Package scala.scala

package scala

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

Classlikes

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object #::

Source@showAsInfix

sealed abstract class *:[+H, +T <: Tuple] extends NonEmptyTuple

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object *:

Source@implicitNotFound(msg = "Cannot prove that ${From} <:>

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.

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.
}

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object <:<

Source@implicitNotFound(msg = "Cannot prove that ${From} =:= ${To}.")

sealed abstract class =:=[From, To] extends From <:< To with Serializable

An instance of A =:= B witnesses that the types A and B are equal. It also acts as a A <:< B, but not a B <:< A (directly) due to restrictions on subclassing.

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.

implicit class BufOps[A](private val buf: ArrayBuffer[A]) extends AnyVal {
  def inPlaceTranspose[E]()(implicit ev: A =:= ArrayBuffer[E]) = ???
  // Because ArrayBuffer is invariant, we can't make do with just a A <:< ArrayBuffer[E]
  // Getting buffers *out* from buf would work, but adding them back *in* wouldn't.
}

abstract open class Any

Class Any is the root of the Scala class hierarchy. Every class in a Scala execution environment inherits directly or indirectly from this class.

Starting with Scala 2.10 it is possible to directly extend Any using universal traits. A universal trait is a trait that extends Any, only has defs as members, and does no initialization.

The main use case for universal traits is to allow basic inheritance of methods for value classes. For example,

trait Printable extends Any {
  def print(): Unit = println(this)
}
class Wrapper(val underlying: Int) extends AnyVal with Printable

val w = new Wrapper(3)
w.print()

See the Value Classes and Universal Traits for more details on the interplay of universal traits and value classes.

final abstract class AnyKind

The super-type of all types.

See https://dotty.epfl.ch/docs/reference/other-new-features/kind-polymorphism.html.

class AnyRef

Class AnyRef is the root class of all reference types. All types except the value types descend from this class.

abstract open class AnyVal

AnyVal is the root class of all value types, which describe values not implemented as objects in the underlying host system. Value classes are specified in Scala Language Specification, section 12.2.

The standard implementation includes nine AnyVal subtypes:

scala.Double, scala.Float, scala.Long, scala.Int, scala.Char, scala.Short, and scala.Byte are the numeric value types.

scala.Unit and scala.Boolean are the non-numeric value types.

Other groupings:

• The subrange types are scala.Byte, scala.Short, and scala.Char.

• The integer types include the subrange types as well as scala.Int and scala.Long.

• The floating point types are scala.Float and scala.Double.

Prior to Scala 2.10, AnyVal was a sealed trait. Beginning with Scala 2.10, however, it is possible to define a subclass of AnyVal called a user-defined value class which is treated specially by the compiler. Properly-defined user value classes provide a way to improve performance on user-defined types by avoiding object allocation at runtime, and by replacing virtual method invocations with static method invocations.

User-defined value classes which avoid object allocation...

• must have a single val parameter that is the underlying runtime representation.

• can define defs, but no vals, vars, or nested traitss, classes or objects.

• typically extend no other trait apart from AnyVal.

• cannot be used in type tests or pattern matching.

• may not override equals or hashCode methods.

A minimal example:

class Wrapper(val underlying: Int) extends AnyVal {
  def foo: Wrapper = new Wrapper(underlying * 19)
}

It's important to note that user-defined value classes are limited, and in some circumstances, still must allocate a value class instance at runtime. These limitations and circumstances are explained in greater detail in the Value Classes and Universal Traits.

Source@nowarn("cat=deprecation&origin=scala\\.DelayedInit")

trait App extends DelayedInit

The App trait can be used to quickly turn objects into executable programs. Here is an example:

object Main extends App {
  Console.println("Hello World: " + (args mkString ", "))
}

No explicit main method is needed. Instead, the whole class body becomes the “main method”.

args returns the current command line arguments as an array.

Caveats

It should be noted that this trait is implemented using the DelayedInit functionality, which means that fields of the object will not have been initialized before the main method has been executed.

Future versions of this trait will no longer extend DelayedInit.

