package ocaml-protoc

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Pure OCaml compiler for .proto files


Dune Dependency







protoc protobuf codegen

Published: 22 Jul 2024




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

  • 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.mli
.. Generating
  • example.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 ->
(** [default_person ()] is the default value for type [person] *)

(** {2 Protobuf Encoding} *)

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

(** {2 Protobuf Decoding} *)

val decode_pb_person : Pbrt.Decoder.t -> person
(** [decode_pb_person decoder] decodes a [person] binary value from [decoder] *)
  • in, write the following to encode a person value and save it to a file:

let () =

  (* Create OCaml value of generated type *) 
  let person = Example.({ 
    name = "John Doe"; 
    id = 1234l;
    email = Some ""; 
    phone = ["123-456-7890"];
  }) in 
  (* Create a Protobuf encoder and encode value *)
  let encoder = Pbrt.Encoder.create () in 
  Example.encode_pb_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 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; 
  (* Decode the person and Pretty-print it *)
  Example.decode_pb_person (Pbrt.Decoder.of_bytes bytes)
  • :heavy_exclamation_mark: 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


ocaml-protoc only depends on

  • the OCaml compiler distribution (byte code/native compiler).

  • dune

  • stdlib-shims for the compiler itself

  • a C99 compiler for the runtime library's stubs

Intall from OPAM

$ opam install ocaml-protoc

Or from source

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

Build your program

Using dune, the program can be compiled with:

  (name main)
  (modules main example)
  (libraries pbrt))

More manually, the program can be built directly using ocamlfind:

$ ocamlfind ocamlopt -linkpkg -package pbrt \
    -o example \
    example.mli \

🏁 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:

Command line switch Description Runtime
Type definition along with a default constructor function to conveniently create values of that type
--make make constructor functions
--binary Binary encodings pbrt
--yojson JSON encoding using the widely popular yojson library pbrt_yojson
--bs BuckleScript encoding using the BuckleScript core binding to JS json library bs-ocaml-protoc-json
--pp pretty printing functions based on the Format module. pbrt
--services RPC definitions. pbrt_services

Protobuf <-> OCaml mapping

see here.


With the --services option, ocaml-protoc now generates stubs for service declarations.

For example with the given calculator.proto file:

syntax = "proto3";

message I32 {
  int32 value = 0;

message AddReq {
  int32 a = 1;
  int32 b = 2;

service Calculator {
  rpc add(AddReq) returns (I32);

  rpc add_stream(stream I32) returns (I32);

Using ocaml-protoc --binary --services --ml_out=. calculator.proto, we get the normal type definitions, but also this service definition:

(** Calculator service *)
module Calculator : sig
  open Pbrt_services
  open Pbrt_services.Value_mode

  module Client : sig
    val add : (add_req, unary, i32, unary) Client.rpc
    val add_stream : (i32, stream, i32, unary) Client.rpc
  module Server : sig
    (** Produce a server implementation from handlers *)
    val make : 
      add:((add_req, unary, i32, unary) Server.rpc -> 'handler) ->
      add_stream:((add_req, stream, i32, unary) Server.rpc -> 'handler) ->
      unit -> 'handler Pbrt_services.Server.t

This can then potentially be used with libraries that implement specific protobuf-based network protocols, such as ocaml-grpc or ocaml-twirp, or other custom protocols.

Protobuf service endpoints take a single type and return a single type, but they have the ability to stream either side. We represent this ability with the Pbrt_services.Value_mode types:

(** Whether there's a single value or a stream of them *)
module Value_mode = struct
  type unary
  type stream

A (req, req_kind, res, res_kind) Client.rpc is a bundle describing a single RPC endpoint, from the client perspective. It contains the RPC name, service, etc. alongside encoders for the request type req, and decoders for the response type res.

The phantom types req_kind and res_kind represent the value mode for request, respectively response. Here we see that Calculator.Client.add is unary for both (it takes a single argument and returns a single value) but Calculator.Client.add_stream takes a string of i32 as parameters before returning a single result.

With transports such as grpc, all 4 combinations are possible. With twirp over HTTP 1.1, only unary mode is supported.


On the server side, ocaml-protoc generates individual stubs, like on the client side; but it also generates services as bundles of endpoints. One service corresponds to a service declaration in the .proto file.

Detailed explanation of how server-side services work

In practice, in something like twirp, a service could be added to a web server by adding each endpoint to a single HTTP route; or a twirp-aware router could directly map incoming HTTP queries to services.

The trickiest part here is that the type 'handler Pbrt_services.Server.t is parametric. Indeed it'd be hard for the generated code to cater to every possible combination of network transport and concurrency library (eio, lwt, async, etc.).

Instead, the code is generic over 'handler (the type of a query handler for a single endpoint; e.g. a HTTP endpoint for a single route). The function

  module Server : sig
    val make : 
      add:((add_req, unary, i32, unary) Server.rpc -> 'handler) ->
      add_stream:((add_req, stream, i32, unary) Server.rpc -> 'handler) ->
      unit -> 'handler Pbrt_services.Server.t

seen previously is used to build the 'handler service by asking the user to provide a handler for each method. The builder for add is given a description of the add endpoint (with decoders for requests; and encoders for responses), and must return a handler that knows how to decode the request, add numbers, and turn that back into a response.

Libraries will provide facilities to build such handlers, so that the user only has to provide the actual logic (here, adding numbers). For example in twirp_tiny_httpd (part of ocaml-twirp), implementing a server looks like this[^1]:

[^1]: we use a different .proto because twirp doesn't handle streams.

syntax = "proto3";

message I32 {
  int32 value = 0;

message AddReq {
  int32 a = 1;
  int32 b = 2;

message AddAllReq {
  repeated int32 ints = 1;

service Calculator {
  rpc add(AddReq) returns (I32);

  rpc add_all(AddAllReq) returns (I32);
let add (a : add_req) : i32 = default_i32 ~value:Int32.(add a.a a.b) ()

let add_all (a : add_all_req) : i32 =
  let l = ref 0l in
  List.iter (fun x -> l := Int32.add !l x) a.ints;
  default_i32 ~value:!l ()

let calc_service : Twirp_tiny_httpd.handler Pbrt_services.Server.t =
    ~add:(fun rpc -> Twirp_tiny_httpd.mk_handler rpc add)
    ~add_all:(fun rpc -> Twirp_tiny_httpd.mk_handler rpc add_all)

let() =
  let server = Tiny_httpd.create ~port:1234 () in
  Twirp_tiny_httpd.add_service ~prefix:(Some "twirp") server calc_service;
  Tiny_httpd.run_exn server

Here we see that all the logic is in add and add_all, which know nothing about protobuf or serialization. A calc_service bundle, using the Twirp_tiny_httpd.handler type for each handler, is built from them. Finally, a HTTP server is created, the service is added to it (binding some routes), and we enter the server's main loop.

Compiler Internals

see here

Protobuf Extensions

see here


see here

Dependencies (3)

  1. ocaml >= "4.08"
  2. pbrt = version
  3. dune >= "2.0"

Dev Dependencies (3)

  1. pbrt_services = version & with-test
  2. pbrt_yojson = version & with-test
  3. odoc with-doc




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