Re: [Caml-list] Re: How to read different ints from a Bigarray?

[Goswin von Brederlow <goswin-v-b@web.de>]

Goswin von Brederlow <goswin-v-b@web.de> writes:

> blue storm <bluestorm.dylc@gmail.com> writes:
>
>> On Wed, Oct 28, 2009 at 6:57 PM, Goswin von Brederlow <goswin-v-b@web.de> wrote:
>>> Maybe ideal would be a format string based interface that calls C with
>>> a format string and a record of values. Because what I really need is
>>> to read/write records in an architecture independend way. Something
>>> like
>>>
>>> type t = { x:int; y:char; z:int64 }
>>> let t_format = "%2u%c%8d"
>>>
>>> put_formated buf t_format t
>>>
>>> But how to get that type safe? Maybe a camlp4 module that generates
>>> the format string and type from a single declaration so they always
>>> match.
>>
>> It's possibly off-topic, but you might be interested in Richard
>> Jones's Bitstring project [1] wich deals with similar issues quite
>> nicely in my opinion.
>>
>> [1] http://code.google.com/p/bitstring/
>
> No, quite on-topic.
>
> I glanced at the examples and code and it looks to me though as if
> this can only parse bitstrings but not create them from a pattern.
> You have
>
> let parse_foo bits =
>   bitmatch bits with
>   | { x : 16 : littleendian; y : 16 : littleendian } -> fun x y -> (x, y)
>
> but no
>
> let unparse_foo (x, y) =
>   bitmake { x : 16 : littleendian; y : 16 : littleendian } x y
>
>
> Idealy would be something along
>
> let pattern = make_pattern { x : 16 : littleendian; y : 16 : littleendian }
> let parse_foo bits = parse pattern (fun x y -> (x, y))
> let unparse_foo (x, y) = unparse pattern x y
>
> But I know how to do that with CPS already. I just need the primitives
> to get/set the basic types.
>
> MfG
>         Goswin

And I was wrong. There is

http://code.google.com/p/bitstring/source/browse/trunk/examples/make_ipv4_header.ml

as an example. Not ideal since parsing and unparsing will duplicate
the pattern definition but that will be locale for each type.

MfG
        Goswin