So I think I've extracted the important points here: 1. seek needs to flush the buffer, so it needs to know whether it is on a buffered handle or not. My plan was to do it like this: class Seekable s where seek :: s -> SeekLocation -> IO () instance Seekable s => Seekable (BufferedInputStream s) where seek (BufferedInputStream s buf) loc = do .. flush the buffer .. seek s loc 2. performance of putByte, getByte. You have the ByteStream interface, as far as I can tell, just to improve the performance of these operations. I didn't have such a class because I believe that shovelling large numbers of bytes is more important from a performance perspective than shovelling one byte at a time. For instance, I gave the byte reader that works on any InputStream earlier: -- | Reads a single 'Word8' from a stream. streamGet :: InputStream s => s -> IO Word8 streamGet s = alloca $ \p -> do r <- streamReadBufferNonBlocking s 1 p if r == 0 then ioe_EOF else peek p Still, you can have full speed single byte operations by just adding methods to the MemInputStream/MemOutputStream class: class MemInputStream s where withStreamInputBuffer :: s -> (Ptr Word8 -> Int -> IO (a,Int)) -> IO a memStreamGet :: s -> IO Word8 memStreamGet s = withStreamInputBuffer s $ \p i -> do x <- peek p return (x, i+1) (the default obviously isn't very fast, but individual instances would provide faster implementations). 3. code explosion. Firstly, removing ByteStream will reduce the code explosion. In my design I had separate types for BufferedInputStream and BufferedOutputStream, and there is hardly any duplication because buffering input is just different to buffering output. Read/write buffering is just a combination of these two. I still don't see why you need another layer of buffering over a memory stream. It supports direct operations on the memory already. As I said above, if you want fast(er) single byte read on these, then add it to the MemInputStream class. There is some boilerplate: the need to translate Stream, Seekable, Buffered, InputBuffered etc. through upper layers to lower layers means some boilerplate wrapper instances. But I don't think that's a big deal, and splitting up these classes hasn't made it any worse. Admittedly though, separating input and output has doubled the amount of boilerplate wrappers. I don't see a way around that, other than using some preprocessing or TH to generate the instances. -------------- So here's how I see the layering for a file reading stream using iconv: file or socket (fd-based) :: FileInputStream instance of: InputStream, Stream, Seekable byte buffer :: BufferedInputStream instance of: MemInputStream, Stream, Seekable, Buffered, InputBuffered iconv :: BufferedTextInputStream instance of: MemTextInputStream, TextInputStream, Stream, Seekable, Buffered, InputBuffered lock :: LockedTextInputStream instance of: MemTextInputStream, TextInputStream, Stream, Seekable, Buffered, InputBuffered Two layers of buffering aren't strictly necessary, because the byte buffer will usually be emptied immediately by the decoder, except that it might leave a few bytes when there isn't a complete character to decode. It is simpler to do it this way though, because the decoding layer then works with any MemInputStream (eg. a memory mapped file or ByteString, as well as a buffered file), and it doesn't lose any efficiency. A memory-mapped file becomes a MemInputStream directly, so it only gets 3 layers instead of 4, similarly for a ByteString or UArray Word8. I haven't said what the MemTextInputStream or TextInputStream classes look like. I imagine something like this: class MemTextInputStream s where withCharInputBuffer :: s -> (Ptr Char -> Int -> IO (a,Int)) -> IO a class TextInputStream s where getChar :: s -> IO Char getContents ... getLine ... A StringReader can be made an instance of TextInputStream directly. So do you see any problem with this design? Performance should be fine: getContents reads directly from the buffer filled by iconv, and all other operations are done at buffer sizes. Fast byte and multibyte operations are supported on MemInputStreams so fast Binary I/O and serialisation/deserialisation are possible. Cheers, Simon Bulat Ziganshin wrote:
Hello Simon,
Friday, April 21, 2006, 2:20:26 PM, you wrote:
well, your last answer shows that you don't understand my problems. i'm entirely want to have precise classes, but when i run into IMPLEMENTATION, Haskell restrictions bite me again and again.
Ok, I think you need to describe these problems in more detail. The message you just wrote describes the structure of the library which I think is mostly fine, and corresponds fairly well with what I had in mind.
Could you provide a .zip or .tar.gz instead?
yes, the last version is http://freearc.narod.ru/StreamsBeta.zip
vPutBuf should be non-blocking too.
so it should return number of bytes written? and it should be named vPutBufNonBlocking? you are already said in February that vGetBuf should be used in buffering transformer gently - i.e. i should use just the returned number of bytes and don't enforce filling of whole buffer. how should be the policy of using vPutBufNonBlocking? what should be vGEtBuf/vPutBuf (i.e. functions provided to users) - are they should be blocking or non-blocking?
which is equivalent to my MemInputStream/MemOutputStream. I believe the with-style interface that I use is better though.
main benefit of my structure is that MemoryStream and BlockStream are very close so i hope to implement common buffering layer. your variant makes this much harder. on the other side, i don't see any benefits in using your scheme
each buffering transformer implements ByteStream via BlockStream or MemoryStream:
I didn't have an equivalent to this class in my design. Why is it necessary? vGetByte/vPutByte can be implemented for an arbitrary BlockStream or indeed a MemoryStream.
