Would you dare to change HashMap implementation?

Sky-Tiger

AnyVal is the root class of all value types, which describe values not implemented as objects in the underlying host system. Value classes are specified in Scala Language Specification, section 12.2.

The standard implementation includes nine AnyVal subtypes:

scala.Double, scala.Float, scala.Long, scala.Int, scala.Char, scala.Short, and scala.Byte are the numeric value types.

scala.Unit and scala.Boolean are the non-numeric value types.

Other groupings:

The subrange types are scala.Byte, scala.Short, and scala.Char.
The integer types include the subrange types as well as scala.Int and scala.Long.
The floating point types are scala.Float and scala.Double.
Prior to Scala 2.10, AnyVal was a sealed trait. Beginning with Scala 2.10, however, it is possible to define a subclass of AnyVal called a user-defined value class which is treated specially by the compiler. Properly-defined user value classes provide a way to improve performance on user-defined types by avoiding object allocation at runtime, and by replacing virtual method invocations with static method invocations.

User-defined value classes which avoid object allocation...

must have a single, public val parameter that is the underlying runtime representation.
can define defs, but no vals, vars, or nested traitss, classes or objects.
typically extend no other trait apart from AnyVal.
cannot be used in type tests or pattern matching.
may not override equals or hashCode methods.
A minimal example:

class Wrapper(val underlying: Int) extends AnyVal {
def foo: Wrapper = new Wrapper(underlying * 19)
}
It's important to note that user-defined value classes are limited, and in some circumstances, still must allocate a value class instance at runtime. These limitations and circumstances are explained in greater detail in the Value Classes Guide as well as in SIP-15: Value Classes, the Scala Improvement Proposal.
Source
AnyVal.scala
Linear Supertypes
Known Subclasses
Type Hierarchy

Sky-Tiger

The Application trait can be used to quickly turn objects into executable programs, but is not recommended. Here is an example:

object Main extends Application {
Console.println("Hello World!")
}
Here, object Main inherits the main method of Application. The body of the Main object defines the main program. This technique does not work if the main program depends on command-line arguments (which are not accessible with the technique presented here).

It is possible to time the execution of objects that inherit from class Application by setting the global scala.time property. Here is an example for benchmarking object Main:

java -Dscala.time Main
In practice the Application trait has a number of serious pitfalls:

Threaded code that references the object will block until static initialization is complete. However, because the entire execution of an object extending Application takes place during static initialization, concurrent code will always deadlock if it must synchronize with the enclosing object.
As described above, there is no way to obtain the command-line arguments because all code in body of an object extending Application is run as part of the static initialization which occurs before Application's main method even begins execution.
Static initializers are run only once during program execution, and JVM authors usually assume their execution to be relatively short. Therefore, certain JVM configurations may become confused, or simply fail to optimize or JIT the code in the body of an object extending Application. This can lead to a significant performance degradation.
It is recommended to use the scala.App trait instead.

object Main {
def main(args: Array[String]) {
  //..
}
}
Annotations
@deprecated
Deprecated
(Since version 2.9.0) use App instead
Source
Application.scala
Version
1.0, 10/09/2003

Sky-Tiger

Defines a finite set of values specific to the enumeration. Typically these values enumerate all possible forms something can take and provide a lightweight alternative to case classes.

Each call to a Value method adds a new unique value to the enumeration. To be accessible, these values are usually defined as val members of the evaluation.

All values in an enumeration share a common, unique type defined as the Value type member of the enumeration (Value selected on the stable identifier path of the enumeration instance).
Self Type
Enumeration
Annotations
@SerialVersionUID( uid = 8476000850333817230L )
Source
Enumeration.scala
Example:
object Main extends App {

  object WeekDay extends Enumeration {
    type WeekDay = Value
    val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
  }
  import WeekDay._

  def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)

  WeekDay.values filter isWorkingDay foreach println
}
// output:
// Mon
// Tue
// Wed
// Thu
// Fri

Sky-Tiger

The scala.language object controls the language features available to the programmer, as proposed in the SIP-18 document.

Each of these features has to be explicitly imported into the current scope to become available:

import language.postfixOps // or language._
List(1, 2, 3) reverse
The language features are:

dynamics enables defining calls rewriting using the Dynamic trait
postfixOps enables postfix operators
reflectiveCalls enables using structural types
implicitConversions enables defining implicit methods and members
higherKinds enables writing higher-kinded types
existentials enables writing existential types
experimental contains newer features that have not yet been tested in production
Source
language.scala

Sky-Tiger

A marker trait that enables dynamic invocations. Instances x of this trait allow method invocations x.meth(args) for arbitrary method names meth and argument lists args as well as field accesses x.field for arbitrary field names field.

If a call is not natively supported by x (i.e. if type checking fails), it is rewritten according to the following rules:

foo.method("blah")      ~~> foo.applyDynamic("method")("blah")
foo.method(x = "blah")  ~~> foo.applyDynamicNamed("method")(("x", "blah"))
foo.method(x = 1, 2)    ~~> foo.applyDynamicNamed("method")(("x", 1), ("", 2))
foo.field           ~~> foo.selectDynamic("field")
foo.varia = 10      ~~> foo.updateDynamic("varia")(10)
foo.arr(10) = 13    ~~> foo.selectDynamic("arr").update(10, 13)
foo.arr(10)         ~~> foo.applyDynamic("arr")(10)
As of Scala 2.10, defining direct or indirect subclasses of this trait is only possible if the language feature dynamics is enabled.
Source
Dynamic.scala