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:

class Object
trait Matchable
class Any

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.

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.

Language Features

implicit lazy val dynamics: dynamics

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.

implicit lazy val postfixOps: postfixOps

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.

implicit lazy val reflectiveCalls: reflectiveCalls

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.

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.

@deprecated("higherKinds no longer needs to be imported explicitly", "2.13.1")
implicit lazy val higherKinds: higherKinds

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.

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. 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.

Type members


Where imported, ad hoc extensions of non-open classes in other compilation units are allowed.

Why control the feature? Ad-hoc extensions should usually be avoided since they typically cannot rely on an "internal" contract between a class and its extensions. Only open classes need to specify such a contract. Ad-hoc extensions might break for future versions of the extended class, since the extended class is free to change its implementation without being constrained by an internal contract.

Why allow it? An ad-hoc extension can sometimes be necessary, for instance when mocking a class in a testing framework, or to work around a bug or missing feature in the original class. Nevertheless, such extensions should be limited in scope and clearly documented. That's why the language import is required for them.

object future

Where imported, auto-tupling is disabled.

Why control the feature? Auto-tupling can lead to confusing and brittle code in presence of overloads. In particular, surprising overloads can be selected, and adding new overloads can change which overload is selected in suprising ways.

Why allow it? Not allowing auto-tupling is difficult to reconcile with operators accepting tuples.

Where imported, loose equality using eqAny is disabled.

Why allow and control the feature? For compatibility and migration reasons, strict equality is opt-in. See linked documentation for more information.

object unsafeNulls