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object Array

Utility methods for operating on arrays. For example:

val a = Array(1, 2)
val b = Array.ofDim[Int](2)
val c = Array.concat(a, b)

where the array objects a, b and c have respectively the values Array(1, 2), Array(0, 0) and Array(1, 2, 0, 0).

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final class Array[T](_length: Int) extends Serializable with Cloneable

Arrays are mutable, indexed collections of values. Array[T] is Scala's representation for Java's T[].

val numbers = Array(1, 2, 3, 4)
val first = numbers(0) // read the first element
numbers(3) = 100 // replace the 4th array element with 100
val biggerNumbers = numbers.map(_ * 2) // multiply all numbers by two

Arrays make use of two common pieces of Scala syntactic sugar, shown on lines 2 and 3 of the above example code. Line 2 is translated into a call to apply(Int), while line 3 is translated into a call to update(Int, T).

Two implicit conversions exist in scala.Predef that are frequently applied to arrays: a conversion to scala.collection.ArrayOps (shown on line 4 of the example above) and a conversion to scala.collection.mutable.ArraySeq (a subtype of scala.collection.Seq). Both types make available many of the standard operations found in the Scala collections API. The conversion to ArrayOps is temporary, as all operations defined on ArrayOps return an Array, while the conversion to ArraySeq is permanent as all operations return a ArraySeq.

The conversion to ArrayOps takes priority over the conversion to ArraySeq. For instance, consider the following code:

val arr = Array(1, 2, 3)
val arrReversed = arr.reverse
val seqReversed : collection.Seq[Int] = arr.reverse

Value arrReversed will be of type Array[Int], with an implicit conversion to ArrayOps occurring to perform the reverse operation. The value of seqReversed, on the other hand, will be computed by converting to ArraySeq first and invoking the variant of reverse that returns another ArraySeq.

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final abstract class Boolean extends AnyVal

Boolean (equivalent to Java's boolean primitive type) is a subtype of scala.AnyVal. Instances of Boolean are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Boolean => scala.runtime.RichBoolean which provides useful non-primitive operations.

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object Boolean

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final abstract class Byte extends AnyVal

Byte, a 8-bit signed integer (equivalent to Java's byte primitive type) is a subtype of scala.AnyVal. Instances of Byte are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Byte => scala.runtime.RichByte which provides useful non-primitive operations.

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object Byte

Source@implicitNotFound("Values of types ${L} and ${R} cannot be compared with == or !=")

sealed trait CanEqual[-L, -R]

A marker trait indicating that values of type L can be compared to values of type R.

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object CanEqual

Companion object containing a few universally known CanEqual instances. CanEqual instances involving primitive types or the Null type are handled directly in the compiler (see Implicits.synthesizedCanEqual), so they are not included here.

Source@experimental @implicitNotFound("The capability to throw exception ${E} is missing.\nThe capability can be provided by one of the following:\n - Adding a using clause `(using CanThrow[${E}])` to the definition of the enclosing method\n - Adding `throws ${E}` clause after the result type of the enclosing method\n - Wrapping this piece of code with a `try` block that catches ${E}")

erased class CanThrow[-E <: Exception]

A capability class that allows to throw exception E. When used with the experimental.saferExceptions feature, a throw Ex() expression will require a given of class CanThrow[Ex] to be available.

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final abstract class Char extends AnyVal

Char, a 16-bit unsigned integer (equivalent to Java's char primitive type) is a subtype of scala.AnyVal. Instances of Char are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Char => scala.runtime.RichChar which provides useful non-primitive operations.

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object Char

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object Console extends AnsiColor

Implements functionality for printing Scala values on the terminal. For reading values use StdIn. Also defines constants for marking up text on ANSI terminals.

Console Output

Use the print methods to output text.

scala> Console.printf(
  "Today the outside temperature is a balmy %.1f°C. %<.1f°C beats the previous record of %.1f°C.\n",
  -137.0,
  -135.05)
Today the outside temperature is a balmy -137.0°C. -137.0°C beats the previous record of -135.1°C.

