Dotty Documentation


object language
extends Object with Serializable

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

and, for dotty:

  • [[Scala2 Scala2] backwards compatibility mode for Scala2
  • [[noAtoTupling noAutoTupling]] disable auto-tupling
  • [[strictEquality strictEquality]] enable strick equality

[-] Constructors

[-] Members

[+] object Scala2

Where imported, a backwards compatibility mode for Scala2 is enabled

[+] object experimental

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

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.

[+] object noAutoTupling

Where imported, auto-tupling is disabled

[+] object strictEquality

Where imported loose equality using eqAny is disabled

[+] @volatile implicit lazy val dynamics : dynamics

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.

[+] @volatile 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. 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.

[+] @volatile implicit lazy val higherKinds : higherKinds

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.

[+] @volatile implicit lazy val implicitConversions : implicitConversions

Only where enabled, definitions of legacy implicit conversions and certain uses of implicit conversions are allowed.

A legacy 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. In particular, delegates of the scala.Conversion class can be defined without having to import the language feature.

The language import is also required to enable uses of implicit conversions unless the conversion in question is co-defined with the type to which it maps. Co-defined means: defined in the companion object of the class of the result type. Examples:

{{{ class A class B object B { delegate a2b for Conversion[A, B] { ... } } object C { delegate b2a for Conversion[B, A] { ... } } import delegate B._ import delegate C._ val x: A = new B // language import required val x: B = new A // no import necessary since a2b is co-defined with B }}}

'''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. This holds in particular for implicit conversions defined after the fact between unrelated types.

[+] @volatile implicit lazy val postfixOps : postfixOps

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.

[+] @volatile implicit lazy val reflectiveCalls : reflectiveCalls

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.