Skip to content


Subversion checkout URL

You can clone with HTTPS or Subversion.

Download ZIP
Fetching contributors…

Cannot retrieve contributors at this time

174 lines (162 sloc) 8.454 kb
package scala
* 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
* @groupname production Language Features
* @groupname experimental Experimental Language Features
* @groupprio experimental 10
object language {
import languageFeature._
/** 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.
* @group production
implicit lazy val dynamics: dynamics = languageFeature.dynamics
/** 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.
* @group production
implicit lazy val postfixOps: postfixOps = languageFeature.postfixOps
/** 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.
* @group production
implicit lazy val reflectiveCalls: reflectiveCalls = languageFeature.reflectiveCalls
/** 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.
* @group production
implicit lazy val implicitConversions: implicitConversions = languageFeature.implicitConversions
/** 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.
* @group production
implicit lazy val higherKinds: higherKinds = languageFeature.higherKinds
/** 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.
* @group production
implicit lazy val existentials: existentials = languageFeature.existentials
/** 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.
* @group experimental
object experimental {
import languageFeature.experimental._
/** Where enabled, macro definitions are allowed. Macro implementations and
* macro applications are unaffected; they can be used anywhere.
* '''Why introduce the feature?''' Macros promise to make the language more regular,
* replacing ad-hoc language constructs with a general powerful abstraction
* capability that can express them. Macros are also a more disciplined and
* powerful replacement for compiler plugins.
* '''Why control it?''' For their very power, macros can lead to code that is hard
* to debug and understand.
implicit lazy val macros: macros = languageFeature.experimental.macros
Jump to Line
Something went wrong with that request. Please try again.