MissingH-0.18.6: Large utility libraryContentsIndex
System.IO.HVIO
Portabilityportable
Stabilityprovisional
MaintainerJohn Goerzen <jgoerzen@complete.org>
Contents
Implementation Classes
Standard HVIO Implementations
Handle
Stream Reader
Memory Buffer
Haskell Pipe
Description

Haskell Virtual I/O -- a system to increase the flexibility of input and output in Haskell

Copyright (c) 2004-2005 John Goerzen, jgoerzen@complete.org

HVIO provides the following general features:

  • The ability to use a single set of functions on various different types of objects, including standard Handles, in-memory buffers, compressed files, network data streams, etc.
  • The ability to transparently add filters to the I/O process. These filters could include things such as character set conversions, compression or decompression of a data stream, and more.
  • The ability to define new objects that have the properties of I/O objects and can be used interchangably with them.
  • Specification compatibility with, and complete support for, existing I/O on Handles.
  • Provide easier unit testing capabilities for I/O actions

HVIO defines several basic type classes that you can use. You will mostly be interested in HVIO.

It's trivial to adapt old code to work with HVIO. For instance, consider this example of old and new code: 

printMsg :: Handle -> String -> IO ()
printMsg h msg = hPutStr h ("msg: " ++ msg)

And now, the new way: 

printMsg :: HVIO h => h -> String -> IO ()
printMsg h msg = vPutStr h ("msg: " ++ msg)

There are several points to note about this conversion:

  • The new method can still accept a Handle in exactly the same way as the old method. Changing your functions to use HVIO will require no changes from functions that call them with Handles.
  • Most "h" functions have equivolent "v" functions that operate on HVIO classes instead of the more specific Handle. The "v" functions behave identically to the "h" functions whenever possible.
  • There is no equivolent of "openFile" in any HVIO class. You must create your Handle (or other HVIO object) using normal means. This is because the creation is so different that it cannot be standardized.

In addition to Handle, there are several pre-defined classes for your use. StreamReader is a particularly interesting one. At creation time, you pass it a String. Its contents are read lazily whenever a read call is made. It can be used, therefore, to implement filters (simply initialize it with the result from, say, a map over hGetContents from another HVIO object), codecs, and simple I/O testing. Because it is lazy, it need not hold the entire string in memory. You can create a StreamReader with a call to newStreamReader.

MemoryBuffer is a similar class, but with a different purpose. It provides a full interface like Handle (it implements HVIOReader, HVIOWriter, and HVIOSeeker). However, it maintains an in-memory buffer with the contents of the file, rather than an actual on-disk file. You can access the entire contents of this buffer at any time. This can be quite useful for testing I/O code, or for cases where existing APIs use I/O, but you prefer a String representation. You can create a MemoryBuffer with a call to newMemoryBuffer.

Finally, there are pipes. These pipes are analogous to the Unix pipes that are available from System.Posix, but don't require Unix and work only in Haskell. When you create a pipe, you actually get two HVIO objects: a PipeReader and a PipeWriter. You must use the PipeWriter in one thread and the PipeReader in another thread. Data that's written to the PipeWriter will then be available for reading with the PipeReader. The pipes are implemented completely with existing Haskell threading primitives, and require no special operating system support. Unlike Unix pipes, these pipes cannot be used across a fork(). Also unlike Unix pipes, these pipes are portable and interact well with Haskell threads. A new pipe can be created with a call to newHVIOPipe.

Together with System.IO.HVFS, this module is part of a complete virtual filesystem solution.

