Scala隐含于包含Map的Option

Question

I am trying to write the following implicit:

implicit class ExtractOrElse[K, V](o: Option[Map[K, V]]) {
  def extractOrElse(key: K)(f: => V): V = { if (o.isDefined) o.get(key) else f }
}

Which I want to use in this way:

normalizationContexts.extractOrElse(shardId)(defaultNormalizationContext)

to avoid a clunkier syntax ( normalizationContexts is an Option[Map[String, NormzalitionContext]] ).

Also, let me add that it is intentional that there is only one default value: it will be used if the Option isEmpty , but if the Option isDefined , then the behavior of the Map is not changed, and it will throw an exception if the key is not found - so the default value won't be used in that case, and this is all intentional.

However, I get an error when passing in None in unit tests:

assertEquals(None.extractOrElse('a')(0), 0)

results in:

Error:(165, 37) type mismatch;
 found   : Char('a')
 required: K
  assertEquals(None.extractOrElse('a')(0), 0)

I realize that None is not parametric, as it is defined as:

case object None extends Option[Nothing] {
  def isEmpty = true
  def get = throw new NoSuchElementException("None.get")

What is the best way to make this work?

Answer 1

Instead of None.extractOrElse(...) , try Option.empty[Map[Char, Int]].extractOrElse(...) .

If you always use the same types for your test cases, you could also create a type alias in the specs class in order to reduce the clutter:

type OpMap = Option[Map[Char, Int]]
// ...
assertEquals(Option.empty[OpMap].extractOrElse('a')(0), 0)

Just in case, you can use flatMap and getOrElse to achieve the same thing without writing a new method:

val n = Option.empty[Map[String, Int]]
val s = Some(Map("x" → 1, "y" → 2))

n.flatMap(_.get("x")).getOrElse(3)  // 3
s.flatMap(_.get("x")).getOrElse(3)  // 1
s.flatMap(_.get("z")).getOrElse(3)  // 3

Answer 2

The type system doesn't have enough information about the types K and V . There is no way to know what the type of A would be in the case where your None was Some[A] .

When I create an example with explicit types, the code works as expected:

// Like this
val e = new ExtractOrElse(Option.empty[Map[Char, Int]])
e.extractOrElse('a')(0) // Equals 0

// Or like this
val e = new ExtractOrElse[Char, Int](None)
println(e.extractOrElse('a')(0))

// Or like this
val m: Option[Map[Char, Int]] = None
val e = new ExtractOrElse(m)
println(e.extractOrElse('a')(0))