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
traitpostfixOps
enables postfix operatorsreflectiveCalls
enables using structural typesimplicitConversions
enables defining implicit methods and membershigherKinds
enables writing higher-kinded typesexistentials
enables writing existential 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
Where enabled, direct or indirect subclasses of trait scala.Dynamic can be defined.
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.
-
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.
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[_]
, 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
higherKinds: higherKinds
Only where this flag is enabled, higher-kinded types can be written.
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.
-
implicit
lazy val
implicitConversions: implicitConversions
Only where enabled, definitions of implicit conversions are allowed.
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.
-
implicit
lazy val
postfixOps: postfixOps
Only where enabled, postfix operator notation
(expr op)
will be allowed.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.
-
implicit
lazy val
reflectiveCalls: reflectiveCalls
Only where enabled, accesses to members of structural types that need reflection are supported.
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 }
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.
Experimental Language Features
-
object
experimental
The experimental object contains features that have been recently added but have not been thoroughly tested in production yet.
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.
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.collection.parallel.immutable
- Immutable, parallel data-structures such asParVector
,ParRange
,ParHashMap
orParHashSet
scala.collection.parallel.mutable
- Mutable, parallel data-structures such asParArray
,ParHashMap
,ParTrieMap
orParHashSet
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.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)scala.util.parsing
- Parser combinators, including an example implementation of a JSON parser (scala-parser-combinators.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
.