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.
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.
(defined at scala.language)
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. 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.
(defined at scala.language)
implicit lazy val higherKinds: higherKinds
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.
(defined at scala.language)
implicit lazy val implicitConversions: implicitConversions
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 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.
(defined at scala.language)
implicit lazy val postfixOps: postfixOps
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.
(defined at scala.language)
implicit lazy val reflectiveCalls: reflectiveCalls
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.
(defined at scala.language)
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