:) yes, you never mind about implementation issues! how the naked FD, for example, can implement vGetByte? i'm not Copperfield to extract buffer from nowhere. Buffering transformer is not just a bunch of functions, it's a whole DATA STRUCTURE what joins stream handle and buffer pointers together! see my previous letter what described this
the main idea of the whole Streams library is what low-level stream types don't carry any data that will be used only in high-level routines. FD don't carry buffer or CharEncoding information. it's just can read or write block of data - it's all! all other data items are added by corresponding transformers. so FD CAN'T implement vGetByte with buffering and its implementation without buffering will be imho just source of errors - users will constantly asking why it is so slow. moreover i can't add vGetByte/vPutByte to the BlockStream or MemoryStream class (because both can implement it), so it anyway should be in separate class
instance (MemoryStream IO h) => ByteStream IO (BufferedMemoryStream h)
what's a BufferedMemoryStream for? Isn't a memory stream already buffered by definition?
we just use word "buffering" in different meanings. for me buffering means that underlying stream give us data in large enough chunks and this can be implemented inefficiently. buffering transformer efficiently implements byte-oriented or char-oriented operations by working with buffer (either provided by MemoryStream or allocated explicitly for BlockStream) and make rather infrequent calls to the block i/o operations of underlying stream
so, for me MemoryStream is not buffered, it just support block i/o operations in slightly different way than BlockStream. going your way, as i understand, we should call withStreamInputBuffer on each i/o operation what mean that withStreamInputBuffer should be implemented very efficiently in each MemoryStream. well, it's also possible but i prefer to use the same algorithms (and i hope to join implementations) for byte&char i/o over both MemoryStreams and BlockStreams
on the other side, now i think about making BlockStream->MemoryStream transformer (it should just alloc it's own buffer) and then your's withStreamInputBuffer may be used for any I/O through buffer (i.e. both for BlockStreams and MemoryStreams). it will be great for implementing functions requiring lookahead such as vGetLine
class (Stream m h) => TextStream m h | h->m where vGetChar :: h -> m Char vGetLine :: h -> m String vGetContents :: h -> m String vPutChar :: h -> Char -> m () vPutStr :: h -> String -> m ()
Ok, I wouldn't put all those method in the class, I think.
it is because you never mind about fast implementation :) i already moved vPutStrLn/vPrint out of this class.
Also it is necessary to have buffering at the TextStream level, as Marcin pointed out. I haven't thought through the design carefully here.
implementation is rather obvious and therefore boring :) we should convert data to the UCS-4 and then work with buffer containing 4-byte chars. are you not done this in 6.6 compiler when you read UTF-8 sources? we will lose vTell operation (on the other side, we anyway lose it on text streams in windows :) )
all these can be named a canonical Streams hierarchy and it already works.
So what are the problems you were referring to?
i said only about working things. as you see, i don't mentioned, for example, InTextStream or SeekableStream classes, because i had problems with implementing this
2) separation of Stream classes make some automatic definitions impossible. for example, released version contains vGetBuf implementation that is defined via vGetChar and works ok for streams that provide only vGetChar as base function.
it's a minor problem, so you can skip it. just for completeness: my first lib contained the following definitions:
class Stream m h where vGetChar :: ... vGetPuf :: ...
-- default implementations vGetPuf = ... some code using vGetChar
instance Stream IO StringBuffer vGetChar = ... instance Stream IO StringReader vGetChar = ...
and vGetBuf for StringBuffer and StringReader was defined automatically. try to make this effect for current design. it should define:
instance (TextStream m h) => BlockStream m h vGetPuf = ... some code using vGetChar
but such code seen by Haskell as potential overlapping instance definition
i will be glad to add BufferedStream and SeekableStream classes and split BlockStream..TextStream to the reading and writing ones, but this is, as i said, limited by implementation issues
which implementation issues?
well, let's we have 3 different FDs - ReadFD, WriteFD and ReadWriteFD. it's essentially the same Ints, just supporting different sets of I/O operations:
instance InBlockStream ReadFD where ... -- implements vGetBuf instance InBlockStream ReadWriteFD where ... instance OutBlockStream WriteFD where ... -- implements vPutBuf instance OutBlockStream ReadWriteFD where ...
now i'm going to implement InByteStream interface (vGetByte) and OutByteStream (vPutByte) for buffered FDs. i want to use the same data type constructor to buffer all 3 types of FD streams. well, we can write something like:
instance (InBlockStream h) => InByteStream (Buffered h) where vGetByte = ... something involving vGetBuf at last
instance (OutBlockStream h) => OutByteStream (Buffered h) where vPutByte = ... something involving vPutBuf at last
at the first look, it seems fine. but we also need to implement some common functionality:
instance (Stream h) => Stream (Buffered h) where vSeek = ...
let's examine vSeek implementation. it should flush buffer on output streams, i.e. it should call vPutBuf. but that's impossible - vPutBuf is not in Streams dictionary! well, let's replicate implementation of Stream class:
instance (InBlockStream h) => Stream (Buffered h) where ... instance (OutBlockStream h) => Stream (Buffered h) where ...
but this, besides repeating almost the same code two times, again creates overlapped instances! what instance should use compiler for ReadWriteFD?
yes, i can create 3 different data types for buffering input, output and input-output streams but this means even more code duplication. and multiplying these 3 i/o modes to the 3 buffering transformers i already have, i got limitless code growth!
the same problem strikes me on each transformation stage: BlockStream->ByteStream, ByteStream->TextStream, ByteStream->BitsStream. it also bites me when i try to separate SeekableStream operations - vSeek operation required to discard input buffer, what is required in vSetBuf, for example. implementation of different Stream operations contains numerous dependencies from each other. may be it is great that i discover these dependencies at the early development stage instead of bothering with numerous errors when library will be really used in various environments
btw, that you think about specifying buffering mode at the Stream creation? so vSetBuf will just gone away. to be exact, it will be still possible to use different buffering schemes, by creating new buffered Streams over the one raw Stream:
h <- openRawFD "text" h1 <- withBuffering (BlockBuffering 512) h ... working with h1 vFlush h1 -- discards buffer and returns `h` pointer to exact the -- position of first byte that is not consumed by `h1` h2 <- withBuffering LineBuffering h ... working with h2