scala

# language

#### object language

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

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### Value Members

1. #### final def !=(arg0: Any): Boolean

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2. #### final def ##(): Int

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3. #### final def ==(arg0: Any): Boolean

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4. #### final def asInstanceOf[T0]: T0

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5. #### def clone(): AnyRef

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     protected[java.lang]
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     @throws( ... )

6. #### implicit lazy val dynamics: dynamics

   Where enabled, direct or indirect subclasses of trait scala.

   Where enabled, direct or indirect subclasses of trait scala.Dynamic can be defined. Unless dynamics is enabled, a definition of a class, trait, or object that has Dynamic as a base trait is rejected. Dynamic member selection of existing subclasses of trait Dynamic are unaffected; they can be used anywhere.

   Why introduce the feature? To enable flexible DSLs and convenient interfacing with dynamic languages.

   Why control it? Dynamic member selection can undermine static checkability of programs. Furthermore, dynamic member selection often relies on reflection, which is not available on all platforms.

7. #### final def eq(arg0: AnyRef): Boolean

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8. #### def equals(arg0: Any): Boolean

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9. #### implicit lazy val existentials: existentials

   Only where enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods.

   Only where enabled, existential types that cannot be expressed as wildcard types can be written and are allowed in inferred types of values or return types of methods. Existential types with wildcard type syntax such as `List[_]`, or `Map[String, _]` are not affected.

   Why keep the feature? Existential types are needed to make sense of Java’s wildcard types and raw types and the erased types of run-time values.

   Why control it? Having complex existential types in a code base usually makes application code very brittle, with a tendency to produce type errors with obscure error messages. Therefore, going overboard with existential types is generally perceived not to be a good idea. Also, complicated existential types might be no longer supported in a future simplification of the language.

10. #### object experimental

    The experimental object contains features that have been recently added but have not been thoroughly tested in production yet.

11. #### def finalize(): Unit

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      @throws( classOf[java.lang.Throwable] )

12. #### final def getClass(): Class[_]

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13. #### def hashCode(): Int

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14. #### implicit lazy val higherKinds: higherKinds

    Only where this flag is enabled, higher-kinded types can be written.

    Only where this flag is enabled, higher-kinded types can be written.

    Why keep the feature? Higher-kinded types enable the definition of very general abstractions such as functor, monad, or arrow. A significant set of advanced libraries relies on them. Higher-kinded types are also at the core of the scala-virtualized effort to produce high-performance parallel DSLs through staging.

    Why control it? Higher kinded types in Scala lead to a Turing-complete type system, where compiler termination is no longer guaranteed. They tend to be useful mostly for type-level computation and for highly generic design patterns. The level of abstraction implied by these design patterns is often a barrier to understanding for newcomers to a Scala codebase. Some syntactic aspects of higher-kinded types are hard to understand for the uninitiated and type inference is less effective for them than for normal types. Because we are not completely happy with them yet, it is possible that some aspects of higher-kinded types will change in future versions of Scala. So an explicit enabling also serves as a warning that code involving higher-kinded types might have to be slightly revised in the future.

15. #### implicit lazy val implicitConversions: implicitConversions

    Only where enabled, definitions of implicit conversions are allowed.

    Only where enabled, definitions of implicit conversions are allowed. An implicit conversion is an implicit value of unary function type `A => B`, or an implicit method that has in its first parameter section a single, non-implicit parameter. Examples:

        implicit def stringToInt(s: String): Int = s.length
        implicit val conv = (s: String) => s.length
        implicit def listToX(xs: List[T])(implicit f: T => X): X = ...

    implicit values of other types are not affected, and neither are implicit classes.

    Why keep the feature? Implicit conversions are central to many aspects of Scala’s core libraries.

    Why control it? Implicit conversions are known to cause many pitfalls if over-used. And there is a tendency to over-use them because they look very powerful and their effects seem to be easy to understand. Also, in most situations using implicit parameters leads to a better design than implicit conversions.

16. #### final def isInstanceOf[T0]: Boolean

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17. #### final def ne(arg0: AnyRef): Boolean

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18. #### final def notify(): Unit

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19. #### final def notifyAll(): Unit

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20. #### implicit lazy val postfixOps: postfixOps

    Only where enabled, postfix operator notation `(expr op)` will be allowed.

    Only where enabled, postfix operator notation `(expr op)` will be allowed.

    Why keep the feature? Several DSLs written in Scala need the notation.

    Why control it? Postfix operators interact poorly with semicolon inference. Most programmers avoid them for this reason.

21. #### implicit lazy val reflectiveCalls: reflectiveCalls

    Only where enabled, accesses to members of structural types that need reflection are supported.

    Only where enabled, accesses to members of structural types that need reflection are supported. Reminder: A structural type is a type of the form `Parents { Decls }` where `Decls` contains declarations of new members that do not override any member in `Parents`. To access one of these members, a reflective call is needed.

    Why keep the feature? Structural types provide great flexibility because they avoid the need to define inheritance hierarchies a priori. Besides, their definition falls out quite naturally from Scala’s concept of type refinement.

    Why control it? Reflection is not available on all platforms. Popular tools such as ProGuard have problems dealing with it. Even where reflection is available, reflective dispatch can lead to surprising performance degradations.

22. #### final def synchronized[T0](arg0: ⇒ T0): T0

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23. #### def toString(): String

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24. #### final def wait(): Unit

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      @throws( ... )

25. #### final def wait(arg0: Long, arg1: Int): Unit

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26. #### final def wait(arg0: Long): Unit

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### Language Features

### Experimental Language Features

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