Synopsis
class Show a => HVIO a where
vClose :: a -> IO ()
vIsOpen :: a -> IO Bool
vIsClosed :: a -> IO Bool
vTestOpen :: a -> IO ()
vIsEOF :: a -> IO Bool
vShow :: a -> IO String
vMkIOError :: a -> IOErrorType -> String -> Maybe FilePath -> IOError
vThrow :: a -> IOErrorType -> IO b
vGetFP :: a -> IO (Maybe FilePath)
vTestEOF :: a -> IO ()
vGetChar :: a -> IO Char
vGetLine :: a -> IO String
vGetContents :: a -> IO String
vReady :: a -> IO Bool
vIsReadable :: a -> IO Bool
vPutChar :: a -> Char -> IO ()
vPutStr :: a -> String -> IO ()
vPutStrLn :: a -> String -> IO ()
vPrint :: Show b => a -> b -> IO ()
vFlush :: a -> IO ()
vIsWritable :: a -> IO Bool
vSeek :: a -> SeekMode -> Integer -> IO ()
vTell :: a -> IO Integer
vRewind :: a -> IO ()
vIsSeekable :: a -> IO Bool
vSetBuffering :: a -> BufferMode -> IO ()
vGetBuffering :: a -> IO BufferMode
vPutBuf :: a -> Ptr b -> Int -> IO ()
vGetBuf :: a -> Ptr b -> Int -> IO Int
data StreamReader
newStreamReader :: String -> IO StreamReader
data MemoryBuffer
newMemoryBuffer :: String -> (String -> IO ()) -> IO MemoryBuffer
mbDefaultCloseFunc :: String -> IO ()
getMemoryBuffer :: MemoryBuffer -> IO String
data PipeReader
data PipeWriter
newHVIOPipe :: IO (PipeReader, PipeWriter)
Implementation Classes
class Show a => HVIO a where

This is the generic I/O support class. All objects that are to be used in the HVIO system must provide an instance of HVIO.

Functions in this class provide an interface with the same specification as the similar functions in System.IO. Please refer to that documentation for a more complete specification than is provided here.

Instances of HVIO must provide vClose, vIsEOF, and either vIsOpen or vIsClosed.

Implementators of readable objects must provide at least vGetChar and vIsReadable. An implementation of vGetContents is also highly suggested, since the default cannot implement proper partial closing semantics.

Implementators of writable objects must provide at least vPutChar and vIsWritable.

Implementators of seekable objects must provide at least vIsSeekable, vTell, and vSeek.

Methods
vClose :: a -> IO ()
Close a file
vIsOpen :: a -> IO Bool
Test if a file is open
vIsClosed :: a -> IO Bool
Test if a file is closed
vTestOpen :: a -> IO ()
Raise an error if the file is not open. This is a new HVIO function and is implemented in terms of vIsOpen.
vIsEOF :: a -> IO Bool
Whether or not we're at EOF. This may raise on exception on some items, most notably write-only Handles such as stdout. In general, this is most reliable on items opened for reading. vIsEOF implementations must implicitly call vTestOpen.
vShow :: a -> IO String
Detailed show output.
vMkIOError :: a -> IOErrorType -> String -> Maybe FilePath -> IOError
Make an IOError.
vThrow :: a -> IOErrorType -> IO b
Throw an IOError.
vGetFP :: a -> IO (Maybe FilePath)
Get the filename/object/whatever that this corresponds to. May be Nothing.
vTestEOF :: a -> IO ()
Throw an isEOFError if we're at EOF; returns nothing otherwise. If an implementation overrides the default, make sure that it calls vTestOpen at some point. The default implementation is a wrapper around a call to vIsEOF.
vGetChar :: a -> IO Char
Read one character
vGetLine :: a -> IO String
Read one line
vGetContents :: a -> IO String

Get the remaining contents. Please note that as a user of this function, the same partial-closing semantics as are used in the standard hGetContents are encouraged from implementators, but are not required. That means that, for instance, a vGetChar after a vGetContents may return some undefined result instead of the error you would normally get. You should use caution to make sure your code doesn't fall into that trap, or make sure to test your code with Handle or one of the default instances defined in this module. Also, some implementations may essentially provide a complete close after a call to vGetContents. The bottom line: after a call to vGetContents, you should do nothing else with the object save closing it with vClose.

For implementators, you are highly encouraged to provide a correct implementation.

