control.Monad.Fix
import Data.IORef
import Data.Maybe
import System.Mem.Weak

{-| A signal can be thought of as a function of type @Nat -> a@, where
the argument is the sampling time, and the 'Monad' instance agrees
with the intuition (bind corresponds to extracting the current
sample). -}

newtype Signal a = S (IO a) deriving (Functor, Applicative, Monad)

{-| A dynamic set of actions to update a network without breaking
consistency. -}

type UpdatePool = [Weak (IO (),IO ())]

{-| A signal generator is the only source of stateful signals.  It can
be thought of as a function of type @Nat -> a@, where the result is an
arbitrary data structure that can potentially contain new signals, and
the argument is the creation time of these new signals.  It exposes
the 'MonadFix' interface, which makes it possible to define signals in
terms of each other. -}

newtype SignalGen a = SG { unSG :: IORef UpdatePool -> IO a }

{-| The phases every signal goes through during a superstep. -}

data Phase a = Ready a | Updated a a

instance Functor SignalGen where
  fmap = (<*>).pure

instance Applicative SignalGen where
  pure = return
  (<*>) = ap
instance Monad SignalGen where
  return = SG . const . return
  SG g >>= f = SG $ \p -> g p >>= \x -> unSG (f x) p

instance MonadFix SignalGen where
  mfix f = SG $ \p -> mfix (($p).unSG.f)

{-| Embedding a signal into an 'IO' environment.  Repeated calls to
the computation returned cause the whole network to be updated, and
the current sample of the top-level signal is produced as a result.
This is the only way to extract a signal generator outside the
network, and it is equivalent to passing zero to the function
representing the generator. -}

start :: SignalGen (Signal a) -- ^ the generator of the top-level signal
      -> IO (IO a)            -- ^ the computation to sample the signal
start (SG gen) = do
  pool <- newIORef []
  S sample <- gen pool
  return $ do
    let deref ptr = (fmap.fmap) ((,) ptr) (deRefWeak ptr)
    res <- sample
    (ptrs,acts) <- unzip.catMaybes <$> (mapM deref =<< readIORef pool)
    writeIORef pool ptrs
    mapM_ fst acts
    mapM_ snd acts
    return res

{-| Auxiliary function used by all the primitives that create a
mutable variable. -}

addSignal :: (a -> IO a)      -- ^ sampling function
          -> (a -> IO ())     -- ^ aging function
          -> IORef (Phase a)  -- ^ the mutable variable behind the signal
          -> IORef UpdatePool -- ^ the pool of update actions
          -> IO (Signal a)    -- ^ the signal created
addSignal sample update ref pool = do
  let  upd = readIORef ref >>= \v -> case v of
               Ready x  -> update x
               _        -> return ()

       fin = readIORef ref >>= \v -> case v of
               Updated x _  -> writeIORef ref $! Ready x
               _            -> error "Signal not updated!"

       sig = S $ readIORef ref >>= \v -> case v of
               Ready x      -> sample x
               Updated _ x  -> return x
  updateActions <- mkWeak sig (upd,fin) Nothing
  modifyIORef pool (updateActions:)
  return sig

{-| The 'delay' transfer function emits the value of a signal from the
previous superstep, starting with the filler value given in the first
argument.  It can be thought of as the following function (which
should also make it clear why the return value is 'SignalGen'):

 delay x0 s t_start t_sample
   | t_start == t_sample = x0
   | t_start < t_sample  = s (t_sample-1)
   | otherwise           = error \"Premature sample!\"

The way signal generators are extracted ensures that the error can
never happen. -}

delay :: a                    -- ^ initial output at creation time
      -> Signal a             -- ^ the signal to delay
      -> SignalGen (Signal a) -- ^ the delayed signal
delay x0 (S s) = SG $ \pool -> do
  ref <- newIORef (Ready x0)

  let update x = s >>= \x' -> x' `seq` writeIORef ref (Updated x' x)

  addSignal return update ref pool

{-| A reactive signal that takes the value to output from a signal
generator carried by its input with the sampling time provided as the
time of generation.  It is possible to create new signals in the
monad.  It can be thought of as the following function:

 generator g t_start t_sample = g t_sample t_sample

It has to live in the 'SignalGen' monad, because it needs to maintain
an internal state to be able to cache the current sample for
efficiency reasons. However, this state is not carried between
samples, therefore starting time doesn't matter and can be ignored.


