{-# LANGUAGE BangPatterns #-}
-----------------------------------------------------------------------------
-- |
-- Module      :  Control.Concurrent.Chan.Strict
-- Copyright   :  (c) The University of Glasgow 2001, Don Stewart 2007
-- License     :  BSD-style
-- 
-- Maintainer  :  dons@galois.com
-- Stability   :  experimental
-- Portability :  non-portable (concurrency)
--
-- Unbounded, element-strict channels. Elements will be evaluated to
-- WHNF on entering the channel. For some concurrency applications, this
-- is more desirable than passing an unevaluted thunk through the channel
-- (for instance, it guarantees the node willl be evaluated to WHNF in a
-- worker thead).
--
-- Element-strict channes may potentially use more memory than lazy
-- channels
--
-----------------------------------------------------------------------------

module Control.Concurrent.Chan.Strict (
          -- * The 'Chan' type
        Chan,                   -- abstract

          -- * Operations
        newChan,                -- :: IO (Chan a)
        writeChan,              -- :: Chan a -> a -> IO ()
        readChan,               -- :: Chan a -> IO a
        dupChan,                -- :: Chan a -> IO (Chan a)
        unGetChan,              -- :: Chan a -> a -> IO ()
        isEmptyChan,            -- :: Chan a -> IO Bool

          -- * Stream interface
        getChanContents,        -- :: Chan a -> IO [a]
        writeList2Chan,         -- :: Chan a -> [a] -> IO ()
   ) where

import Prelude

import System.IO.Unsafe         ( unsafeInterleaveIO )
import Control.Concurrent.MVar.Strict
import Control.DeepSeq

-- A channel is represented by two @MVar@s keeping track of the two ends
-- of the channel contents,i.e.,  the read- and write ends. Empty @MVar@s
-- are used to handle consumers trying to read from an empty channel.

-- |'Chan' is an abstract type representing an unbounded FIFO channel.
data Chan a
 = Chan (MVar (Stream a))
        (MVar (Stream a))

type Stream a = MVar (ChItem a)

data ChItem a = ChItem !a (Stream a)

instance NFData a => NFData (ChItem a) where
    rnf :: ChItem a -> ()
rnf (ChItem a
a Stream a
s) = a -> ()
forall a. NFData a => a -> ()
rnf a
a () -> () -> ()
`seq` Stream a -> ()
forall a. NFData a => a -> ()
rnf Stream a
s

-- @newChan@ sets up the read and write end of a channel by initialising
-- these two @MVar@s with an empty @MVar@.

-- |Build and returns a new instance of 'Chan'.
newChan :: NFData a => IO (Chan a)
newChan :: IO (Chan a)
newChan = do
   MVar (ChItem a)
hole  <- IO (MVar (ChItem a))
forall a. IO (MVar a)
newEmptyMVar
   MVar (MVar (ChItem a))
readm <- MVar (ChItem a) -> IO (MVar (MVar (ChItem a)))
forall a. NFData a => a -> IO (MVar a)
newMVar MVar (ChItem a)
hole
   MVar (MVar (ChItem a))
write <- MVar (ChItem a) -> IO (MVar (MVar (ChItem a)))
forall a. NFData a => a -> IO (MVar a)
newMVar MVar (ChItem a)
hole
   Chan a -> IO (Chan a)
forall (m :: * -> *) a. Monad m => a -> m a
return (MVar (MVar (ChItem a)) -> MVar (MVar (ChItem a)) -> Chan a
forall a. MVar (Stream a) -> MVar (Stream a) -> Chan a
Chan MVar (MVar (ChItem a))
readm MVar (MVar (ChItem a))
write)

-- To put an element on a channel, a new hole at the write end is created.
-- What was previously the empty @MVar@ at the back of the channel is then
-- filled in with a new stream element holding the entered value and the
-- new hole.

-- |Write a value to a 'Chan'.
writeChan :: NFData a => Chan a -> a -> IO ()
writeChan :: Chan a -> a -> IO ()
writeChan (Chan MVar (Stream a)
_read MVar (Stream a)
write) a
val = do
  Stream a
new_hole <- IO (Stream a)
forall a. IO (MVar a)
newEmptyMVar
  MVar (Stream a) -> (Stream a -> IO (Stream a)) -> IO ()
forall a. NFData a => MVar a -> (a -> IO a) -> IO ()
modifyMVar_ MVar (Stream a)
write ((Stream a -> IO (Stream a)) -> IO ())
-> (Stream a -> IO (Stream a)) -> IO ()
forall a b. (a -> b) -> a -> b
$ \Stream a
old_hole -> do
    Stream a -> ChItem a -> IO ()
forall a. NFData a => MVar a -> a -> IO ()
putMVar Stream a
old_hole (ChItem a -> IO ()) -> ChItem a -> IO ()
forall a b. (a -> b) -> a -> b
$! a -> Stream a -> ChItem a
forall a. a -> Stream a -> ChItem a
ChItem a
val Stream a
new_hole
    Stream a -> IO (Stream a)
forall (m :: * -> *) a. Monad m => a -> m a
return Stream a
new_hole

-- |Read the next value from the 'Chan'.
readChan :: NFData a => Chan a -> IO a
readChan :: Chan a -> IO a
readChan (Chan MVar (Stream a)
readm MVar (Stream a)
_write) = do
  MVar (Stream a) -> (Stream a -> IO (Stream a, a)) -> IO a
forall a b. NFData a => MVar a -> (a -> IO (a, b)) -> IO b
modifyMVar MVar (Stream a)
readm ((Stream a -> IO (Stream a, a)) -> IO a)
-> (Stream a -> IO (Stream a, a)) -> IO a
forall a b. (a -> b) -> a -> b
$ \Stream a
read_end -> do
    (ChItem a
val Stream a
new_read_end) <- Stream a -> IO (ChItem a)
forall a. NFData a => MVar a -> IO a
readMVar Stream a
read_end
        -- Use readMVar here, not takeMVar,
        -- else dupChan doesn't work
    (Stream a, a) -> IO (Stream a, a)
forall (m :: * -> *) a. Monad m => a -> m a
return (Stream a
new_read_end, a
val)

