[Haskell-cafe] Re: [Haskell] installing streams library

[Jeremy Shaw]

At Thu, 25 May 2006 13:42:11 +0400,
Bulat Ziganshin wrote:
> 
> Hello Jeremy,
> 
> Monday, May 22, 2006, 12:20:54 AM, you wrote:
> 
> > For my own needs, I cabalized and debianized the Streams library. It
> > generates binary debs for ghc6 and hugs -- but I think the hugs
> > version is broken. In any case, it is a start, you can download the
> > packaging at:
> 
> > http://www.n-heptane.com/nhlab/tmp/Streams_packaging.tar.gz
> 
> can i include your work in the library itself? 

Absolutely.

> is it better to include 'debian' directory to my archive or left
> this to the debian packagers?

If someone volunteers to maintain the package -- then it is probably
better to not keep a copy of the debian directory in your archive --
because it will often be out of date and confuse people -- and debian
users will be able to get the debianized source easily by typing,
'apt-get source haskell-streams'.

On the other hand -- if there is no one officially maintaing it -- it
would be useful to provide the debian directory (with a disclaimer) so
that debian users can easily build and install the .deb, since
subverting the debian package system tends to lead to long-term
complications.

> can you say how you are use my library? it's both interesting for me
> and can be helpful in deciding how it should be further developed

I am using it to serialize/deserialize haskell data structures so I
can store them in a Berkeley database.

To get them into BDB I need to convert the haskell data structure into
a C structure that looks like this:

struct __db_dbt {
	void	 *data;			/* Key/data */
	u_int32_t size;			/* key/data length */
};


Currently I am doing it like this -- but this will clearly fail if the
serialized data structure is longer than 512 bytes...

withDBT :: (Binary a) => a -> (Ptr DBT -> IO b) -> IO b
withDBT thedata f =
    allocaBytes #{size DBT} $ \dbtPtr ->
        allocaBytes 512 $ \dataPtr ->
            do h <- openMemBuf dataPtr 512
               withByteAlignedLE h $ flip put_ thedata
               wrote <- vTell h
               vClose h
               #{poke DBT, data} dbtPtr (castPtr dataPtr)
               #{poke DBT, size} dbtPtr ((fromIntegral wrote) :: Int)
               f dbtPtr

I don't really need the file-system interface for this project -- what
would be nice is something like 'withCStringLen' and 'peekCString' for
the encode/decode functions:

type PtrLen a = (Ptr a, Int)
encodePtrLen :: (Binary a) => a -> (PtrLen a -> IO b) -> IO b
decodePtr :: (Binary a) => Ptr a -> IO a

I could simulate this by using 'encode' to convert the data structure
to a String and then use 'withCStringLen' to get the pointer and
length -- but having the intermediate String seems like it could be a
big performance hit.

Two alternative ideas are:

 (1) accurately pre-calculate the size of the serialized structure and
     allocate the correct amount of memory from the start 

 (2) start with a 'guess' and realloc the memory if the initial guess
     is too small.

Both of those alternatives have their own problems -- so I think only
testing will tell what works best...

I have not looked at the library exhaustively, so if there is already
a good way to do this, let me know.

Thanks!
j.