protoc-gen-elm

Elm Plugin for Protocol Buffers

This protoc plug-in generates Elm modules from .proto specification files. The generated modules make use of the elm-protocol-buffers library to handle the (de)serialization. They can be used to transmit bytes over HTTP(S) or via web-sockets.

Remote Procedure Call (RPC) generation is supported and possible to disable with --elm_opt=grpc=false. If used, you need to add a dependency on the elm-grpc library.

Take a look here for a general introduction on Protocol Buffers.

Installation

This package is a plug-in for protoc, make sure you have installed it and protoc is available on your path. After installing protoc-gen-elm globally from NPM, protoc will automatically find the binary when you add the --elm_out flag to your command.

npm install --global protoc-gen-elm

Alternatively, you can add protoc as a dev-dependency to your project. This should be the preferred way if you want to build your project in CI. If you wrap the call to protoc in some npm script, it should still work as expected.

npm install --save-dev protoc-gen-elm

You can now turn any .proto file into an Elm module. A similar approach can be used to generate code for C++, Dart, Go, Java, Python, Ruby, C#, Rust, JavaScript, PHP or another language to build a compliant back-end server!

protoc --elm_out=. api.proto

Overview

The following table gives an overview of how .proto types correspond to Elm types and what their default values are.

.proto type	Elm type	Default value**
package	The name of the module	Proto
double	Float	0
float	Float	0
int32	Int	0
int64	Int64*	0
uint32	Int	0
uint64	Int64*	0
sint32	Int	0
sint64	Int64*	0
fixed32	Int	0
fixed64	Int64*	0
bool	Bool	False
string	String	""
bytes	Bytes.Bytes	Empty bytes sequence
required a	a	No default
optional a	a	Default of a
repeated a	List a	[]
enum	Custom type	First element
message	Record	All fields take their default value
a	Maybe Record	Nothing
oneof	Custom type with an associated data	Nothing
map<k, v>	Dict.Dict k v	Dict.empty
service	Grpc.Rpc req res***	No default
reserved	N/A
extensions	N/A

*) 64-bit integers are defined in elm-protocol-buffers in Protobuf.Types.Int64.

**) Some default values can be overridden in proto2 specifications. This is currently not supported.

***) Rpc is implemented via the elm-grpc library.

Json Encoding

Protocol Buffers specify a canonical json encoding. When you pass the additional option --elm_opt=json to the protoc invocation, you will get JSON Encoders and Decoders generated. If you want to be more granular --elm_opt=json=encode or --elm_opt=json=decode will only generate one or the other.

The canonical json encoding has quite a few special cases. Below is a list which ones have been implemented so far.

• camelCased json field names as a default ✔️
• Using json_name option to override field names ✔️
• Enum decoding via enum variant names in .proto file ✔️
• Enum decoding via field numbers in .proto file ✔️
• Accepting null as the empty list ✔️
• Accepting Infinity, -Infinity, NaN as floats/doubles ✔️
• Accepting floats/doubles in String format ✔️
• Accepting int32 in String format ✔️
• Accepting int64 in Number format ✔️
• Accepting floats/doubles in Exponent Notation ✔️
• Encoding/Decoding Timestamps in ISO Format ✔️
• Encoding/Decoding Durations in fractional second-based format ✔️
• Encoding/Decoding google.protobuf.Struct as a JSON object ✔️
• Encoding/Decoding google.protobuf.ListValue as a JSON list ✔️
• Encoding/Decoding google.protobuf.ListValue as a JSON list ✔️
• Encoding/Decoding google.protobuf.NullValue as JSON null ✔️
• Encoding/Decoding google.protobuf.Empty as {} ✔️
• Encoding/Decoding google.protobuf.Value into/from any JSON object ✔️
• Encoding/Decoding google.protobuf.FieldMask as a comma-seperated list of dot-seperated field names ✔️
• Special support for google.protobuf.Any ❌

Grpc Dev Tools

This plugin supports code generation for the grpc-dev-tools, which should primarily be used as debugging information for local development. Since it adds code bloat to your bundle and comes with a performance overhead, you should make sure it stays out of your production bundle.

To enable the extra code generation, enable the grpcDevTools flag by adding --elm_opt=grpcDevTools to your protoc invocation. This will generate additional JsonEncoders for your data types according to the canonical json encoding, a Proto/DevToolsWorker.elm file and a Proto/dev-tools.mjs file.

The dev-tools.mjs file internally imports the Elm file, so assuming you are using some sort of bundler with Elm support (like vite or webpack), all you need to do is import the file at the top of your html file or your main JS bundle, e.g.

<body>
  <script type="module" src="/generated/Proto/dev-tools.mjs"></script>
  ...
</body>

You can see a vite setup example in the /example directory, which also makes sure it does not get added in the production bundle.

Explanations about the generated code

In general, the generated code tries to be close to what the code looks like in other languages while still being ideomatic Elm code. Elm's concept of "Only one solution to solve" a problem has several consequences here.

