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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:

Source
language.scala
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Value Members

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

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    AnyRef → Any
  2. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  4. final def asInstanceOf[T0]: T0

    Cast the receiver object to be of type T0.

    Cast the receiver object to be of type T0.

    Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at runtime, while the expression List(1).asInstanceOf[List[String]] will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.

    returns

    the receiver object.

    Definition Classes
    Any
    Exceptions thrown

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  5. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.CloneNotSupportedException]) @native()
    Note

    not specified by SLS as a member of AnyRef

  6. implicit lazy val dynamics: dynamics

    Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined.

    Only where this feature is enabled, can direct or indirect subclasses of trait scala.Dynamic be defined. If dynamics is not enabled, a definition of a class, trait, or object that has Dynamic as a base trait is rejected by the compiler.

    Selections of dynamic members 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

    Tests whether the argument (that) is a reference to the receiver object (this).

    Tests whether the argument (that) is a reference to the receiver object (this).

    The eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).

    returns

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  8. def equals(arg0: AnyRef): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  9. implicit lazy val existentials: existentials

    Where this feature is 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.

    Where this feature is 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. If existentials is not enabled, those cases will trigger a warning from the compiler.

    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. def finalize(): Unit

    Called by the garbage collector on the receiver object when there are no more references to the object.

    Called by the garbage collector on the receiver object when there are no more references to the object.

    The details of when and if the finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.Throwable])
    Note

    not specified by SLS as a member of AnyRef

  11. final def getClass(): Class[_ <: AnyRef]

    Returns the runtime class representation of the object.

    Returns the runtime class representation of the object.

    returns

    a class object corresponding to the runtime type of the receiver.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  12. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  13. implicit lazy val higherKinds: higherKinds

    Where this feature is enabled, higher-kinded types can be written.

    Where this feature is enabled, higher-kinded types can be written. If higherKinds is not enabled, a higher-kinded type such as F[A] will trigger a warning from the compiler.

    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.

  14. implicit lazy val implicitConversions: implicitConversions

    Where this feature is enabled, definitions of implicit conversions are allowed.

    Where this feature is enabled, definitions of implicit conversions are allowed. If implicitConversions is not enabled, the definition of an implicit conversion will trigger a warning from the compiler.

    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 classes and implicit values of other types are not governed by this language feature.

    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.

  15. final def isInstanceOf[T0]: Boolean

    Test whether the dynamic type of the receiver object is T0.

    Test whether the dynamic type of the receiver object is T0.

    Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression 1.isInstanceOf[String] will return false, while the expression List(1).isInstanceOf[List[String]] will return true. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.

    returns

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  16. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  17. final def notify(): Unit

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Wakes up a single thread that is waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  18. final def notifyAll(): Unit

    Wakes up all threads that are waiting on the receiver object's monitor.

    Wakes up all threads that are waiting on the receiver object's monitor.

    Definition Classes
    AnyRef
    Annotations
    @native()
    Note

    not specified by SLS as a member of AnyRef

  19. implicit lazy val postfixOps: postfixOps

    Only where this feature is enabled, is postfix operator notation (expr op) permitted.

    Only where this feature is enabled, is postfix operator notation (expr op) permitted. If postfixOps is not enabled, an expression using postfix notation is rejected by the compiler.

    Why keep the feature? Postfix notation is preserved for backward compatibility only. Historically, 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 alone. Postfix syntax is associated with an abuse of infix notation, a op1 b op2 c op3, that can be harder to read than ordinary method invocation with judicious use of parentheses. It is recommended not to enable this feature except for legacy code.

  20. implicit lazy val reflectiveCalls: reflectiveCalls

    Where this feature is enabled, accesses to members of structural types that need reflection are supported.

    Where this feature is enabled, accesses to members of structural types that need reflection are supported. If reflectiveCalls is not enabled, an expression requiring reflection will trigger a warning from the compiler.

    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.

  21. final def synchronized[T0](arg0: => T0): T0
    Definition Classes
    AnyRef
  22. def toString(): String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    AnyRef → Any
  23. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
  24. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
  25. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException]) @native()
  26. object experimental

    The experimental object contains features that are known to have unstable API or behavior that may change in future releases.

    The experimental object contains features that are known to have unstable API or behavior that may change in future releases.

    Experimental features may undergo API changes in future releases, so production code should not rely on them.

    Programmers are encouraged to try out experimental features and report any bugs or API inconsistencies they encounter so they can be improved in future releases.

Inherited from AnyRef

Inherited from Any

Language Features

Experimental Language Features

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