Scala Implicit class and inheritance

Question

let's say that I would like to add some method to String class. But concrete implicit class that should be applied is known during runtime (strategy pattern). Lets say we have

trait StringExtensions {
  def doSth(str: String): String
}

class Strategy1 extends StringExtensions {
   override def doSth(str: String): String = "a"
}

class Strategy2 extends StringExtensions {
   override def doSth(str: String): String = "b"
}

now my client code looks like:

def someMethod(strategy: StringExtensions) : String{
  val name = "Pawel"
  return strategy.doSth(name)
}
...
String ret = someMethod(new Strategy1())

but I would like to have code like:

def someMethod(strategy: StringExtensions) : String{
  val name = "Pawel"
  return name.doSth() // Here is the tricky line
}
...
String ret = someMethod(new Strategy1())

I played a bit with implicits but when comes to this use case with inheritance I cannot find proper solution, any help?

Jasper-M · Accepted Answer · 2016-12-06 14:09:29Z

5

I'm not sure you should really use implicits like this, but maybe in some DSL this might be a valid use case.

class StringExtensions(str: String, strategy: StringExtensionsStrategy) {
  def doSth() = strategy.doSth(str)
}

trait StringExtensionsStrategy extends (String => StringExtensions) {
  final def apply(str: String) = new StringExtensions(str, this)
  def doSth(str: String): String
}

class Strategy1 extends StringExtensionsStrategy {
   override def doSth(str: String) = "a"
}

class Strategy2 extends StringExtensionsStrategy {
   override def doSth(str: String) = "b"
}

def someMethod(implicit strategy: StringExtensionsStrategy) = {
  val name = "Pawel"
  name.doSth()
}

val ret: String = someMethod(new Strategy1())

As mentioned in the comments, an alternative encoding would be this:

class StringExtensions(str: String, strategy: StringExtensionsStrategy) {
  def doSth() = strategy.doSth(str)
}

trait StringExtensionsStrategy {
  implicit final def apply(str: String) = new StringExtensions(str, this)
  def doSth(str: String): String
}

class Strategy1 extends StringExtensionsStrategy {
   override def doSth(str: String) = "a"
}

class Strategy2 extends StringExtensionsStrategy {
   override def doSth(str: String) = "b"
}

def someMethod(strategy: StringExtensionsStrategy) = {
  import strategy._
  val name = "Pawel"
  name.doSth()
}

val ret: String = someMethod(new Strategy1())

edited Dec 6, 2016 at 14:09

answered Dec 6, 2016 at 13:49

Jasper-M

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5 Comments

adamwy Over a year ago

Implicit strategy parameter could be replaced with implicit val s = strategy as a first line in someMethod.

adamwy Over a year ago

Or import strategy._ if you make apply method in StringExtensionsStrategy trait implicit.

Jasper-M Over a year ago

You could do that, but after having to add some extra implicit vals and imports, you could just as easily have written strategy.doSth(name). That's why I say that this is probably not a good idea, but there might be some use case, and I assumed that you will want to eliminate as much boilerplate as you can in that hypothetical use case.

Paweł Kaczorowski Over a year ago

Thank you guys, of course my example is pretty trivial, I will use it to construct some dsl. Could give me some hints where should I look for the explanation of this code?

Jasper-M Over a year ago

@PawełKaczorowski It just uses functions and implicits. You can look here for an overview.

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