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Scala implicits

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Introduction to

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Transcript

  • 1. Implicits 2 Features, 1 Keyword Nick Partridge @nkpart
  • 2. 2 Features • Implicit Conversions • Implicit Parameters
  • 3. Conversions • int a = 5.3f; // Implicit conversion
  • 4. Conversions • val a: A = ...; • val b: B = a; // Compile Fail. Of course.
  • 5. Conversions • val a: A = ...; • implicit def (a: A): B = ...; • val b: B = a; • // Profit.
  • 6. Conversions Compiler searches the ‘implicit scope’ • val b: B = a; • val b: B = bToA(a);
  • 7. Pimp My Library Live Demo.
  • 8. Conversions • Also triggered by a method search case class Foo(s: String) { def pl = s + “es” }
  • 9. Conversions implicit def toFoo(s: String) = Foo(s) “peach”.pl // == “peaches” toFoo(s).pl
  • 10. Not Transitive Implicit A => B Implicit B => C Does not mean Implicit A => C
  • 11. View Bounds Live coding!
  • 12. The End. Of Part 1.
  • 13. Parameters
  • 14. Parameters • Scala functions have parameter groups • def foo(a: Int)(b: Int)(c: Int) = ... • foo(1)(2)(3)
  • 15. Parameters • The *last* group can be `implicit`. • def mungeData(d: Data)(implicit m: Munger) • mungeData(myData)(myMunger)
  • 16. Parameters • implicit val myMunger = new Munger • mungeData(myData)(myMunger) Compiler figures out the last group
  • 17. Parameters • def mungeData[D](d: D)(implicit m: Munger[D]) • mungeData(“foo”) // Compiler finds a Munger[String]
  • 18. Parameters • Higher kinds! • def mungeData[M[_], D](d: D)(implicit m: Munger[D], each: Each[M]) = ... • mungeData(List(1,2,3)) • Munger[Int] • Each[List]
  • 19. Live demo! Monoid!
  • 20. Conversions Parameters
  • 21. Where to get the ‘implicits’ from?
  • 22. Implicit Search Scope • Similar to value resolution scope • local variables, values in current object, super classes. • Imported object members • Function parameters
  • 23. Implicit Search Scope // COMPANION OBJECTS trait Monoid[T] object Monoid { // SAME NAME implicit val s = new Monoid[String] } // `s` will be available anywhere a Monoid[String] is needed. Without imports.
  • 24. Implicit Search Scope def foo[T](t: T)(implicit m: Monoid[T]) = { // m is available for any implicit parameters needed within this method body }
  • 25. More! • Anonymous functions • implicit r => { ... } • Import instances • case class Z(a: A) { implicit ia = a } • val z = new Z(someA) • import z._
  • 26. Wonderful and Terrible.
  • 27. La fin.