[project @ 2002-05-10 13:17:27 by simonmar]

[haskell-directory.git] / Control / Concurrent / Chan.hs

diff --git a/Control/Concurrent/Chan.hs b/Control/Concurrent/Chan.hs
index e756d1a..09afdb5 100644 (file)
--- a/Control/Concurrent/Chan.hs
+++ b/Control/Concurrent/Chan.hs
@@ -6,31 +6,29 @@
 -- 
 -- Maintainer  :  libraries@haskell.org
 -- Stability   :  experimental
--- Portability :  non-portable
+-- Portability :  non-portable (concurrency).
 --
--- Standard, unbounded channel abstraction.
+-- Unbounded channels.
 --
 -----------------------------------------------------------------------------
 
 module Control.Concurrent.Chan
-       ( Chan                  -- abstract
-
-         -- creator
-       , newChan               -- :: IO (Chan a)
-
-         -- operators
-       , 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
+  ( 
+         -- * 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
 
@@ -41,6 +39,7 @@ import Control.Concurrent.MVar
 -- 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))
@@ -55,6 +54,7 @@ data ChItem a = ChItem a (Stream a)
 -- @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 :: IO (Chan a)
 newChan = do
    hole  <- newEmptyMVar
@@ -67,6 +67,7 @@ newChan = do
 -- filled in with a new stream element holding the entered value and the
 -- new hole.
 
+-- |Write a value to a 'Chan'.
 writeChan :: Chan a -> a -> IO ()
 writeChan (Chan _read write) val = do
   new_hole <- newEmptyMVar
@@ -74,6 +75,7 @@ writeChan (Chan _read write) val = do
     putMVar old_hole (ChItem val new_hole)
     return new_hole
 
+-- |Read the next value from the 'Chan'.
 readChan :: Chan a -> IO a
 readChan (Chan read _write) = do
   modifyMVar read $ \read_end -> do
@@ -82,12 +84,17 @@ readChan (Chan read _write) = do
        -- else dupChan doesn't work
     return (new_read_end, 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 :: Chan a -> IO (Chan a)
 dupChan (Chan _read write) = do
    hole     <- readMVar write
    new_read <- newMVar hole
    return (Chan new_read write)
 
+-- |Put a data item back onto a channel, where it will be the next item read.
 unGetChan :: Chan a -> a -> IO ()
 unGetChan (Chan read _write) val = do
    new_read_end <- newEmptyMVar
@@ -95,6 +102,7 @@ unGetChan (Chan read _write) val = do
      putMVar new_read_end (ChItem val read_end)
      return new_read_end
 
+-- |Returns 'True' if the supplied 'Chan' is empty.
 isEmptyChan :: Chan a -> IO Bool
 isEmptyChan (Chan read write) = do
    withMVar read $ \r -> do
@@ -104,6 +112,8 @@ isEmptyChan (Chan read write) = do
 
 -- Operators for interfacing with functional streams.
 
+-- |Return a lazy list representing the contents of the supplied
+-- 'Chan', much like 'IO.hGetContents'.
 getChanContents :: Chan a -> IO [a]
 getChanContents ch
   = unsafeInterleaveIO (do
@@ -112,6 +122,6 @@ getChanContents ch
        return (x:xs)
     )
 
--------------
+-- |Write an entire list of items to a 'Chan'.
 writeList2Chan :: Chan a -> [a] -> IO ()
 writeList2Chan ch ls = sequence_ (map (writeChan ch) ls)