Type class derivation in Scala 3

Chris Birchall

47 Degrees

Agenda

Type class derivation in Scala 2
In Haskell and Rust
New features in Scala 3
Show
~~Functor~~
Macros: generating gRPC clients and servers

Status quo in Scala 2

Implementation choices
- Magnolia
- Shapeless
- Hand-rolled macros

A few standard usage patterns

define in TC's companion object
```
import io.circe.generic.auto._
```
```
import io.circe.generic.semiauto._
```

Haskell

`deriving` keyword to generate TC instances
Language extensions galore
- DeriveFunctor, DeriveFoldable, DeriveTraversable
- DeriveGeneric
- DeriveAnyClass
- DeriveDataTypeable
- DerivingStrategies
Sky's the limit with DerivingVia
Print derived instances with -ddump-deriv

Rust

#[derive(...)] attribute
Auto-derivation of stock traits (e.g. Eq)
Derive custom traits using procedural macros

New features in Scala 3

derives keyword
Mirror

Live coding 😅

gRPC client/server generation

Feature of Mu, a microservices framework
Currently implemented as a macro annotation

gRPC client/server generation

(in Scala 2)

message HelloRequest {
  string name = 1;
}

message HelloResponse {
  string name = 1;
}

service Greeter {
  rpc SayHello(HelloRequest)
      returns (HelloResponse);
}

greeter.proto

case class HelloRequest(name: String)

case class HelloResponse(greeting: String)

@service(Protobuf)
trait Greeter[F[_]] {
  def SayHello(req: HelloRequest): F[HelloResponse]
}

Greeter.scala

srcgen using sbt plugin
write by hand

gRPC client/server generation

(in Scala 2)

@service(Protobuf)
trait Greeter[F[_]] {
  def SayHello(req: HelloRequest): F[HelloResponse]
}

// Generated by macro annotation
object Greeter {

  def toServerServiceDefinition[F: ConcurrentEffect](
    service: Greeter[F]
  ): io.grpc.ServerServiceDefinition
  
  def client[F: ConcurrentEffect](
    host: String, port: Int
  ): Resource[F, Greeter[F]]

}

gRPC client/server generation

(in Scala 2)

Macro takes care of

mapping methods to RPC endpoints
Protobuf marshalling of requests/responses

gRPC client/server generation

Plan for Scala 3

move server/client factory methods to type classes
derive instances using macros

trait GrpcServerSide[S[_[_]]] {

  def [F[_]: ConcurrentEffect] (service: S[F])
    .toServerServiceDefinition: io.grpc.ServerServiceDefinition
  
}

trait GrpcClientSide[S[_[_]]] {

  def client[F: ConcurrentEffect](
    host: String, port: Int
  ): Resource[F, S[F]]

}

gRPC client/server generation

(in Scala 3)

message HelloRequest {
  string name = 1;
}

message HelloResponse {
  string name = 1;
}

service Greeter {
  rpc SayHello(HelloRequest)
      returns (HelloResponse);
}

greeter.proto

case class HelloRequest(name: String)

case class HelloResponse(greeting: String)

@service(Protobuf)
trait Greeter[F[_]] derives GrpcServerSide, GrpcClientSide {
  def SayHello(req: HelloRequest): F[HelloResponse]
}

Greeter.scala

srcgen using sbt plugin
write by hand

gRPC client/server generation

(in Scala 3)

class MyGreeter[F[_]: Applicative] extends Greeter[F] {
  def SayHello(req: HelloRequest): F[HelloResponse] =
    HelloResponse(s"Hi, ${req.name}!").pure
}

val serverServiceDef =
  new MyGreeter[IO].toServerServiceDefinition

val client =
  summon[GrpcClientSide[Greeter]].client[IO]("localhost", 8080)

Summary

`derives` clause + `derived` method
Extension methods
`using` and `given`
`inline`
Derivation using mirrors
Derivation using a macro

Slides: https://slides.com/cb372/type-class-derivation-in-scala-3

Code: https://github.com/cb372/type-class-derivation-in-scala-3