Scala Language
Dynamischer Aufruf

Einführung

Syntax

• Klasse Foo erweitert Dynamic
• foo.field
• foo.field = Wert
• foo.method (args)
• foo.method (namedArg = x, y)

Bemerkungen

Feldzugriffe

Diese:

class Foo extends Dynamic {
  // Expressions are only rewritten to use Dynamic if they are not already valid
  // Therefore foo.realField will not use select/updateDynamic
  var realField: Int = 5
  // Called for expressions of the type foo.field
  def selectDynamic(fieldName: String) = ???
  def updateDynamic(fieldName: String)(value: Int) = ???
}

ermöglicht den einfachen Zugriff auf Felder:

val foo: Foo = ???
foo.realField // Does NOT use Dynamic; accesses the actual field
foo.realField = 10 // Actual field access here too
foo.unrealField // Becomes foo.selectDynamic(unrealField)
foo.field = 10  // Becomes foo.updateDynamic("field")(10)
foo.field = "10" // Does not compile; "10" is not an Int.
foo.x() // Does not compile; Foo does not define applyDynamic, which is used for methods.
foo.x.apply() // DOES compile, as Nothing is a subtype of () => Any
// Remember, the compiler is still doing static type checks, it just has one more way to
// "recover" and rewrite otherwise invalid code now.

Methodenaufrufe

Diese:

class Villain(val minions: Map[String, Minion]) extends Dynamic {
  def applyDynamic(name: String)(jobs: Task*) = jobs.foreach(minions(name).do)
  def applyDynamicNamed(name: String)(jobs: (String, Task)*) = jobs.foreach {
    // If a parameter does not have a name, and is simply given, the name passed as ""
    case ("", task) => minions(name).do(task)
    case (subsys, task) => minions(name).subsystems(subsys).do(task)
  }
}

erlaubt Aufrufe von Methoden mit und ohne benannte Parameter:

val gru: Villain = ???
gru.blu() // Becomes gru.applyDynamic("blu")()
// Becomes gru.applyDynamicNamed("stu")(("fooer", ???), ("boomer", ???), ("", ???),
//         ("computer breaker", ???), ("fooer", ???))
// Note how the `???` without a name is given the name ""
// Note how both occurrences of `fooer` are passed to the method
gru.stu(fooer = ???, boomer = ???, ???, `computer breaker` = ???, fooer = ???)
gru.ERR("a") // Somehow, scalac thinks "a" is not a Task, though it clearly is (it isn't)

Interaktion zwischen Feldzugriff und Aktualisierungsmethode

Etwas uninteressant (aber auch der einzige vernünftige Weg, damit es funktioniert):

val dyn: Dynamic = ???
dyn.x(y) = z

ist äquivalent zu:

dyn.selectDynamic("x").update(y, z)

während

dyn.x(y)

ist immer noch

dyn.applyDynamic("x")(y)

Es ist wichtig, sich dessen bewusst zu sein, andernfalls könnte es unbemerkt schleichen und seltsame Fehler verursachen.