Based on this mongodb-dev thread, it would help driver implementers to reserve a BSON binary subtype for language-specific types, along with reserving the first byte of the binary payload of that subtype to indicate a specific language. We would also add a language registry to the BSON spec.

For example, if the Perl 6 driver wants to store a Rat type (rational with separate numerator and denominator), it could do so by using the language-specific subtype, a registry-assigned byte for "Perl 6" and a Perl 6 driver defined type byte to identify the Rat type and then 16 bytes of data (2 unint64s).

Specific proposal for changes to the BSON Spec:

• Add binary subtype "0x06" to be "language-specific arbitrary type (binary_lang_type)".
• Define language specific type as: binary_lang_type := lang_tag (byte*)
• Define lang_tag as

  lang_tag := 0x01 C
            | 0x02 C++
            | 0x03 C#
            | 0x04 Go
            | 0x05 Java
            | 0x06 Node.js
            | 0x07 Perl 5
            | 0x08 PHP
            | 0x09 Python
            | 0x0A Ruby
            | 0x0B Scala
            | 0x0C Perl 6
  

• Add additional language tags over time by request, if any

Usage example:

For example, if Perl 6 wanted to use the first byte to determine the type, it could choose 0x01 for the Rat type. Then BSON::Document.new("a" => (1/3)) could be encoded as:

1f00000005610012000000060c010100000000000000030000000000000000

Broken up for visibility, those parts are:

1f000000 05   "a"00 12000000   06      0C   0101000000000000000300000000000000 00
length   type key   datalength subtype lang lang-specific-payload              term