Scala doesn't have type-safe enum
s like Java has. If I have a set of related constants then what is the best way in Scala to represent those constants?
views:
3465answers:
5http://www.scala-lang.org/docu/files/api/scala/Enumeration.html
Example use
object Main extends Application {
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 filter isWorkingDay foreach println
}
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 object
s).
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
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.
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 => replaceRow(where, newValue)
case 'col | 'column => replaceCol(where, newValue)
case _ => throw new IllegalArgumentException
}
// At REPL:
scala> val a = unitMatrixInt(3)
a: teste7.MatrixInt =
/ 1 0 0 \
| 0 1 0 |
\ 0 0 1 /
scala> a('row, 1) = a.row(0)
res41: teste7.MatrixInt =
/ 1 0 0 \
| 1 0 0 |
\ 0 0 1 /
scala> 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 => replaceRow(where, newValue)
case Column => replaceCol(where, newValue)
}
// At REPL:
scala> a(Row, 2) = a.row(1)
<console>:13: error: not found: value Row
a(Row, 2) = a.row(1)
^
scala> a(Dimension.Row, 2) = a.row(1)
res1: teste.MatrixInt =
/ 1 0 0 \
| 0 1 0 |
\ 0 1 0 /
scala> import Dimension._
import Dimension._
scala> 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> val a = unitMatrixInt(3)
a: teste3.MatrixInt =
/ 1 0 0 \
| 0 1 0 |
\ 0 0 1 /
scala> 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.
Looks like the original example now works correctly with Scala 2.8 beta