A small library for `sealed trait` enums.
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README.md

rbricks itemized

A small library (<300 loc) that provides macros and typeclasses for enums encoded as sealed trait hierarchies.

Part of rbricks, a collection of composable, small-footprint libraries for scala.

Build Status Maven Central Changelog

Features

  1. (Optional) macro annotations for compact "enum" syntax.
import io.rbricks.itemized.annotation.enum

@enum trait Planet {
  object Earth
  object Venus
  object Mercury
}

This expands to:

sealed trait Planet extends io.rbricks.itemized.Itemized
object Planet {
  case object Earth extends Planet
  case object Venus extends Planet
  case object Mercury extends Planet
}
  1. Typeclass instances to convert to and from String are automatically derived:
scala> import io.rbricks.itemized.ItemizedCodec

scala> ItemizedCodec[Planet].fromRep("Earth")
res0: Option[Planet] = Some(Earth)

scala> val planet: Planet = Planet.Earth

scala> import io.rbricks.itemized.ItemizedCodec.ops._

scala> planet.toRep
  1. Pattern matching against the sealed hierarchy supports exhaustiveness checking, for added safety.
scala> (Planet.Earth : Planet) match {
     |   case Planet.Earth => "close"
     |   case Planet.Venus => "far"
     | }
<console>:19: warning: match may not be exhaustive.
It would fail on the following input: Mercury
       (Planet.Earth : Planet) match {
                     ^
res1: String = close
  1. Support for serialization libraries, currently (click on links for usage/installation instructions):

Install

Artifacts for Scala 2.11 and 2.12 are available on Maven Central.

Add the dependency to your build.sbt

libraryDependencies += "io.rbricks" %% "itemized" % "0.2.0"

To enable the macro paradise plugin (for the @enum annotation), also add

addCompilerPlugin("org.scalamacros" % "paradise" % "2.1.0" cross CrossVersion.full)

Convention and marker trait

The @enum annotation builds enumerations that follow the library's convention for how ADT-based enums should be encoded. However, usage of the macro annotation can be avoided by manually writing out the ADT. Here's an example that serves as an informal definition of the convention.

sealed trait Planet extends io.rbricks.itemized.Itemized
object Planet {
  case object Earth extends Planet
  case object Venus extends Planet
  case object Mercury extends Planet
}

This is equivalent to the following (which expands to the same encoding).

@enum trait Planet {
  object Earth
  object Venus
  object Mercury
}

Usage of the @enum macro annotations requires the macro paradise plugin to be enabled in your project. Refer to the Install section for how to set it up.

To and from String

The ItemizedCodec typeclass provides operations to convert ADT-based enumerations to and from Strings.

Implemetors of encoding and decoding (serialization) protocols may use it as follows:

implicit def itemizedJsonEncoding[T <: Itemized](implicit instance: ItemizedCodec[T]) = new JsonEncoding[T] {
  def write(value: T): JsonObject = JsonString(instance.caseToString(value))
  def read(jsonObject: JsonObject) = ... instance.itemizedFromString(str).get
}

Additionally, ItemizedCodec[T] exposes the Map[String, T] as .stringMap, for other use-cases (such as retrieving the set of admissible elements or strings).

Enumerations with an associated value

The @indexedEnum annotation builds enumerations that follow the library's convention for ADT-based enums with an associated value.

sealed trait Planet extends io.rbricks.itemized.IndexedEnum {
  type Index = Int
}
object Planet {
  case object Earth extends Planet { val index = 1 }
  case object Venus extends Planet { val index = 2 }
  case object Mercury extends Planet { val index = 3 }
}

This is equivalent to the following.

import io.rbricks.itemized.annotation.indexedEnum

@indexedEnum trait Planet {
  type Index = Int
  object Earth   { 1 }
  object Venus   { 2 }
  object Mercury { 3 }
}

Usage of the @indexedEnum macro annotations requires the macro paradise plugin to be enabled in your project. Refer to the Install section for how to set it up.

To and from the associated value ("index")

The ItemizedIndex typeclass provides operations to convert ADT-based enums to and from their associated values.

import io.rbricks.itemized.annotation.indexedEnum

@indexedEnum trait Planet {
  type Index = Int
  object Earth   { 1 }
  object Venus   { 2 }
  object Mercury { 3 }
}

Examples of usage follow.

scala> import io.rbricks.itemized.ItemizedIndex

scala> ItemizedIndex[Planet].fromIndex(2)
res0: Option[Planet] = Some(Venus)

scala> Planet.Mercury.index
res1: Int = 3

scala> import io.rbricks.itemized.ItemizedIndex.ops._

scala> val planet: Planet = Planet.Mercury

scala> planet.toIndex
res2: Planet#Index = 3

Additionally, ItemizedIndex[T] exposes the Map[T#Index, T] as .indexMap, for other use-cases (such as retrieving the set of admissible elements or indices).

Changelog

0.2.0

  • circe encoders and decoders (@utaal, 6ac3e9a)
  • More info in macro errors (@esarbe, 0de55b3)
  • ItemizedCodec.stringMap and ItemizedIndex.indexMap retrieve the underlying string<->object or index<->object mappings (@esarbe, 25013c4)