Streams internal structure

[Bulat Ziganshin]

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


-- 
Best regards,
 Bulat                            mailto:Bulat.Ziganshin at gmail.com