+%*********************************************************
+%* *
+\subsection{Types @Handle@, @Handle__@}
+%* *
+%*********************************************************
+
+The type for @Handle@ is defined rather than in @IOHandle@
+module, as the @IOError@ type uses it..all operations over
+a handles reside in @IOHandle@.
+
+\begin{code}
+
+{-
+ Sigh, the MVar ops in ConcBase depend on IO, the IO
+ representation here depend on MVars for handles (when
+ compiling a concurrent way). Break the cycle by having
+ the definition of MVars go here:
+
+-}
+data MVar a = MVar (SynchVar# RealWorld a)
+
+#if defined(__CONCURRENT_HASKELL__)
+type Handle = MVar Handle__
+#else
+type Handle = MutableVar RealWorld Handle__
+#endif
+
+data Handle__
+ = ErrorHandle IOError
+ | ClosedHandle
+ | SemiClosedHandle ForeignObj (Addr, Int)
+ | ReadHandle ForeignObj (Maybe BufferMode) Bool
+ | WriteHandle ForeignObj (Maybe BufferMode) Bool
+ | AppendHandle ForeignObj (Maybe BufferMode) Bool
+ | ReadWriteHandle ForeignObj (Maybe BufferMode) Bool
+
+-- Standard Instances as defined by the Report..
+
+instance Eq Handle {-partain:????-}
+instance Show Handle where {showsPrec p h = showString "<<Handle>>"}
+
+\end{code}
+
+%*********************************************************
+%* *
+\subsection[BufferMode]{Buffering modes}
+%* *
+%*********************************************************
+
+Three kinds of buffering are supported: line-buffering,
+block-buffering or no-buffering. These modes have the following
+effects. For output, items are written out from the internal
+buffer according to the buffer mode:
+
+\begin{itemize}
+\item[line-buffering] the entire output buffer is written
+out whenever a newline is output, the output buffer overflows,
+a flush is issued, or the handle is closed.
+
+\item[block-buffering] the entire output buffer is written out whenever
+it overflows, a flush is issued, or the handle
+is closed.
+
+\item[no-buffering] output is written immediately, and never stored
+in the output buffer.
+\end{itemize}
+
+The output buffer is emptied as soon as it has been written out.
+
+Similarly, input occurs according to the buffer mode for handle {\em hdl}.
+\begin{itemize}
+\item[line-buffering] when the input buffer for {\em hdl} is not empty,
+the next item is obtained from the buffer;
+otherwise, when the input buffer is empty,
+characters up to and including the next newline
+character are read into the buffer. No characters
+are available until the newline character is
+available.
+\item[block-buffering] when the input buffer for {\em hdl} becomes empty,
+the next block of data is read into this buffer.
+\item[no-buffering] the next input item is read and returned.
+\end{itemize}
+For most implementations, physical files will normally be block-buffered
+and terminals will normally be line-buffered.
+
+\begin{code}
+data BufferMode
+ = NoBuffering | LineBuffering | BlockBuffering (Maybe Int)
+ deriving (Eq, Ord, Read, Show)
+\end{code}
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