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 theDynamic
traitexistentials
enables writing existential typeshigherKinds
enables writing higher-kinded typesimplicitConversions
enables defining implicit methods and memberspostfixOps
enables postfix operators (not recommended)reflectiveCalls
enables using structural typesexperimental
contains newer features that have not yet been tested in production
- Source
- language.scala
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Language Features
- 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 hasDynamic
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.
- 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[_]
, orMap[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.
- 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.
- 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. IfpostfixOps
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. - 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 }
whereDecls
contains declarations of new members that do not override any member inParents
. 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.
- 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 asF[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.
- Annotations
- @deprecated
- Deprecated
(Since version 2.13.1) higherKinds no longer needs to be imported explicitly
Experimental Language Features
- 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.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.