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Numerato: easy deluxe enums for Scala

enumerato, adj.: Italian for enumerated (sans the extra e because I lack the gift of tongues)

scala.Enumeration sucks

Everyone knows this, but I felt the need to point out the obvious:

// - singleton type tedious to use as an actual *type*
// - why should I extend some mysterious class?
object CrustyStatus extends Enumeration {
  type CrustyStatus = Value // far from DRY: I already called it `CrustyStatus`!

  val Enabled, Disabled = Value // this is probably the nicest part of `Enumeration`
// defined object CrustyStatus

// without this import I won't be able to access the `CrustyStatus` non-singleton type
import CrustyStatus._
// import CrustyStatus._

def isEnabled(s: CrustyStatus) = s match {
  case Enabled => true
  // non-exhaustive match not caught at compile time...
  // ...seriously, doesn't even emit a warning
// isEnabled: (s: CrustyStatus.CrustyStatus)Boolean

Could we do better?

Of course! When in doubt, generate it with macros:

import numerato._

Declaring an enumeration

Simply create a plain class, annotate it with @enum, and use the familiar val ... = Value declaration to define a few enum values.

The @enum annotation invokes a macro, which will:

  • Replace your Status class with a sealed Status class suitable for acting as a base type for enum values. Specifically, it'll grow a (val index: Int, val name: String) constructor. These parameters will be supplied by the macro, so you don't have to worry about it.
  • Generate a Status companion object, which will contain most of the pieces that now make Status an enumeration. This includes a values: List[Status], plus lookup methods.

Give the above Status enum, here's what the generated code looks like:

scala> @enum(debug = true) class Status {
     |   val Enabled, Disabled = Value
     | }
  sealed abstract class Status(val index: Int, val name: String)(implicit sealant: Status.Sealant);
  object Status {
    @scala.annotation.implicitNotFound(msg = "Enum types annotated with ".+("@enum can not be extended directly. To add another value to the enum, ").+("please adjust your `def ... = Value` declaration.")) sealed abstract protected class Sealant;
    implicit protected object Sealant extends Sealant;
    case object Enabled extends Status(0, "Enabled") with scala.Product with scala.Serializable;
    case object Disabled extends Status(1, "Disabled") with scala.Product with scala.Serializable;
    val values: List[Status] = List(Enabled, Disabled);
    val fromIndex: _root_.scala.Function1[Int, Status] = Map(Enabled.index.->(Enabled), Disabled.index.->(Disabled));
    val fromName: _root_.scala.Function1[String, Status] = Map(>(Enabled),>(Disabled));
    def switch[A](pf: PartialFunction[Status, A]): _root_.scala.Function1[Status, A] = macro numerato.SwitchMacros.switch_impl[Status, A]
defined class Status
defined object Status

Note that you don't need to supply the debug = true part during normal use. This is only useful for debugging, and is passed here to facilitate output of generated code.

Enumeration flavors: plain vs parametric

Parametric enumerations can carry arbitrary paylod data a-la Java's enum-s, while plain enumerations don't carry any payload.

Plain enumerations

scala> @enum class LocationPlain {
     |   val UWS, Chelsea = Value
     | }
defined class LocationPlain
defined object LocationPlain

A plain enumeration exposes only the auto-generated index and name fields. It has no other accessors, and you can't make it carry around any additional values.

Parametric enumerations

scala> @enum class Neighborhood(zip: Int, elevation: Double) {
     |   val UWS = Value(zip = 10024, elevation = 40) // use named args
     |   val Chelsea = Value(10011, 19)               // or not!
     | }
defined class Neighborhood
defined object Neighborhood

A parametric enumeration is declared with a bunch of constructor parameters, which are turned into public accessors on the generated enum type. These accessors can be called like so:

res3: Int = 10024

scala> Neighborhood.Chelsea.elevation
res4: Double = 19.0

Please note that you cannot declare enum fields with the names index and name, because currently these fields are auto-generated by the @enum macro annotation, and are thus reserved.

scala> @enum class Dumb(name: Double)
<console>:16: error: `name` is reserved & can not be used as a enum field
object $iw {
scala> @enum class Dumber(index: AnyRef)
<console>:16: error: `index` is reserved & can not be used as a enum field
object $iw {
Duplicate values

@enum doesn't currently perform uniqueness checks on the values of parametric enumerations. In practice this means that the following is a valid enumeration:

scala> @enum class Duplicity(mandatory: Boolean) {
     |   val Mandatory = Value(true)
     |   val Required = Value(true)
     |   val HellNo = Value(false)
     | }
defined class Duplicity
defined object Duplicity

Incidentally, this is also the behavior of Java's enumeration. If you have an opinion on the subject, please drop by #3 and let your voice be heard.

