package ocaml-protoc-plugin
Install
Dune Dependency
Authors
Maintainers
Sources
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README.md.html
Ocaml protoc plugin
The goal of Ocaml protoc plugin is to create an up to date plugin for the google protobuf compiler (protoc) to generate ocaml types and serialization and de-serialization.
The main features include:
Messages are mapped to idiomatic OCaml types, using modules
Support service descriptions
proto3 compliant
proto2 compliant
Support includes
Comparisson with other OCaml protobuf handlers.
Feature | ocaml-protoc | ocaml-pb | ocaml-protoc-plugin |
---|---|---|---|
Ocaml types | Supported | Defined runtime(*) | Supported |
Service endpoints | Not supported | N/A | Supported |
proto3 | Partly supported(**) | Supported | Supported |
proto2 | Supported | Supported | Supported |
(*) ocaml-bp has a sister project ocaml-bp-plugin
which emit ocaml-pb definitions from a .proto
. The plugin parses files are proto2 ocaml type definitions (all fields are option types), and repeated fields are not packed by default.
(**) ocaml-protoc
release 1.2.0 does not yet fully support proto3, the master branch does, however.
Types
Basic types are mapped trivially to ocaml types:
Primitive types:
Protobuf Type | Ocaml type |
---|---|
int32, int64, uint32, uint64, sint32, sint64 | int(*) |
fixed64, sfixed64, fixed32, sfixed32 | int32, int64(*) |
bool | bool |
float, double | float |
string | string |
bytes | bytes |
(*) The plugin supports changing the type for scalar types to int/int64/int32. See options section below.
A message t
. However, messages without any fields are mapped to unit.
Packages are trivially mapped to modules. Included proto files (<name>.proto
) are assumed to have been compiled to <name>.ml
, and types in included proto files are referenced by their fill name.
Compound types are mapped like:
Protobuf Type | Ocaml type |
---|---|
oneof | Polymorphic variants: [ Field1 of fieldtype1, Field1 of fieldtype2 ] |
repeated 'a | 'a list |
message | message option |
enum | Abstract data types: Enum1, Enum2, Enum3 |
map<'a, 'b> | ('a * 'b) list |
Proto2 type support
The specification for proto2 states that when deserializing a message, fields which are not transmitted should be set the the default value (either 0, or the value of the default option).
However, It seems to be the defacto standard that in proto2 it should be possible to determine if a field was transmitted or not. Thefore all non-repeated fields in proto2 are default - unless it has a default value, or is a required field.
The specification is vague when it comes to serializing fields for proto2 syntax. (i.e. should fields with default values be serialized?). This implementation chooses to always serialize values, default or otherwise, for all fields which are set (i.e. Some x
). This differs from proto3, which explicitly states that fields with default values does not need to be transmitted).
Invocation
If the plugin is available in the path as protoc-gen-ocaml
, then you can generate the ocaml code by running
protoc --ocaml_out=. --ocaml_opt=<options> file.proto
Options control the code generated.
Option | Description | Example | Default |
---|---|---|---|
annot | Type annotations. | annot=[@@deriving show] |
"" |
debug | Enable debugging | debug |
Not set |
open | Add open at top of generated files. May be given multiple times | open=Base.Sexp |
[] |
int64_as_int | Map *int64 types to int instead of int64 |
int64_as_int=false |
true |
int32_as_int | Map *int32 types to int instead of int32 |
int32_as_int=false |
true |
fixed_as_int | Map *fixed* types to int |
fixed_as_int=true |
false |
Parameters are seperated by ;
If protoc-gen-ocaml
is not located in the path, it is possible to specify the exact path to the plugin:
protoc --plugin=protoc-gen-ocaml=../plugin/ocaml-protocol-plugin.exe --ocaml_out=. <file>.proto
Older versions of protoc
It seems that the --ocaml_opt
flag may not be supported by older versions of the proto compiler. As an alternative, options can also be passed with the --ocaml_out
flag:
protoc --plugin=protoc-gen-ocaml=../plugin/ocaml.exe --ocaml_out=annot=debug;[@@deriving show { with_path = false }, eq]:. <file>.proto
Using dune
Below is a dune rule for generating code for test.proto
:
(rule
(targets test.ml)
(deps
(:proto test.proto))
(action
(run protoc -I . "--ocaml_opt=annot=[@@deriving show { with_path = false }, eq]" --ocaml_out=. %{proto})))
Service interface
Service interfaces create a module with values that just references the request and reply pair. These binding can then be used with function in Protobuf.Service
.
The call function will take a string -> string
function, which implement message sending -> receiving.
The service function is a string -> string
function which takes a handler working over the actual message types.
Example:
test.proto
syntax = "proto3";
message Address {
enum Planet {
Earth = 0; Mars = 1; Pluto = 2;
}
string street = 1;
uint64 number = 2;
Planet planet = 3;
}
message Person {
uint64 id = 1;
string name = 2;
Address address = 3;
}
$ protoc --ocaml_out=. test.proto
Generates a file test.ml
with the following signature:
module Address : sig
module rec Planet : sig
type t = Earth | Mars | Pluto
val to_int: t -> int
val from_int: int -> t Protobuf.Deserialize.result
end
val name: unit -> string
type t = {
street: string;
number: int;
planet: Planet.t;
}
val to_proto: t -> Protobuf.Writer.t
val from_proto: Protobuf.Reader.t -> (t, Protobuf.Deserialize.error) result
end
module Person : sig
val name: unit -> string
type t = {
id: int;
name: string;
address: Address.t option;
}
val to_proto: t -> Protobuf.Writer.t
val from_proto: Protobuf.Reader.t -> (t, Protobuf.Deserialize.error) result
end = struct
Protobuf.Reader
and Protobuf.Writer
are used then reading or writing protobuf binary format. Below is an example on how to decode a message and how to read a message.
let string_of_planet = function
| Address.Earth -> "earth"
| Mars -> "mars"
| Pluto -> "pluto"
in
let read_person binary_message =
let reader = Protobuf.Reader.create binary_message in
match Person.from_proto from_proto reader in
| Ok Person.{ id; name; address = Some Address { street; number; planet } } ->
Printf.printf "P: %d %s - %s %s %d\n" id name (string_of_planet planet) street number
| Ok Person.{ id; name; address = None } ->
Printf.printf "P: %d %s - Address unknown\n" id name
| Error _ -> failwith "Could not decode"