[erlang-questions] using variable in bit syntax

[Haobo Yu]

Hi,

I'm rather new to erlang, so please pardon my ignorance.

I'm trying to parse a packet header using the bit syntax.  There is a  
byte in the header that indicates the endianness of the subsequent  
integers in the packet header.  For example:

endianness : 8/unsigned-integer
%% The following integers follow the above endianness
value1     : 32/integer
value2	   : 32/integer
...

Because the endianness is specified in the bit syntax, such as  
unsigned-little-integer, it would be nice to encode the endianness in  
a variable when I parse that byte, then use this variable to parse  
the following integers in the packet header.  I would like to have  
something like this:

if Endianness == 1 ->
   Spec = "32/little-integer";
    true ->
   Spec = "32/big-integer";
end,
<<Value1:Spec, Value2:Spec>> = Header,

However, putting a variable in the place of bit type specifier does  
not seem to work.  I can't put a BIF, such as list_to_atom, there  
either.

Any suggestions whether and how this might be done?

Thanks very much.

- Haobo