Packages

  • package root

    This is the documentation for the Scala standard library.

    This is the documentation for the Scala standard library.

    Package structure

    The scala package contains core types like Int, Float, Array or Option which are accessible in all Scala compilation units without explicit qualification or imports.

    Notable packages include:

    Other 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 for scala.collection.immutable.List.

    Other aliases refer to classes provided by the underlying platform. For example, on the JVM, String is an alias for java.lang.String.

    Definition Classes
    root
  • package scala

    Core Scala types.

    Core Scala types. They are always available without an explicit import.

    Definition Classes
    root
  • object Predef extends LowPriorityImplicits

    The Predef object provides definitions that are accessible in all Scala compilation units without explicit qualification.

    The Predef object provides definitions that are accessible in all Scala compilation units without explicit qualification.

    Commonly Used Types

    Predef provides type aliases for types which are commonly used, such as the immutable collection types scala.collection.immutable.Map, scala.collection.immutable.Set, and the scala.collection.immutable.List constructors (scala.collection.immutable.:: and scala.collection.immutable.Nil).

    Console Output

    For basic console output, Predef provides convenience methods print and println, which are aliases of the methods in the object scala.Console.

    Assertions

    A set of assert functions are provided for use as a way to document and dynamically check invariants in code. Invocations of assert can be elided at compile time by providing the command line option -Xdisable-assertions, which raises -Xelide-below above elidable.ASSERTION, to the scalac command.

    Variants of assert intended for use with static analysis tools are also provided: assume, require and ensuring. require and ensuring are intended for use as a means of design-by-contract style specification of pre- and post-conditions on functions, with the intention that these specifications could be consumed by a static analysis tool. For instance,

    def addNaturals(nats: List[Int]): Int = {
  require(nats forall (_ >= 0), "List contains negative numbers")
  nats.foldLeft(0)(_ + _)
} ensuring(_ >= 0)

    The declaration of addNaturals states that the list of integers passed should only contain natural numbers (i.e. non-negative), and that the result returned will also be natural. require is distinct from assert in that if the condition fails, then the caller of the function is to blame rather than a logical error having been made within addNaturals itself. ensuring is a form of assert that declares the guarantee the function is providing with regards to its return value.

    Implicit Conversions

    A number of commonly applied implicit conversions are also defined here, and in the parent type scala.LowPriorityImplicits. Implicit conversions are provided for the "widening" of numeric values, for instance, converting a Short value to a Long value as required, and to add additional higher-order functions to Array values. These are described in more detail in the documentation of scala.Array.

    Definition Classes
    scala
  • <:<
  • =:=
  • ArrayCharSequence
  • ArrowAssoc
  • DummyImplicit
  • Ensuring
  • SeqCharSequence
  • StringFormat
  • any2stringadd

sealed abstract class <:<[-From, +To] extends (From) ⇒ To with scala.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
Predef.scala
Example:

  1. scala.Option#flatten

    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

Linear Supertypes
java.io.Serializable, (From) ⇒ To, AnyRef, Any
Known Subclasses
=:=
Type Hierarchy
Ordering
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Inherited
  1. <:<
  2. Serializable
  3. Function1
  4. AnyRef
  5. Any
Implicitly
  1. by any2stringadd
  2. by StringFormat
  3. by Ensuring
  4. by ArrowAssoc
  5. by any2stringadd
  6. by StringFormat
  7. by Ensuring
  8. by ArrowAssoc
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Visibility
  1. Public
  2. All

Abstract Value Members

  1. abstract def substituteBoth[F[-_, +_]](ftf: F[To, From]): F[From, To]

    Substitute To for From and From for To in the type F[To, From], given that F is contravariant in the first argument and covariant in the second.

    Substitute To for From and From for To in the type F[To, From], given that F is contravariant in the first argument and covariant in the second. Essentially swaps To and From in ftf's type.

    Equivalent in power to each of substituteCo and substituteContra.

    This method is impossible to implement without throwing or otherwise "cheating" unless From <: To, so it ensures that this really represents a subtyping relationship.

    returns

    ftf, but with a (potentially) different type

Concrete Value Members

  1. final def !=(arg0: Any): Boolean

    Test two objects for inequality.

    Test two objects for inequality.

    returns

    true if !(this == that), false otherwise.

    Definition Classes
    AnyRef → Any
  2. final def ##(): Int

    Equivalent to x.hashCode except for boxed numeric types and null.

