Binary and XML serialization of types

[hanslad@gmail.com]

Hello,
I am an Ada beginner who is working on a private project.
The project is to implement a protocol which either does binary or xml serialization of the different defined records. 
I have tried to separate all the "encoding" details from the type declaration in a sub package like this:

package A.Types is

   type Guid_Array is array(1 .. 8) of Unsigned_8;
   type Guid is  record
      Data1 : Unsigned_32;
      Data2 : Unsigned_16;
      Data3 : Unsigned_16;
      Data4 : Guid_Array;
   end record;

end A.Types;


with with Ada.Streams; use Ada.Streams;
package A.Types.BinaryEncoder is

   procedure Guid_Write(
   Stream : access Ada.Streams.Root_Stream_Type'Class; Item : Guid );
   for Guid'Write use Guid_Write;

end A.Types.Encoders;


I get the following error:

7:8 entity must be declared in this Scope

How can I separate all the encoding and decoding details from the type declaration? I like the idea of splitting this into packages with different functionality. Is this possible at all?
Sorry for posting such a beginners question.

Thanks,

HP