pb is an OCaml library for describing and serializing Protocol buffers (Protobuf) messages.

Message descriptions can be written by hand, or generated from .proto files using the pb-plugin protoc compiler plugin.

Protocol buffers provide both a file format for describing messages and a serialization (wire) format. The pb library supports only the wire format, but there is a straightforward mapping from the file format into pb code. Here is a description of a Protobuf message with two fields, a number field with the tag 1, and a PhoneType field with the tag 2 and default value HOME:

message PhoneNumber {
  required string number = 1;
  optional PhoneType type = 2 [default = HOME];
}

And here is the equivalent pb code that defines the PhoneNumber message:

module PhoneNumber = (val message "PhoneNumber")
let number = PhoneNumber.required string "number" 1
let type_  = PhoneNumber.optional "type" 2 ~default:home

The type field of the PhoneNumber message has the type PhoneType, an enumeration value. Here is a description of the PhoneType enumeration with its three values:

enum PhoneType {
  MOBILE = 0;
  HOME = 1;
  WORK = 2;
}

And here is the equivalent pb code that defines the PhoneType enumeration:

module PhoneType = (val enum "PhoneType")
let mobile = PhoneType.constant "MOBILE" 0l
let home   = PhoneType.constant "HOME" 1l
let work   = PhoneType.constant "WORK" 2l

Messages described by pb can be serialized using the Faraday serialization library. The following code creates a PhoneNumber message, assigns values to its two fields, and writes it to a Faraday serializer:

let pn = create PhoneNumber.t in
  setf pn number ("+1-541-754-3010");
  setf pn type_ work;
  write pn

Messages can be deserialized (parsed) using the Angstrom parser-combinator library. The following code reads a message using Angstrom and retrieves the values of its fields:

let pn = match Angstrom.parse_string (read PhoneNumber.t) s) with
         | Ok m -> m
         | Error s -> failwith s
  in (getf pn number, getf pn type_)

Messages can also be pretty-printed:

pp_msg PhoneNumber.t Format.std_formatter pn