    Equivalent to x.hashCode except for boxed numeric types and null. 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. For null returns a hashcode where null.hashCode throws a NullPointerException.

    returns

    a hash value consistent with ==

    Definition Classes
    AnyRef → Any
  3. final def ==(arg0: Any): Boolean

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  4. def andThen[C](r: <:<[To, C]): <:<[From, C]

    If From <: To and To <: C, then From <: C (subtyping is transitive)

  5. def andThen[C](r: (To) ⇒ C): (From) ⇒ C

    Composes two instances of Function1 in a new Function1, with this function applied first.

    Composes two instances of Function1 in a new Function1, with this function applied first.

    returns

    a new function f such that f(x) == g(apply(x))

    Definition Classes
    <:< → Function1
  6. def apply(f: From): To

    Coerce a From into a To.

    Coerce a From into a To. This is guaranteed to be the identity function.

    This method is often called implicitly as an implicit A <:< B doubles as an implicit view A => B.

    f

    some value of type From

    returns

    f, but with a (potentially) different type

    Definition Classes
    <:< → Function1
  7. 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 a ClassCastException at runtime, while the expression List(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

    ClassCastException if the receiver object is not an instance of the erasure of type T0.

  8. def clone(): AnyRef

    Create a copy of the receiver object.

    Create a copy of the receiver object.

    The default implementation of the clone method is platform dependent.

    returns

    a copy of the receiver object.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @native() @throws(classOf[java.lang.CloneNotSupportedException])
    Note

    not specified by SLS as a member of AnyRef

  9. def compose[C](r: <:<[C, From]): <:<[C, To]

    If From <: To and C <: From, then C <: To (subtyping is transitive)

  10. def compose[C](r: (C) ⇒ From): (C) ⇒ To

    Composes two instances of Function1 in a new Function1, with this function applied last.

    Composes two instances of Function1 in a new Function1, with this function applied last.

    returns

    a new function f such that f(x) == apply(g(x))

    Definition Classes
    <:< → Function1
  11. 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 eq method implements an equivalence relation on non-null instances of AnyRef, and has three additional properties:

    • It is consistent: for any non-null instances x and y of type AnyRef, multiple invocations of x.eq(y) consistently returns true or consistently returns false.
    • For any non-null instance x of type AnyRef, x.eq(null) and null.eq(x) returns false.
    • null.eq(null) returns true.

    When overriding the equals or hashCode methods, 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

    true if the argument is a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  12. def equals(arg0: Any): Boolean

    The equality method for reference types.

    The equality method for reference types. Default implementation delegates to eq.

    See also equals in scala.Any.

    returns

    true if the receiver object is equivalent to the argument; false otherwise.

    Definition Classes
    AnyRef → Any
  13. 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 finalize method is invoked, as well as the interaction between finalize and non-local returns and exceptions, are all platform dependent.

    Attributes
    protected[java.lang]
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.Throwable])
    Note

    not specified by SLS as a member of AnyRef

  14. final def getClass(): java.lang.Class[_]

    Returns the runtime class representation of the object.

    Returns the runtime class representation of the object.

    returns

    a class object corresponding to the runtime type of the receiver.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  15. def hashCode(): Int

    The hashCode method for reference types.

    The hashCode method for reference types. See hashCode in scala.Any.

    returns

    the hash code value for this object.

    Definition Classes
    AnyRef → Any
    Annotations
    @native()
  16. 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 return false, while the expression List(1).isInstanceOf[List[String]] will return true. 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

    true if the receiver object is an instance of erasure of type T0; false otherwise.

    Definition Classes
    Any
  17. def liftCo[F[+_]]: <:<[F[From], F[To]]

    Lift this evidence over a covariant type constructor F.

  18. def liftContra[F[-_]]: <:<[F[To], F[From]]

    Lift this evidence over a contravariant type constructor F.

  19. final def ne(arg0: AnyRef): Boolean

    Equivalent to !(this eq that).

    Equivalent to !(this eq that).

    returns

    true if the argument is not a reference to the receiver object; false otherwise.

    Definition Classes
    AnyRef
  20. 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

  21. 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

  22. def substituteCo[F[+_]](ff: F[From]): F[To]

    Substitute the From in the type F[From], where F is a covariant type constructor, for To.

    Substitute the From in the type F[From], where F is a covariant type constructor, for To.

    Equivalent in power to each of substituteBoth and substituteContra.