General

• Protobufs messages are product types, enums and oneofs are union types.
• Each message and enum generates encode[name] and decode[name] functions, which integrate seamlessly with elm-protocol-buffers
• Each message and enum generates a default[name] function, which sets the defaults as seen in the table above
• enums and oneofs generate seperate modules, to avoid naming collisions.
• oneofs come in two forms, one where every constructor includes a generic type and one where all types are applied. These are needed for use inside of other messages (you will see why in the section "Module Nesting")

Module Nesting

Protobufs have their own module system, which is different from Elms. Here are some interesting points about it:

• Modules are defined by packages and not by files
• Protoc disallows circular imports of packages
• Declarations can be nested inside of messages, which pretty much makes an inline module
• There are no visibility modifiers. You can access all declarations inside of a message from outside and the other way around

Elm disallows circular imports as well, luckily protoc helps us out here on the package front. However, Elm does not have supported for nested modules, which is a problem.

For an illustration why, see the following example

// file: test.proto
package test;

enum Outer { A = 0 }

message Scope {
  enum Inner { B = 0 }
  message InnerMsg {
    Outer outer = 1;
    Inner inner = 2;
  }

  InnerMsg msg = 1;
}

Obviously there are two modules here: test and test.Scope. We generate two Elm files:

// file: Test.elm
import Test.Scope

type Outer = A

type alias Scope = {
  inner : Test.Scope.InnerMsg
}

// file: Test/Scope.elm
import Test

type Inner = B

type alias InnerMsg = {
  outer : Test.Outer,
  inner : Inner
}

This might look fine on first glance, but if we try to compile this we get a compile error. Why? Because the two modules are mutually recursive.

The only solution to this problem is making a large module for each package, so this is exactly what we do. But if we want to keep the nice, short names, we will get name conflicts. Protoc has no problems with identical names as long as they are in different scopes.

Therefore, we hide the large modules as .Internals_.elm modules, which you should not need to use and re-export from other modules with nicer names from there. The only downside: We lose the ability to pattern match on types, since we can not alias constructors. To not have to generate mapping functions (our solution from v3.x), we instead generate generic union types and apply them in the .Internals_.elm module.

Recursive Data Types

For ease of construction, protoc-gen-elm prefers to generate type aliases instead of nominal types. Type aliases have one downside though: they cannot be recursive. Otherwise, the Elm compiler would have to do infinite work to expand the type. So if you have a recursive type like this:

message Rec {
  repeated Rec rec = 1;
}

we generate

type alias Rec = { rec : List Rec_ }

type Rec_ = Rec_ Rec

and corresponding wrapRec and unwrapRec functions.

gRPC

If your .proto file includes a service declaration, an Elm module will be generated based on package and the services name.

This file:

package some_package

service SomeService {}

will generate a Proto/SomePackage/SomeService.elm module.

The code that needs to be generated inside is actually rather small. A gRPC call just needs

• the package name
• the method name
• the service name
• references to the en/decoder functions

The rest of the work is done by the elm-grpc package. It provides functions to convert the generated Grpc.Rpc instances into Cmds and Tasks, as well as setting the usual Http Request fields (headers, timeout, tracker etc.)

Live Example

To run a minimal live example in your browser, follow the instructions in /example/grpc/README.md. For a more advanced/realistic example, look at /example/tonic_vite/README.md.

Well-known types

If you want to use protobufs well-known-types, you need to install the pre-built package elm-protoc-types or include the paths to the proto files in the compilation.

Example: If this is your proto file test.proto which uses the well-known type Timestamp,

import "google/protobuf/timestamp.proto";

message TestMessage {
  google.protobuf.Timestamp timestamp = 1;
}

the protoc invocation will need to include the path to the well-known types .proto file.

protoc --elm_out=. test.proto /usr/local/include/google/protobuf/timestamp.proto

Limitations

• All limitations of elm-protocol-buffers apply

Development

Note: Currently, this project won't run on Windows (WSL works) because of shell scripts/executable js files.

Execute npm install, npm run build and npm test and you should be good to go. You will need protoc installed and on your PATH.

• The plugin logic is written in Elm itself. To be executable via node, there is a index.js wrapper. It converts the incoming bytes to base64, because there currently is no way to directly send the type Bytes through a port.
• Main.elm essentially wires up the binding to JS: A request is received through a port, gets decoded, processed and then sent through another port.
• For decoding the protoc request, it uses "itself", meaning that upgrading protoc versions should be done by running the plugin against the new include files from protoc to generate the new encoders/decoders (use the upgrade.sh script).
• A Mapper converts the request into a convenient internal structure
• A Generator then uses this internal structure to build an Elm AST which is then pretty-printed to a file.

Run build.sh to build the elm code into index.min.js (which is imported by the entrypoint index.js).

To analyse the protoc requests, there are debug.js, DebugMain and build_debug.sh files. Run build_debug.sh, then use debug.js in place of index.js when running protoc. This should dump the deserialized request into debug.log. You can then put this into the Elm repl for example or use it as input for tests.