Dhall Scala Generic

Load Dhall config directly into Scala case class

libraryDependencies += "us.oyanglul" %% "dhall-generic" % "0.3.+"

There is a dhall config

let Shape = <Rectangle: {width: Double, height: Double}| Circle: {radius: Double}>
in Shape.Circle {radius = 1.2}

And you can parse them into dhall Expr

import org.dhallj.syntax._

val expr = """
let Shape = <Rectangle: {width: Double, height: Double}| Circle: {radius: Double}>
in Shape.Circle {radius = 1.2}
""".parseExpr

Scala 3

Supposed that you have some Scala case classes

enum Shape:
  case Rectangle(width: Double, height: Double)
  case Circle(radius: Double)

to load a dhall expr into Scala case classes, simply just

1. derives Decoder

import us.oyanglul.dhall.generic.Decoder

enum Shape derives Decoder:  // derive will generate decoder inline
  case Rectangle(width: Double, height: Double)
  case Circle(radius: Double)

2. as[Shape]

import us.oyanglul.dhall.generic.as

expr.normalize.as[Shape]
// => Right(Circle(1.2)): Either[DecodingFailure, Shape]

Scala 2

Supposed that you have some Scala case classes

sealed trait Shape
object Shape {
  case class Rectangle(width: Double, height: Double) extends Shape
  case class Circle(radius: Double) extends Shape
}

to load a dhall expr into Scala case classes, simply just

import org.dhallj.codec.syntax._   // for `.as[A]` syntax
import org.dhallj.codec.Decoder._  // Decoders for primity types
import us.oyanglul.dhall.generic._ // generate generic decoders

expr.normalize.as[Shape]
// => Right(Circle(1.2)): Either[DecodingFailure, Shape]

Use cases

Load application config

It is very simple to replace HOCON application.conf with dhall.

To load src/main/resources/application.dhall from classpath your will need dhall-imports

import org.dhallj.syntax._
import org.dhallj.codec.syntax._
import us.oyanglul.dhall.generic._
import org.dhallj.imports.syntax._

  BlazeClientBuilder[IO](ExecutionContext.global).resource.use { implicit c =>
        IO.fromEither("classpath:/application.dhall".parseExpr)
          .flatMap(_.resolveImports[IO])
          .flatMap{expr => IO.fromEither(expr.normalize.as[Config])}
    }

Load dhall to sbt libraryDependencies

This project itself is an example of how to load dhall into build.sbt

It is recursively using previous version to load ./build.dhall to libraryDependencies

libraryDependencies ++= dhall.load.modules.map{case Module(o, n, v) => o %% n % v},

Custom Decoder

Create new decoder from existing decoder

i.e. UUID

Scala 3

given (using d: Decoder[String]): Decoder[UUID] = d.map(UUID.fromString)

Scala 2

implicit val decodeUUID: Decoder[UUID] = decodeString.map(UUID.from)