-- |Duplicate a 'Chan': the duplicate channel begins empty, but data written to
-- either channel from then on will be available from both.  Hence this creates
-- a kind of broadcast channel, where data written by anyone is seen by
-- everyone else.
dupChan :: NFData a => Chan a -> IO (Chan a)
dupChan :: Chan a -> IO (Chan a)
dupChan (Chan MVar (Stream a)
_read MVar (Stream a)
write) = do
   Stream a
hole     <- MVar (Stream a) -> IO (Stream a)
forall a. NFData a => MVar a -> IO a
readMVar MVar (Stream a)
write
   MVar (Stream a)
new_read <- Stream a -> IO (MVar (Stream a))
forall a. NFData a => a -> IO (MVar a)
newMVar Stream a
hole
   Chan a -> IO (Chan a)
forall (m :: * -> *) a. Monad m => a -> m a
return (MVar (Stream a) -> MVar (Stream a) -> Chan a
forall a. MVar (Stream a) -> MVar (Stream a) -> Chan a
Chan MVar (Stream a)
new_read MVar (Stream a)
write)

-- |Put a data item back onto a channel, where it will be the next item read.
unGetChan :: NFData a => Chan a -> a -> IO ()
unGetChan :: Chan a -> a -> IO ()
unGetChan (Chan MVar (Stream a)
readm MVar (Stream a)
_write) a
val = do
   Stream a
new_read_end <- IO (Stream a)
forall a. IO (MVar a)
newEmptyMVar
   MVar (Stream a) -> (Stream a -> IO (Stream a)) -> IO ()
forall a. NFData a => MVar a -> (a -> IO a) -> IO ()
modifyMVar_ MVar (Stream a)
readm ((Stream a -> IO (Stream a)) -> IO ())
-> (Stream a -> IO (Stream a)) -> IO ()
forall a b. (a -> b) -> a -> b
$ \Stream a
read_end -> do
     Stream a -> ChItem a -> IO ()
forall a. NFData a => MVar a -> a -> IO ()
putMVar Stream a
new_read_end (a -> Stream a -> ChItem a
forall a. a -> Stream a -> ChItem a
ChItem a
val Stream a
read_end)
     Stream a -> IO (Stream a)
forall (m :: * -> *) a. Monad m => a -> m a
return Stream a
new_read_end

-- |Returns 'True' if the supplied 'Chan' is empty.
isEmptyChan ::NFData a =>  Chan a -> IO Bool
isEmptyChan :: Chan a -> IO Bool
isEmptyChan (Chan MVar (Stream a)
readm MVar (Stream a)
write) = do
   MVar (Stream a) -> (Stream a -> IO Bool) -> IO Bool
forall a b. NFData a => MVar a -> (a -> IO b) -> IO b
withMVar MVar (Stream a)
readm ((Stream a -> IO Bool) -> IO Bool)
-> (Stream a -> IO Bool) -> IO Bool
forall a b. (a -> b) -> a -> b
$ \Stream a
r -> do
     Stream a
w <- MVar (Stream a) -> IO (Stream a)
forall a. NFData a => MVar a -> IO a
readMVar MVar (Stream a)
write
     let eq :: Bool
eq = Stream a
r Stream a -> Stream a -> Bool
forall a. Eq a => a -> a -> Bool
== Stream a
w
     Bool
eq Bool -> IO Bool -> IO Bool
`seq` Bool -> IO Bool
forall (m :: * -> *) a. Monad m => a -> m a
return Bool
eq

-- Operators for interfacing with functional streams.

-- |Return a lazy list representing the contents of the supplied
-- 'Chan', much like 'System.IO.hGetContents'.
getChanContents ::NFData a =>  Chan a -> IO [a]
getChanContents :: Chan a -> IO [a]
getChanContents Chan a
ch = IO [a] -> IO [a]
forall a. IO a -> IO a
unsafeInterleaveIO (IO [a] -> IO [a]) -> IO [a] -> IO [a]
forall a b. (a -> b) -> a -> b
$ do
    a
x  <- Chan a -> IO a
forall a. NFData a => Chan a -> IO a
readChan Chan a
ch
    [a]
xs <- Chan a -> IO [a]
forall a. NFData a => Chan a -> IO [a]
getChanContents Chan a
ch
    [a] -> IO [a]
forall (m :: * -> *) a. Monad m => a -> m a
return (a
xa -> [a] -> [a]
forall a. a -> [a] -> [a]
:[a]
xs)

-- |Write an entire list of items to a 'Chan'.
writeList2Chan ::NFData a =>  Chan a -> [a] -> IO ()
writeList2Chan :: Chan a -> [a] -> IO ()
writeList2Chan = (a -> IO ()) -> [a] -> IO ()
forall (t :: * -> *) (m :: * -> *) a b.
(Foldable t, Monad m) =>
(a -> m b) -> t a -> m ()
mapM_ ((a -> IO ()) -> [a] -> IO ())
-> (Chan a -> a -> IO ()) -> Chan a -> [a] -> IO ()
forall b c a. (b -> c) -> (a -> b) -> a -> c
. Chan a -> a -> IO ()
forall a. NFData a => Chan a -> a -> IO ()
writeChan