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  1. import scala.language.{higherKinds, implicitConversions}
  2.  
  3. trait Counter {
  4.   def value: Int
  5. }
  6.  
  7. class MutableCounter(var value: Int) extends Counter
  8.  
  9. case class ImmutableCounter(value: Int) extends Counter
  10.  
  11. trait CounterOps[C <: Counter] {
  12.   def count(): C
  13. }
  14.  
  15. class MutableCounterOps(counter: MutableCounter)
  16.     extends CounterOps[MutableCounter] {
  17.   override def count(): MutableCounter = {
  18.     counter.value += 1
  19.     counter
  20.   }
  21. }
  22.  
  23. object MutableCounterOps {
  24.   implicit def counterToOps(counter: MutableCounter): MutableCounterOps =
  25.     new MutableCounterOps(counter)
  26. }
  27.  
  28. class ImmutableCounterOps(counter: ImmutableCounter)
  29.     extends CounterOps[ImmutableCounter] {
  30.   override def count(): ImmutableCounter =
  31.     ImmutableCounter(counter.value + 1)
  32. }
  33.  
  34. object ImmutableCounterOps {
  35.   implicit def counterToOps(counter: ImmutableCounter): ImmutableCounterOps =
  36.     new ImmutableCounterOps(counter)
  37. }
  38.  
  39. class EchoingCounterOps[C <: Counter](
  40.     baseCounterOpsProvider: C => CounterOps[C],
  41.     counter: C)
  42.     extends CounterOps[C] {
  43.   override def count(): C = {
  44.     println("Counting from: " + counter.value)
  45.     baseCounterOpsProvider(counter).count()
  46.   }
  47. }
  48.  
  49. object EchoingCounterOps {
  50.   trait EchoingCounterOpsProvider[C <: Counter] {
  51.     implicit def counterToOps(counter: C): CounterOps[C]
  52.   }
  53.  
  54.   def compose[C <: Counter, CO <: CounterOps[C]](
  55.       baseCounterOpsProvider: C => CO): EchoingCounterOpsProvider[C] = {
  56.     new EchoingCounterOpsProvider[C] {
  57.       override implicit def counterToOps(counter: C): CounterOps[C] =
  58.         new EchoingCounterOps(baseCounterOpsProvider, counter)
  59.     }
  60.   }
  61. }
  62.  
  63. object Main {
  64.   def main(args: Array[String]): Unit = {
  65.     println("Echoing mutable...")
  66.     echoingMutable()
  67.     println("Echoing immutable...")
  68.     echoingImmutable()
  69.   }
  70.  
  71.   def echoingMutable(): Unit = {
  72.     val composed = EchoingCounterOps.compose(MutableCounterOps.counterToOps)
  73.     import composed.counterToOps
  74.  
  75.     val counter = new MutableCounter(3)
  76.     println("New value: " + counter.count().count().count().value)
  77.     println("Retained value: " + counter.value)
  78.   }
  79.  
  80.   def echoingImmutable(): Unit = {
  81.     val composed = EchoingCounterOps.compose(ImmutableCounterOps.counterToOps)
  82.     import composed.counterToOps
  83.  
  84.     val counter = ImmutableCounter(3)
  85.     println("New value: " + counter.count().count().count().value)
  86.     println("Retained value: " + counter.value)
  87.   }
  88. }
  89.  
Success #stdin #stdout 0.33s 4382720KB
comments (?)
stdin 
Standard input is empty
stdout 
Echoing mutable...
Counting from: 3
Counting from: 4
Counting from: 5
New value: 6
Retained value: 6
Echoing immutable...
Counting from: 3
Counting from: 4
Counting from: 5
New value: 6
Retained value: 3
https://ideone.com/GSIJ89
language:
Scala (scala 2.12.8)
created:
7 years ago
visibility:
public

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