generator :: Signal (SignalGen a) -- ^ the signal of generators to run
          -> SignalGen (Signal a) -- ^ the signal of generated structures
generator (S s) = SG $ \pool -> do
  ref <- newIORef (Ready undefined)
  let sample = do  SG g <- s
                   x <- g pool
                   writeIORef ref (Updated undefined x)
                   return x

  addSignal (const sample) (const (sample >> return ())) ref pool

{-| Memoising combinator.  It can be used to cache results of
applicative combinators in case they are used in several places.  It
is observationally equivalent to 'return' in the 'SignalGen' monad. -}

memo :: Signal a             -- ^ the signal to cache
     -> SignalGen (Signal a) -- ^ a signal observationally equivalent to the argument
memo (S s) = SG $ \pool -> do
  ref <- newIORef (Ready undefined)

  let sample = s >>= \x -> writeIORef ref (Updated undefined x) >> return x

  addSignal (const sample) (const (sample >> return ())) ref pool

{-| A signal that can be directly fed through the sink function
returned.  This can be used to attach the network to the outer
world. -}

external :: a                         -- ^ initial value
         -> IO (Signal a, a -> IO ()) -- ^ the signal and an IO function to feed it
external x = do
  ref <- newIORef x
  return (S (readIORef ref), writeIORef ref)

{-| A pure stateful signal.  The initial state is the first output,
and every subsequent state is derived from the preceding one by
applying a pure transformation.  It is equivalent to the following

 stateful x0 f = 'mfix' $ \sig -> 'delay' x0 (f '<$>' sig)

stateful :: a                    -- ^ initial state
         -> (a -> a)             -- ^ state transformation
         -> SignalGen (Signal a)
stateful x0 f = mfix $ \sig -> delay x0 (f <$> sig)

{-| A stateful transfer function.  The current input affects the
current output, i.e. the initial state given in the first argument is
considered to appear before the first output, and can never be
observed, and subsequent states are determined by combining the
preceding state with the current output of the input signal using the
function supplied.  It is equivalent to the following expression:

 transfer x0 f s = 'mfix' $ \sig -> 'liftA2' f s '<$>' 'delay' x0 sig

transfer :: a                    -- ^ initial internal state
         -> (t -> a -> a)        -- ^ state updater function
         -> Signal t             -- ^ input signal
         -> SignalGen (Signal a)
transfer x0 f s = mfix $ \sig -> liftA2 f s <$> delay x0 sig

{-| The @Show@ instance is only defined for the sake of 'Num'... -}

instance Show (Signal a) where
  showsPrec _ _ s = "<SIGNAL>" ++ s

{-| Equality test is impossible. -}

instance Eq (Signal a) where
  _ == _ = False
{-| Error message for unimplemented instance functions. -}

unimp :: String -> a
unimp = error . ("Signal: "++)

instance Ord t => Ord (Signal t) where
  compare = unimp "compare"
  min = liftA2 min
  max = liftA2 max

instance Enum t => Enum (Signal t) where
  succ = fmap succ
  pred = fmap pred
  toEnum = pure . toEnum
  fromEnum = unimp "fromEnum"
  enumFrom = unimp "enumFrom"
  enumFromThen = unimp "enumFromThen"
  enumFromTo = unimp "enumFromTo"
  enumFromThenTo = unimp "enumFromThenTo"

instance Bounded t => Bounded (Signal t) where
  minBound = pure minBound
  maxBound = pure maxBound

instance Num t => Num (Signal t) where
  (+) = liftA2 (+)
  (-) = liftA2 (-)
  (*) = liftA2 (*)
  signum = fmap signum
  abs = fmap abs
  negate = fmap negate
  fromInteger = pure . fromInteger

instance Real t => Real (Signal t) where
  toRational = unimp "toRational"

instance Integral t => Integral (Signal t) where
  quot = liftA2 quot
  rem = liftA2 rem
  div = liftA2 div
  mod = liftA2 mod
  quotRem a b = (fst <$> qrab,snd <$> qrab)
    where qrab = quotRem <$> a <*> b
  divMod a b = (fst <$> dmab,snd <$> dmab)
    where dmab = divMod <$> a <*> b
  toInteger = unimp "toInteger"

instance Fractional t => Fractional (Signal t) where
  (/) = liftA2 (/)
  recip = fmap recip
  fromRational = pure . fromRational

instance Floating t => Floating (Signal t) where
  pi = pure pi
  exp = fmap exp
  sqrt = fmap sqrt
  log = fmap log
  (**) = liftA2 (**)
  logBase = liftA2 logBase
  sin = fmap sin
  tan = fmap tan
  cos = fmap cos
  asin = fmap asin
  atan = fmap atan
  acos = fmap acos
  sinh = fmap sinh
  tanh = fmap tanh
  cosh = fmap cosh
  asinh = fmap asinh
  atanh = fmap atanh
  acosh = fmap acosh