CODEWITHSUNDEEP
scalaimplicit-conversionscala-option
Vilius Normantas
Vilius Normantas

Reputation: 3758

Scala: using None for other purposes than empty Option

According to the docs, None object is intended to "represent non-existent values". As far as I've seen it's mostly used as an empty Option. But do you think it's a good idea to use it for other purposes. For example, in my library I want to have an universal "Empty" object which could be assigned for various missing values, where I would just implicitly convert the "Empty" value to my types as needed:

// In library:
trait A {
  implicit def noneToT1(none: Option[Nothing]): T1 = defaultT1
  implicit def noneToT2(none: Option[Nothing]): T2 = defaultT2
  def f1: T1
  def f2: T2
}
// In the code that uses the library
class EmptyA extends A {
  def f1 = None      
  def f2 = None      
}

One reason for not (mis)using None in this fashion is that the user would expect that f1 and f2 return Option[T1] and Option[T2] respectively. And they don't. Off course, I could have def f1: Option[T1], but in this case the values are not actually optional, they just can have some default empty value, or a real value, I just want to create the default values "under the hood" and have some uniform way of saying "default" or "empty" through the entire library. So the question is, should I use None to express this "defaultness" or go for some custom type? Right now I'm using my own object Empty, but it feels a bit superfluous.

EDIT: To ilustrate my question I'll add the code I am using right now:

// In library:
trait Empty
object Empty extends Empty

trait A {
  implicit def emptyToT1(none: Empty): T1 = defaultT1
  implicit def emptyToT2(none: Empty): T2 = defaultT2
  def f1: T1
  def f2: T2
}
// In the code that uses the library
class EmptyA extends A {
  def f1 = Empty
  def f2 = Empty
}
class HalfFullA extends A {
  def f1 = Empty
  def f2 = someValue2
}
class FullA extends A {
  def f1 = someValue1
  def f2 = someValue2
}

My question is quite simple: is it a good idea to use scala's None instead of my Empty?

Upvotes: 4

Views: 1542

Answers (3)

kassens
kassens

Reputation: 4485

If you can't or don't want to define the default value using inheritance, I suggest to keep the new object. Reusing None for something else than the Some counterpart seems wrong and doesn't really save you much.

Upvotes: 1

IttayD
IttayD

Reputation: 29163

I would just use typeclasses for this:

trait WithDefault[T] {
  def default: T
}

object WithDefault {
  // if T1 is an existing class
  implicit val t1Default = new WithDefault[T1] {def default = defaultT1}
}

//if T2 is your own class:
class T2 ...
object T2 {
  implicit val withDefault = new WithDefault[T2] {def default = defaultT2}
}

then somewhere convenient:

def default[T : WithDefault] = implicitly[WithDefault[T]].default

and use:

class EmptyA {
  def f1 = default[T1]
  def f2 = default[T2]
}

Update: To accomudate Vilius, one can try this:

def default = new WithDefault[Nothing]{def default = error("no default")}

implicit def toDefault[U, T](dummy: WithDefault[U])(implicit withDefault: WithDefault[T]): T = withDefault.default

class EmptyA {
  def f1: T1 = default
  def f2: T2 = default
}

This has the benefit over the OP's original attempt in that each new class can define its own default (and others in WithDefault), rather than have everything in a trait A.

However, this doesn't work. See https://issues.scala-lang.org/browse/SI-2046

To work around this:

trait A {
    def f1: T1
    def f2: T2

    implicit def toT1Default(dummy: WithDefault[Nothing]) = toDefault[T1](dummy)
    implicit def toT2Default(dummy: WithDefault[Nothing]) = toDefault[T2](dummy)
}

class EmptyA extends A {
   def f1 = default
   def f2 = default
}

Upvotes: 7

Miles Sabin
Miles Sabin

Reputation: 23056

I think you should go for something much simpler. For instance, starting with your example and deleting extraneous stuff we very quickly get to,

trait A {
  def noT1 = defaultT1
  def noT2 = defaultT2
  def f1: T1
  def f2: T2
}

class EmptyA extends A {
  def f1 = noT1      
  def f2 = noT2      
}

I really don't see that the addition of Options or implicits to this would add any value, at least not unless there's some other unstated context for the question.

Upvotes: 3

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