sealed abstract class <:<[-From, +To] extends (From) => To with Serializable
An instance of A <:< B witnesses that A is a subtype of B.
Requiring an implicit argument of the type A <:< B encodes
the generalized constraint A <: B.
To constrain any abstract type T that's in scope in a method's
argument list (not just the method's own type parameters) simply
add an implicit argument of type T <:< U, where U is the required
upper bound; or for lower-bounds, use: L <:< T, where L is the
required lower bound.
In case of any confusion over which method goes in what direction, all the "Co" methods (including
apply) go from left to right in the type ("with" the type), and all the "Contra" methods go
from right to left ("against" the type). E.g., apply turns a From into a To, and
substituteContra replaces the Tos in a type with Froms.
In part contributed by Jason Zaugg.
- From
a type which is proved a subtype of
To- To
a type which is proved a supertype of
From
- Annotations
- @implicitNotFound("Cannot prove that ${From} <:< ${To}.")
- Source
- typeConstraints.scala
sealed trait Option[+A] { // def flatten[B, A <: Option[B]]: Option[B] = ... // won't work, since the A in flatten shadows the class-scoped A. def flatten[B](implicit ev: A <:< Option[B]): Option[B] = if(isEmpty) None else ev(get) // Because (A <:< Option[B]) <: (A => Option[B]), ev can be called to turn the // A from get into an Option[B], and because ev is implicit, that call can be // left out and inserted automatically. }
- See also
=:= for expressing equality constraints
- Alphabetic
- By Inheritance
- <:<
- Serializable
- Function1
- AnyRef
- Any
- by UnliftOps
- by any2stringadd
- by StringFormat
- by Ensuring
- by ArrowAssoc
- Hide All
- Show All
- Public
- All
Abstract Value Members
- abstract def substituteBoth[F[-_, +_]](ftf: F[To, From]): F[From, To]
Substitute
ToforFromandFromforToin the typeF[To, From], given thatFis contravariant in the first argument and covariant in the second.Substitute
ToforFromandFromforToin the typeF[To, From], given thatFis contravariant in the first argument and covariant in the second. Essentially swapsToandFrominftf's type.Equivalent in power to each of substituteCo and substituteContra.
This method is impossible to implement without
throwing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.- returns
ftf, but with a (potentially) different type
Concrete Value Members
- final def !=(arg0: Any): Boolean
Test two objects for inequality.
Test two objects for inequality.
- returns
trueif !(this == that), false otherwise.
- Definition Classes
- AnyRef → Any
- final def ##(): Int
Equivalent to
x.hashCodeexcept for boxed numeric types andnull.Equivalent to
x.hashCodeexcept for boxed numeric types andnull. For numerics, it returns a hash value which is consistent with value equality: if two value type instances compare as true, then ## will produce the same hash value for each of them. Fornullreturns a hashcode wherenull.hashCodethrows aNullPointerException.- returns
a hash value consistent with ==
- Definition Classes
- AnyRef → Any
- def +(other: String): String
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toany2stringadd[<:<[From, To]] performed by method any2stringadd in scala.Predef.
- Definition Classes
- any2stringadd
- def ->[B](y: B): (<:<[From, To], B)
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
- Definition Classes
- ArrowAssoc
- Annotations
- @inline()
- final def ==(arg0: Any): Boolean
The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).The expression
x == thatis equivalent toif (x eq null) that eq null else x.equals(that).- returns
trueif the receiver object is equivalent to the argument;falseotherwise.
- Definition Classes
- AnyRef → Any
- def andThen[C](r: <:<[To, C]): <:<[From, C]
If
From <: ToandTo <: C, thenFrom <: C(subtyping is transitive) - def andThen[C](r: (To) => C): (From) => C
Composes two instances of Function1 in a new Function1, with this function applied first.
- def apply(f: From): To
Coerce a
Frominto aTo. - final def asInstanceOf[T0]: T0
Cast the receiver object to be of type
T0.Cast the receiver object to be of type
T0.Note that the success of a cast at runtime is modulo Scala's erasure semantics. Therefore the expression
1.asInstanceOf[String]will throw aClassCastExceptionat runtime, while the expressionList(1).asInstanceOf[List[String]]will not. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the requested type.- returns
the receiver object.
- Definition Classes
- Any
- Exceptions thrown
ClassCastExceptionif the receiver object is not an instance of the erasure of typeT0.
- def clone(): AnyRef
Create a copy of the receiver object.
Create a copy of the receiver object.
The default implementation of the
clonemethod is platform dependent.- returns
a copy of the receiver object.
- def compose[C](r: <:<[C, From]): <:<[C, To]
If
From <: ToandC <: From, thenC <: To(subtyping is transitive) - def compose[C](r: (C) => From): (C) => To
Composes two instances of Function1 in a new Function1, with this function applied last.
- def ensuring(cond: (<:<[From, To]) => Boolean, msg: => Any): <:<[From, To]
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
- def ensuring(cond: (<:<[From, To]) => Boolean): <:<[From, To]
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
- def ensuring(cond: Boolean, msg: => Any): <:<[From, To]
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
- def ensuring(cond: Boolean): <:<[From, To]
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toEnsuring[<:<[From, To]] performed by method Ensuring in scala.Predef.
- Definition Classes
- Ensuring
- final def eq(arg0: AnyRef): Boolean
Tests whether the argument (
that) is a reference to the receiver object (this).Tests whether the argument (
that) is a reference to the receiver object (this).The
eqmethod implements an equivalence relation on non-null instances ofAnyRef, and has three additional properties:- It is consistent: for any non-null instances
xandyof typeAnyRef, multiple invocations ofx.eq(y)consistently returnstrueor consistently returnsfalse. - For any non-null instance
xof typeAnyRef,x.eq(null)andnull.eq(x)returnsfalse. null.eq(null)returnstrue.
