How to model type-safe enum types

Question

Scala doesn t have type-safe enums like Java has  Given a set of related constants  what would be the best way in Scala to represent those constants

User · Answer

I must say that the example copied out of the Scala documentation by skaffman above is of limited utility in practice  you might as well use case objects    In order to get something most closely resembling a Java Enum  i e  with sensible toString and valueOf methods  -- perhaps you are persisting the enum values to a database  you need to modify it a bit  If you had used skaffman s code   WeekDay valueOf  Sun     returns None WeekDay Tue toString     returns Weekday 2    Whereas using the following declaration   object WeekDay extends Enumeration     type WeekDay   Value   val Mon   Value  Mon     val Tue   Value  Tue          etc     You get more sensible results   WeekDay valueOf  Sun     returns Some Sun  WeekDay Tue toString     returns Tue

User · Answer

Dotty  Scala 3  will have native enums supported  Check here and here

User · Answer

In Scala it is very comfortable with https   github com lloydmeta enumeratum  Project is really good with examples and documentation  Just this example from their docs should makes you interested in  import enumeratum    sealed trait Greeting extends EnumEntry  object Greeting extends Enum Greeting              findValues  is a protected method that invokes a macro to find all  Greeting  object declarations inside an  Enum      You use it to implement the  val values  member        val values   findValues    case object Hello   extends Greeting   case object GoodBye extends Greeting   case object Hi      extends Greeting   case object Bye     extends Greeting        Object Greeting has a  withName name  String   method Greeting withName  Hello        gt  res0  Greeting   Hello  Greeting withName  Haro        gt  java lang IllegalArgumentException  Haro is not a member of Enum  Hello  GoodBye  Hi  Bye      A safer alternative would be to use  withNameOption name  String   method which returns an Option Greeting  Greeting withNameOption  Hello        gt  res1  Option Greeting    Some Hello   Greeting withNameOption  Haro        gt  res2  Option Greeting    None     It is also possible to use strings case insensitively Greeting withNameInsensitive  HeLLo        gt  res3  Greeting   Hello  Greeting withNameInsensitiveOption  HeLLo        gt  res4  Option Greeting    Some Hello      Uppercase-only strings may also be used Greeting withNameUppercaseOnly  HELLO        gt  res5  Greeting   Hello  Greeting withNameUppercaseOnlyOption  HeLLo        gt  res6  Option Greeting    None     Similarly  lowercase-only strings may also be used Greeting withNameLowercaseOnly  hello        gt  res7  Greeting   Hello  Greeting withNameLowercaseOnlyOption  hello        gt  res8  Option Greeting    Some Hello

User · Answer

http   www scala-lang org docu files api scala Enumeration html  Example use    object Main extends App        object WeekDay extends Enumeration         type WeekDay   Value       val Mon  Tue  Wed  Thu  Fri  Sat  Sun   Value           import WeekDay        def isWorkingDay d  WeekDay       d    Sat    d    Sun       WeekDay values filter isWorkingDay foreach println

User · Answer

After doing extensive research on all the options around  enumerations  in Scala  I posted a much more complete overview of this domain on another StackOverflow thread  It includes a solution to the  sealed trait   case object  pattern where I have solved the JVM class object initialization ordering problem

User · Answer

Starting from Scala 3  there is now enum keyword which can represent a set of constants  and other use cases  enum Color     case Red  Green  Blue  scala gt  val red   Color Red val red  Color   Red scala gt  red ordinal val res0  Int   0

User · Answer

You can use a sealed abstract class instead of the enumeration  for example   sealed abstract class Constraint val name  String  val verifier  Int   gt  Boolean   case object NotTooBig extends Constraint  NotTooBig       lt  1000   case object NonZero extends Constraint  NonZero         0   case class NotEquals x  Int  extends Constraint  NotEquals     x        x    object Main      def eval ctrs  Seq Constraint   x  Int   Boolean        true    ctrs   case  accum  ctr    gt  accum  amp  amp  ctr verifier x       def main args  Array String         val ctrs   NotTooBig    NotEquals 5     Nil     val evaluate   eval ctrs         println evaluate 3000       println evaluate 3       println evaluate 5

User · Answer

A slightly less verbose way of declaring named enumerations   object WeekDay extends Enumeration  Sun    Mon    Tue    Wed    Thu    Fri    Sat       type WeekDay   Value   val Sun  Mon  Tue  Wed  Thu  Fri  Sat   Value    WeekDay valueOf  Wed      returns Some Wed  WeekDay Fri toString      returns Fri   Of course the problem here is that you will need to keep the ordering of the names and vals in sync which is easier to do if name and val are declared on the same line

User · Answer

There are many ways of doing   1  Use symbols  It won t give you any type safety  though  aside from not accepting non-symbols where a symbol is expected  I m only mentioning it here for completeness  Here s an example of usage   def update what  Symbol  where  Int  newValue  Array Int    MatrixInt     what match       case  row   gt  replaceRow where  newValue      case  col    column   gt  replaceCol where  newValue      case     gt  throw new IllegalArgumentException         At REPL     scala gt  val a   unitMatrixInt 3  a  teste7 MatrixInt     1 0 0     0 1 0     0 0 1    scala gt  a  row  1    a row 0  res41  teste7 MatrixInt     1 0 0     1 0 0     0 0 1    scala gt  a  column  2    a row 0  res42  teste7 MatrixInt     1 0 1     0 1 0     0 0 0     2  Using class Enumeration   object Dimension extends Enumeration     type Dimension   Value   val Row  Column   Value     or  if you need to serialize or display it   object Dimension extends Enumeration  Row    Column       type Dimension   Value   val Row  Column   Value     This can be used like this   def update what  Dimension  where  Int  newValue  Array Int    MatrixInt     what match       case Row   gt  replaceRow where  newValue      case Column   gt  replaceCol where  newValue          At REPL  scala gt  a Row  2    a row 1   lt console gt  13  error  not found  value Row        a Row  2    a row 1              scala gt  a Dimension Row  2    a row 1  res1  teste MatrixInt     1 0 0     0 1 0     0 1 0    scala gt  import Dimension   import Dimension    scala gt  a Row  2    a row 1  res2  teste MatrixInt     1 0 0     0 1 0     0 1 0     Unfortunately  it doesn t ensure that all matches are accounted for  If I forgot to put Row or Column in the match  the Scala compiler wouldn t have warned me  So it gives me some type safety  but not as much as can be gained   3  Case objects   sealed abstract class Dimension case object Row extends Dimension case object Column extends Dimension   Now  if I leave out a case on a match  the compiler will warn me   MatrixInt scala 70  warning  match is not exhaustive  missing combination         Column      what match         one warning found   It s used pretty much the same way  and doesn t even need an import   scala gt  val a   unitMatrixInt 3  a  teste3 MatrixInt     1 0 0     0 1 0     0 0 1    scala gt  a Row 2    a row 0  res15  teste3 MatrixInt     1 0 0     0 1 0     1 0 0     You might wonder  then  why ever use an Enumeration instead of case objects  As a matter of fact  case objects do have advantages many times  such as here  The Enumeration class  though  has many Collection methods  such as elements  iterator on Scala 2 8   which returns an Iterator  map  flatMap  filter  etc   This answer is essentially a selected parts from this article in my blog

User · Answer

just discovered enumeratum  it s pretty amazing and equally amazing it s not more well known

[scala] How to model type-safe enum types?

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