vReady :: a -> IO Bool
Indicate whether at least one item is ready for reading. This will always be True for a great many implementations.
vIsReadable :: a -> IO Bool
Indicate whether a particular item is available for reading.
vPutChar :: a -> Char -> IO ()
Write one character
vPutStr :: a -> String -> IO ()
Write a string
vPutStrLn :: a -> String -> IO ()
Write a string with newline character after it
vPrint :: Show b => a -> b -> IO ()
Write a string representation of the argument, plus a newline.
vFlush :: a -> IO ()
Flush any output buffers. Note: implementations should assure that a vFlush is automatically performed on file close, if necessary to ensure all data sent is written.
vIsWritable :: a -> IO Bool
Indicate whether or not this particular object supports writing.
vSeek :: a -> SeekMode -> Integer -> IO ()
Seek to a specific location.
vTell :: a -> IO Integer
Get the current position.
vRewind :: a -> IO ()
Convenience function to reset the file pointer to the beginning of the file. A call to vRewind h is the same as vSeek h AbsoluteSeek 0.
vIsSeekable :: a -> IO Bool
Indicate whether this instance supports seeking.
vSetBuffering :: a -> BufferMode -> IO ()
Set buffering; the default action is a no-op.
vGetBuffering :: a -> IO BufferMode
Get buffering; the default action always returns NoBuffering.
vPutBuf :: a -> Ptr b -> Int -> IO ()
Binary output: write the specified number of octets from the specified buffer location.
vGetBuf :: a -> Ptr b -> Int -> IO Int
Binary input: read the specified number of octets from the specified buffer location, continuing to read until it either consumes that much data or EOF is encountered. Returns the number of octets actually read. EOF errors are never raised; fewer bytes than requested are returned on EOF.
show/hide Instances
Standard HVIO Implementations
Handle
Handle is a member of HVIO.
Stream Reader
data StreamReader

Simulate I/O based on a string buffer.

When a StreamReader is created, it is initialized based on the contents of a String. Its contents are read lazily whenever a request is made to read something from the StreamReader. It can be used, therefore, to implement filters (simply initialize it with the result from, say, a map over hGetContents from another HVIO object), codecs, and simple I/O testing. Because it is lazy, it need not hold the entire string in memory. You can create a StreamReader with a call to newStreamReader.

show/hide Instances
newStreamReader
:: StringInitial contents of the StreamReader
-> IO StreamReader
Create a new StreamReader object.
Memory Buffer
data MemoryBuffer

A MemoryBuffer simulates true I/O, but uses an in-memory buffer instead of on-disk storage.

It provides a full interface like Handle (it implements HVIOReader, HVIOWriter, and HVIOSeeker). However, it maintains an in-memory buffer with the contents of the file, rather than an actual on-disk file. You can access the entire contents of this buffer at any time. This can be quite useful for testing I/O code, or for cases where existing APIs use I/O, but you prefer a String representation. You can create a MemoryBuffer with a call to newMemoryBuffer.

The present MemoryBuffer implementation is rather inefficient, particularly when reading towards the end of large files. It's best used for smallish data storage. This problem will be fixed eventually.

show/hide Instances
newMemoryBuffer
:: StringInitial Contents
-> (String -> IO ())close func
-> IO MemoryBuffer

Create a new MemoryBuffer instance. The buffer is initialized to the value passed, and the pointer is placed at the beginning of the file.

You can put things in it by using the normal vPutStr calls, and reset to the beginning by using the normal vRewind call.

The function is called when vClose is called, and is passed the contents of the buffer at close time. You can use mbDefaultCloseFunc if you don't want to do anything.

To create an empty buffer, pass the initial value "".

mbDefaultCloseFunc :: String -> IO ()
Default (no-op) memory buf close function.
getMemoryBuffer :: MemoryBuffer -> IO String
Grab the entire contents of the buffer as a string. Unlike vGetContents, this has no effect on the open status of the item, the EOF status, or the current position of the file pointer.
Haskell Pipe
data PipeReader
The reading side of a Haskell pipe. Please see newHVIOPipe for more details.
show/hide Instances
data PipeWriter
The writing side of a Haskell pipe. Please see newHVIOPipe for more details.
show/hide Instances
newHVIOPipe :: IO (PipeReader, PipeWriter)

Create a Haskell pipe.

These pipes are analogous to the Unix pipes that are available from System.Posix, but don't require Unix and work only in Haskell. When you create a pipe, you actually get two HVIO objects: a PipeReader and a PipeWriter. You must use the PipeWriter in one thread and the PipeReader in another thread. Data that's written to the PipeWriter will then be available for reading with the PipeReader. The pipes are implemented completely with existing Haskell threading primitives, and require no special operating system support. Unlike Unix pipes, these pipes cannot be used across a fork(). Also unlike Unix pipes, these pipes are portable and interact well with Haskell threads.

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