When overriding the
equalsorhashCodemethods, it is important to ensure that their behavior is consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).- returns
trueif the argument is a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- It is consistent: for any non-null instances
- def equals(arg0: AnyRef): Boolean
The equality method for reference types.
- def finalize(): Unit
Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the
finalizemethod is invoked, as well as the interaction betweenfinalizeand non-local returns and exceptions, are all platform dependent. - def formatted(fmtstr: String): String
Returns string formatted according to given
formatstring.Returns string formatted according to given
formatstring. Format strings are as forString.format(@see java.lang.String.format).- Implicit
- This member is added by an implicit conversion from <:<[From, To] toStringFormat[<:<[From, To]] performed by method StringFormat in scala.Predef.
- Definition Classes
- StringFormat
- Annotations
- @inline()
- final def getClass(): Class[_ <: AnyRef]
Returns the runtime class representation of the object.
- def hashCode(): Int
The hashCode method for reference types.
- final def isInstanceOf[T0]: Boolean
Test whether the dynamic type of the receiver object is
T0.Test whether the dynamic type of the receiver object is
T0.Note that the result of the test is modulo Scala's erasure semantics. Therefore the expression
1.isInstanceOf[String]will returnfalse, while the expressionList(1).isInstanceOf[List[String]]will returntrue. In the latter example, because the type argument is erased as part of compilation it is not possible to check whether the contents of the list are of the specified type.- returns
trueif the receiver object is an instance of erasure of typeT0;falseotherwise.
- Definition Classes
- Any
- def liftCo[F[+_]]: <:<[F[From], F[To]]
Lift this evidence over a covariant type constructor
F. - def liftContra[F[-_]]: <:<[F[To], F[From]]
Lift this evidence over a contravariant type constructor
F. - final def ne(arg0: AnyRef): Boolean
Equivalent to
!(this eq that).Equivalent to
!(this eq that).- returns
trueif the argument is not a reference to the receiver object;falseotherwise.
- Definition Classes
- AnyRef
- final def notify(): Unit
Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.
- Definition Classes
- AnyRef
- Annotations
- @native()
- Note
not specified by SLS as a member of AnyRef
- final def notifyAll(): Unit
Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.
- Definition Classes
- AnyRef
- Annotations
- @native()
- Note
not specified by SLS as a member of AnyRef
- def substituteCo[F[+_]](ff: F[From]): F[To]
Substitute the
Fromin the typeF[From], whereFis a covariant type constructor, forTo.Substitute the
Fromin the typeF[From], whereFis a covariant type constructor, forTo.Equivalent in power to each of substituteBoth and substituteContra.
This method is impossible to implement without
throwing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.- returns
ff, but with a (potentially) different type
- def substituteContra[F[-_]](ft: F[To]): F[From]
Substitute the
Toin the typeF[To], whereFis a contravariant type constructor, forFrom.Substitute the
Toin the typeF[To], whereFis a contravariant type constructor, forFrom.Equivalent in power to each of substituteBoth and substituteCo.
This method is impossible to implement without
throwing or otherwise "cheating" unlessFrom <: To, so it ensures that this really represents a subtyping relationship.- returns
ft, but with a (potentially) different type
- final def synchronized[T0](arg0: => T0): T0
- Definition Classes
- AnyRef
- def toString(): String
Creates a String representation of this object.
- def unlift: PartialFunction[From, B]
Converts an optional function to a partial function.
Converts an optional function to a partial function.
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toUnliftOps[From, B] performed by method UnliftOps in scala.Function1.This conversion will take place only if To is a subclass of Option[B] (To <: Option[B]).
- Definition Classes
- UnliftOps
Unlike Function.unlift, this UnliftOps.unlift method can be used in extractors.
val of: Int => Option[String] = { i => if (i == 2) { Some("matched by an optional function") } else { None } } util.Random.nextInt(4) match { case of.unlift(m) => // Convert an optional function to a pattern println(m) case _ => println("Not matched") }
Example: - final def wait(): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws(classOf[java.lang.InterruptedException])
- final def wait(arg0: Long, arg1: Int): Unit
- Definition Classes
- AnyRef
- Annotations
- @throws(classOf[java.lang.InterruptedException])
- final def wait(arg0: Long): Unit
Deprecated Value Members
- def →[B](y: B): (<:<[From, To], B)
- Implicit
- This member is added by an implicit conversion from <:<[From, To] toArrowAssoc[<:<[From, To]] performed by method ArrowAssoc in scala.Predef.This conversion will take place only if To is a subclass of Option[Nothing] (To <: Option[Nothing]).
- Definition Classes
- ArrowAssoc
- Annotations
- @deprecated
- Deprecated
(Since version 2.13.0) Use
->instead. If you still wish to display it as one character, consider using a font with programming ligatures such as Fira Code.
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int,Float,ArrayorOptionwhich are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collectionand its sub-packages contain Scala's collections frameworkscala.collection.immutable- Immutable, sequential data-structures such asVector,List,Range,HashMaporHashSetscala.collection.mutable- Mutable, sequential data-structures such asArrayBuffer,StringBuilder,HashMaporHashSetscala.collection.concurrent- Mutable, concurrent data-structures such asTrieMapscala.concurrent- Primitives for concurrent programming such asFuturesandPromisesscala.io- Input and output operationsscala.math- Basic math functions and additional numeric types likeBigIntandBigDecimalscala.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.Predefobject are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
Listis an alias forscala.collection.immutable.List.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
Stringis an alias forjava.lang.String.