ocaml-protoc

🐪 A protobuf compiler for OCaml 🐪.

Introduction

⇨ ocaml-protoc compiles protobuf message files into OCaml types along with serialization functions for a variety of encodings.

⇨ ocaml-protoc supports both proto syntax 2 and 3 as well as binary and JSON encodings.

⇨ ocaml-protoc supports JavaScript object encoding through the BuckleScript compiler. See here for complete example.

A simple example

This example generates the binary encoding, if you are more interested in a JavaScript example, go here

Write in example.proto

message Person {
  required string name = 1;
  required int32 id = 2;
  optional string email = 3;
  repeated string phone = 4;
}

Run:

❯ ocaml-protoc -binary -ml_out ./ example.proto
.. Generating example_types.mli
.. Generating example_types.ml
.. Generating example_pb.mli
.. Generating example_pb.ml

example_types.mli:

(** example.proto Generated Types *)

(** {2 Types} *)

type person = {
  name : string;
  id : int32;
  email : string;
  phone : string list;
}

(** {2 Default values} *)

val default_person : 
  ?name:string ->
  ?id:int32 ->
  ?email:string ->
  ?phone:string list ->
  unit ->
  person
(** [default_person ()] is the default value for type [person] *)

example_pb.mli:

(** {2 Protobuf Encoding} *)

val encode_person : Example_types.person -> Pbrt.Encoder.t -> unit
(** [encode_person v encoder] encodes [v] with the given [encoder] *)

(** {2 Protobuf Decoding} *)

val decode_person : Pbrt.Decoder.t -> Example_types.person
(** [decode_person decoder] decodes a [person] value from [decoder] *)

in main.ml, write the following to encode a person value and save it to a file:

let () =

  (* Create OCaml value of generated type *) 
  let person = Example_types.({ 
    name = "John Doe"; 
    id = 1234l;
    email = Some "jdoe@example.com"; 
    phone = ["123-456-7890"];
  }) in 
  
  (* Create a Protobuf encoder and encode value *)
  let encoder = Pbrt.Encoder.create () in 
  Example_pb.encode_person person encoder; 

  (* Output the protobuf message to a file *) 
  let oc = open_out "myfile" in 
  output_bytes oc (Pbrt.Encoder.to_bytes encoder);
  close_out oc

then in the same main.ml append the following to read from the same file:

let () = 
  (* Read bytes from the file *) 
  let bytes = 
    let ic = open_in "myfile" in 
    let len = in_channel_length ic in 
    let bytes = Bytes.create len in 
    really_input ic bytes 0 len; 
    close_in ic; 
    bytes 
  in 
  
  (* Decode the person and Pretty-print it *)
  Example_pb.decode_person (Pbrt.Decoder.of_bytes bytes)

❗ Int32 vs int

OCaml users will immediately point to the use of int32 type in the generated code which might not be the most convenient choice. One can modify this behavior using custom extensions.

Install & Build

Prerequesite

ocaml-protoc only depends on

the OCaml compiler distribution (byte code/native compiler).
dune
stdlib-shims

Intall from OPAM

❯ opam install ocaml-protoc

Or from source

❯ mkdir -p tmp/bin
❯ export PREFIX=`pwd`/tmp
❯ make install

Build your program

Using ocamlfind one can build the program above with the following:

❯ ocamlfind ocamlopt -linkpkg -package ocaml-protoc \
    -o example \
    example_types.mli example_types.ml \
    example_types.mli example_types.ml \
    main.ml

🏁 You can now run the example

❯ ./example

Runtime library

The generated code depends on the opam package "pbrt", defining a module Pbrt.

Online documentation here

All Generated Files and Encodings:

file name	Command line switch	Description	Runtime
<name>_types.{ml\|mli}		Type definition along with a constructor function to conveniently create values of that type
<name>_pb.{ml\|mli}	-binary	Binary encodings	ocaml-protoc
<name>_yojson.{ml\|mli}	-yojson	JSON encoding using the widely popular yojson library	ocaml-protoc-yojson
<name>_bs.{ml\|mli}	-bs	BuckleScript encoding using the BuckleScript core binding to JS json library	bs-ocaml-protoc-json
<name>_pp.{ml\|mli}	-pp	pretty printing functions based on the Format module.	ocaml-protoc

Protobuf <-> OCaml mapping

see here.

ddickstein/ocaml-protoc