Async.hs

-- |
-- Copyright  : (c) Ivan Perez and Manuel Baerenz, 2016
-- License    : BSD3
-- Maintainer : ivan.perez@keera.co.uk
--
-- This module contains operations on monadic streams that are asynchronous,
-- i.e. that change the speed at which data enters or leaves the 'MSF'.
module Data.MonadicStreamFunction.Async where

-- Internal
import Data.MonadicStreamFunction.Core
import Data.MonadicStreamFunction.InternalCore
import Data.MonadicStreamFunction.Util         (MStream)

-- |
-- Concatenates a monadic stream of lists to a monadic stream.
-- The stream of lists will be called exactly when new data is needed.
--
-- Example:
--
-- >>> let intstream = constS $ putStrLn "Enter a list of Ints:" >> readLn :: MStream IO [Int]
-- >>> reactimate $ concatS intstream >>> arrM print
-- Enter a list of Ints:
-- [1,2,33]
-- 1
-- 2
-- 33
-- Enter a list of Ints:
-- []
-- Enter a list of Ints:
-- []
-- Enter a list of Ints:
-- [1,2]
-- 1
-- 2
-- Enter a list of Ints:
-- ...
--
-- Beware that @concatS msf@ becomes unproductive when @msf@ starts outputting empty lists forever.
-- This is ok:
--
-- >>> let boolToList b = if b then ["Yes"] else []
-- >>> let everyOddEmpty = count >>> arr (even >>> boolToList)
-- >>> reactimate $ concatS everyOddEmpty >>> arrM print
-- "Yes"
-- "Yes"
-- "Yes"
-- "Yes"
-- "Yes"
-- ...
--
-- But this will be caught in a loop:
--
-- >>> let after3Empty = count >>> arr ((<= 3) >>> boolToList)
-- >>> reactimate $ concatS after3Empty  >>> arrM print
-- "Yes"
-- "Yes"
-- "Yes"
-- ^CInterrupted.
concatS :: Monad m => MStream m [b] -> MStream m b
concatS msf = MSF $ \_ -> tick msf []
  where
    tick msf' (b:bs) = return (b, MSF $ \_ -> tick msf' bs)
    tick msf' []     = do
      (bs, msf'') <- unMSF msf' ()
      tick msf'' bs
-- TODO Maybe this can be written more nicely with exceptions?
-- Similarly takeS :: Int -> MSF m a b -> MSFExcept m a b () throws an exception after n ticks
-- Or with merge?