    This method is impossible to implement without throwing or otherwise "cheating" unless From <: To, so it ensures that this really represents a subtyping relationship.

    returns

    ff, but with a (potentially) different type

  23. def substituteContra[F[-_]](ft: F[To]): F[From]

    Substitute the To in the type F[To], where F is a contravariant type constructor, for From.

    Substitute the To in the type F[To], where F is a contravariant type constructor, for From.

    Equivalent in power to each of substituteBoth and substituteCo.

    This method is impossible to implement without throwing or otherwise "cheating" unless From <: To, so it ensures that this really represents a subtyping relationship.

    returns

    ft, but with a (potentially) different type

  24. final def synchronized[T0](arg0: ⇒ T0): T0
    Definition Classes
    AnyRef
  25. def toString(): java.lang.String

    Creates a String representation of this object.

    Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.

    returns

    a String representation of the object.

    Definition Classes
    Function1 → AnyRef → Any
  26. final def wait(): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
  27. final def wait(arg0: Long, arg1: Int): Unit
    Definition Classes
    AnyRef
    Annotations
    @throws(classOf[java.lang.InterruptedException])
  28. final def wait(arg0: Long): Unit
    Definition Classes
    AnyRef
    Annotations
    @native() @throws(classOf[java.lang.InterruptedException])

Shadowed Implicit Value Members

  1. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: any2stringadd[<:<[From, To]]).+(other)
    Definition Classes
    any2stringadd
  2. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: any2stringadd[<:<[From, To]]).+(other)
    Definition Classes
    any2stringadd
  3. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: ArrowAssoc[<:<[From, To]]).->(y)
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  4. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: ArrowAssoc[<:<[From, To]]).->(y)
    Definition Classes
    ArrowAssoc
    Annotations
    @inline()
  5. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond, msg)
    Definition Classes
    Ensuring
  6. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond)
    Definition Classes
    Ensuring
  7. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond, msg)
    Definition Classes
    Ensuring
  8. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond)
    Definition Classes
    Ensuring
  9. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond, msg)
    Definition Classes
    Ensuring
  10. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond)
    Definition Classes
    Ensuring
  11. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond, msg)
    Definition Classes
    Ensuring
  12. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: Ensuring[<:<[From, To]]).ensuring(cond)
    Definition Classes
    Ensuring
  13. def formatted(fmtstr: String): String

    Returns string formatted according to given format string.

    Returns string formatted according to given format string. Format strings are as for String.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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: StringFormat[<:<[From, To]]).formatted(fmtstr)
    Definition Classes
    StringFormat
    Annotations
    @inline()
  14. def formatted(fmtstr: String): String

    Returns string formatted according to given format string.

    Returns string formatted according to given format string. Format strings are as for String.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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: StringFormat[<:<[From, To]]).formatted(fmtstr)
    Definition Classes
    StringFormat
    Annotations
    @inline()
  15. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: ArrowAssoc[<:<[From, To]]).→(y)
    Definition Classes
    ArrowAssoc
  16. 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.
    Shadowing
    This implicitly inherited member is ambiguous. One or more implicitly inherited members have similar signatures, so calling this member may produce an ambiguous implicit conversion compiler error.
    To access this member you can use a type ascription:
    (<:<: ArrowAssoc[<:<[From, To]]).→(y)
    Definition Classes
    ArrowAssoc

Inherited from java.io.Serializable

Inherited from (From) ⇒ To

Inherited from AnyRef

Inherited from Any

Inherited by implicit conversion any2stringadd from<:<[From, To] to any2stringadd[<:<[From, To]]

Inherited by implicit conversion StringFormat from<:<[From, To] to StringFormat[<:<[From, To]]

Inherited by implicit conversion Ensuring from<:<[From, To] to Ensuring[<:<[From, To]]

Inherited by implicit conversion ArrowAssoc from<:<[From, To] to ArrowAssoc[<:<[From, To]]

Inherited by implicit conversion any2stringadd from<:<[From, To] to any2stringadd[<:<[From, To]]

Inherited by implicit conversion StringFormat from<:<[From, To] to StringFormat[<:<[From, To]]

Inherited by implicit conversion Ensuring from<:<[From, To] to Ensuring[<:<[From, To]]

Inherited by implicit conversion ArrowAssoc from<:<[From, To] to ArrowAssoc[<:<[From, To]]

Ungrouped