[Haskell-cafe] Interpreting fields in a buffer

[Alastair Reid]

> I have a question related to a program I'm writing. I have to
> handle IP packets, which will be read into a buffer. What is the
> best haskell idiom for getting access to the fields in the buffer?

There's no way in Haskell to define a datastructure with a particular memory 
layout.  (Strictly, there's no way to do it in ANSI C either but it's easy 
enough to work with what C gives you.)

To access the fields, you will need to write a bunch of functions to read them 
out (and write them if you need).  There's basically two approaches:

1) Write access functions in C and use the ffi interface to call them.
   For example, you might have a function to read the TTL field given
   a pointer to the IP header.

2) Write access functions in Haskell using functions from the Storable
   class and associated libraries.

http://www.haskell.org/ghc/docs/latest/html/libraries/base/Foreign.Storable.html

   To do this, you need to know the offset (in bytes) of each field 
   of the data structure. hsc2hs is great for doing this kind of thing.


> Is there a way in haskell to declare the format of the header, then
> access the components of the buffer as elements of those types?
> I'm only going to do read access on the buffer (an unboxed array).

I would lean towards leaving the buffer over in C land instead of loading it 
into a Haskell buffer.  (Of course, it depends a bit on how much processing 
of the packet you will be doing in Haskell.)

Hope this helps,

--
Alastair Reid     www.haskell-consulting.com