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Standard library for the Scala Programming Language
/*
* Scala (https://www.scala-lang.org)
*
* Copyright EPFL and Lightbend, Inc.
*
* Licensed under Apache License 2.0
* (http://www.apache.org/licenses/LICENSE-2.0).
*
* See the NOTICE file distributed with this work for
* additional information regarding copyright ownership.
*/
package scala
/**
* The `scala.language` object controls the language features available to the programmer, as proposed in the
* [[https://docs.google.com/document/d/1nlkvpoIRkx7at1qJEZafJwthZ3GeIklTFhqmXMvTX9Q/edit '''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
* - [[existentials `existentials`]] enables writing existential types
* - [[higherKinds `higherKinds`]] enables writing higher-kinded types
* - [[implicitConversions `implicitConversions`]] enables defining implicit methods and members
* - [[postfixOps `postfixOps`]] enables postfix operators (not recommended)
* - [[reflectiveCalls `reflectiveCalls`]] enables using structural 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._
/** 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.
*
* @group production
*/
implicit lazy val dynamics: dynamics = languageFeature.dynamics
/** 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.
*
* @group production
*/
implicit lazy val postfixOps: postfixOps = languageFeature.postfixOps
/** 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.
*
* @group production
*/
implicit lazy val reflectiveCalls: reflectiveCalls = languageFeature.reflectiveCalls
/** 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.
*
* @group production
*/
implicit lazy val implicitConversions: implicitConversions = languageFeature.implicitConversions
/** 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.
*
* @group production
*/
@deprecated("higherKinds no longer needs to be imported explicitly", "2.13.1")
implicit lazy val higherKinds: higherKinds = languageFeature.higherKinds
/** 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.
*
* @group production
*/
implicit lazy val existentials: existentials = languageFeature.existentials
/** 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
* [[https://github.com/scala/bug/issues report any bugs or API inconsistencies]]
* they encounter so they can be improved in future releases.
*
* @group experimental
*/
object experimental {
import languageFeature.experimental._
/** Only where this feature is enabled, are macro definitions allowed.
* If `macros` is not enabled, macro definitions are rejected by the compiler.
*
* Macro implementations and macro applications are not governed by this
* language feature; 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
}
}