Using the enumeration

The switch statement

The @enum-generated companion comes with a switch construct that can be used similarly to Scala's built-in match statement, but comes with additional compile-time safety checks. Here's how it's used:

scala> import Status._
import Status._

scala> val statuses = Enabled :: Disabled :: Enabled :: Enabled :: Disabled :: Nil
statuses: List[Product with Serializable with Status] = List(Enabled, Disabled, Enabled, Enabled, Disabled)

scala> switch {
     |   case Status.Disabled => "not working"
     |   case Status.Enabled  => "working for sure"
     | })
res5: List[String] = List(working for sure, not working, working for sure, working for sure, not working)

From the above follows that the return type of switch is Status => A, where A is the right hand side type representing the match result. In other words, switch converts the provided partial function into a total function, and ensures at compile time that the total function will not throw a MatchError at runtime.

In order to complete its mission, switch imposes certain rules on code that may appear within a switch block:

  • All declared enum values must be matched, either Individually or in Alternate | Groups of Absolutely | Any | Size. In case of an incomplete match an error message will be shown explaining which enum values are missing:
scala> switch {
     |   case Status.Disabled => "not working"
     | })
<console>:28: error: not all values of Status are covered: Enabled switch {
  • If you don't wish to specify all enum values, you must include a wildcard pattern that will act as a catch-all, thus making the match total:
scala> switch {
     |   case Status.Disabled => "not working"
     |   case _ => "-unknown-"
     | })
res7: List[String] = List(-unknown-, not working, -unknown-, -unknown-, not working)
  • Guards cannot be used within a switch construct. There are no restrictions on the right hand side of a => pattern, so feel free to write if statements or other conditionals there. Guards can't be evaluated at compile time, making it impossible to guarantee that the match will be complete:
scala> switch {
     |   case s @ Status.Disabled if == "Enabled" => "not working"
     |   case _ => "-unknown-"
     | })
<console>:29: error: potentially incomplete `Disabled` match - guards not allowed
         case s @ Status.Disabled if == "Enabled" => "not working"
scala> switch {
     |   case Status.Disabled => "not working"
     |   case _ if System.currentTimeMillis % 2 == 0 => "-unknown-"
     | })
<console>:30: error: potentially incomplete wildcard match - guards not allowed
         case _ if System.currentTimeMillis % 2 == 0 => "-unknown-"

Non-exhaustive regular match-es

Scala's built-in match statement works with @enum as usual. The main attraction is being able to match {} against an enum type & be warned by the compiler about non-exhaustive matches. As shown way above, scala.Enumeration doesn't support this, but @enum types do! Check this out:

scala> def isEnabled(s: Status) = s match {
     |   case Status.Enabled => true
     | }
<console>:26: warning: match may not be exhaustive.
It would fail on the following input: Disabled
       def isEnabled(s: Status) = s match {
isEnabled: (s: Status)Boolean

Non-exhaustive matches are still possible, but at least you'll be warned at compile time. You're free to use either switch or match depending on your needs. Just be aware that nothing stops you from ignoring match incompleteness warnings, while switch will refuse to compile if something smells funky.

Reflecting upon the enum

Enumeration values have numeric indexes, string names (no more getClass.getSimpleName), and the @enum-generated companion provides lookup methods that safely and easily turn indexes and names into instances of the annotated type.

// each enum value has a auto-detected name
// res11: String = Disabled

// you can look up values by name
// res13: Status = Enabled

// or by the auto-generated zero-based index
// res15: Status = Disabled

// iterate over all values as needed
// res17: List[Status] = List(Enabled, Disabled)

Safety & hygiene

In order to maintain integrity of the enumeration, @enum discourages intrepid explorers: the enumeration type will be automatically sealed for your safety & convenience.

Attempting to directly extend an @enum-annotated type will result in compile-time errors. As seen above, the generated type is sealed, which makes it impossible to derive from it in another compilation unit (aka file).

Additionally, the generated enum constructor requires an implicit Sealant parameter, which is a protected inner type of the generated companion object. This makes it impossible to derive from the enum type even within the same file.

If you wish to add new values to the enum type, simply adjust the val ... = Value declaration to include more values. @enum will take care of the rest.

Using numerato in your project

First, add the resolver & dependency to your SBT build:

resolvers += "maxaf-releases" at s""

libraryDependencies += "com.bumnetworks" %% "numerato" % "0.0.1"

Next, enable macros & add a dependency on Macro Paradise:

resolvers += Resolver.sonatypeRepo("releases")

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

scalacOptions += "-language:experimental.macros"

At last, proceed to define your enumerations as above:

scala> import numerato._
import numerato._

scala> @enum class Foo {
     |   val Bar, Baz, Quux = Value
     | }
defined class Foo
defined object Foo