ANSI escape codes

Use the ANSI escape codes for colorizing console output either to STDOUT or STDERR.

import Console.{GREEN, RED, RESET, YELLOW_B, UNDERLINED}

object PrimeTest {

  def isPrime(): Unit = {

    val candidate = io.StdIn.readInt().ensuring(_ > 1)

    val prime = (2 to candidate - 1).forall(candidate % _ != 0)

    if (prime)
      Console.println(s"${RESET}${GREEN}yes${RESET}")
    else
      Console.err.println(s"${RESET}${YELLOW_B}${RED}${UNDERLINED}NO!${RESET}")
  }

  def main(args: Array[String]): Unit = isPrime()

}

IO redefinition

Use IO redefinition to temporarily swap in a different set of input and/or output streams. In this example the stream based method above is wrapped into a function.

import java.io.{ByteArrayOutputStream, StringReader}

object FunctionalPrimeTest {

  def isPrime(candidate: Int): Boolean = {

    val input = new StringReader(s"$candidate\n")
    val outCapture = new ByteArrayOutputStream
    val errCapture = new ByteArrayOutputStream

    Console.withIn(input) {
      Console.withOut(outCapture) {
        Console.withErr(errCapture) {
          PrimeTest.isPrime()
        }
      }
    }

    if (outCapture.toByteArray.nonEmpty) // "yes"
      true
    else if (errCapture.toByteArray.nonEmpty) // "NO!"
      false
    else throw new IllegalArgumentException(candidate.toString)
  }

  def main(args: Array[String]): Unit = {
    val primes = (2 to 50) filter (isPrime)
    println(s"First primes: $primes")
  }

}

Source@FunctionalInterface

abstract class Conversion[-T, +U] extends T => U

A class for implicit values that can serve as implicit conversions The implicit resolution algorithm will act as if there existed the additional implicit definition:

def $implicitConversion[T, U](x: T)(c: Conversion[T, U]): U = c(x)

However, the presence of this definition would slow down implicit search since its outermost type matches any pair of types. Therefore, implicit search contains a special case in Implicits#discardForView which emulates the conversion in a more efficient way.

Note that this is a SAM class - function literals are automatically converted to the Conversion values.

Also note that in bootstrapped dotty, Predef.<:< should inherit from Conversion. This would cut the number of special cases in discardForView from two to one.

The Conversion class can also be used to convert explicitly, using the convert extension method.

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final abstract class Double extends AnyVal

Double, a 64-bit IEEE-754 floating point number (equivalent to Java's double primitive type) is a subtype of scala.AnyVal. Instances of Double are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Double => scala.runtime.RichDouble which provides useful non-primitive operations.

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object Double

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final class DummyImplicit

A type for which there is always an implicit value.

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object DummyImplicit

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trait Dynamic

A marker trait that enables dynamic invocations. Instances x of this trait allow method invocations x.meth(args) for arbitrary method names meth and argument lists args as well as field accesses x.field for arbitrary field names field.

If a call is not natively supported by x (i.e. if type checking fails), it is rewritten according to the following rules:

foo.method("blah")      ~~> foo.applyDynamic("method")("blah")
foo.method(x = "blah")  ~~> foo.applyDynamicNamed("method")(("x", "blah"))
foo.method(x = 1, 2)    ~~> foo.applyDynamicNamed("method")(("x", 1), ("", 2))
foo.field           ~~> foo.selectDynamic("field")
foo.varia = 10      ~~> foo.updateDynamic("varia")(10)
foo.arr(10) = 13    ~~> foo.selectDynamic("arr").update(10, 13)
foo.arr(10)         ~~> foo.applyDynamic("arr")(10)

As of Scala 2.10, defining direct or indirect subclasses of this trait is only possible if the language feature dynamics is enabled.

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object EmptyTuple extends Tuple

A tuple of 0 elements.

Source@SerialVersionUID(8476000850333817230L)

abstract class Enumeration(initial: Int) extends Serializable

Defines a finite set of values specific to the enumeration. Typically these values enumerate all possible forms something can take and provide a lightweight alternative to case classes.

Each call to a Value method adds a new unique value to the enumeration. To be accessible, these values are usually defined as val members of the enumeration.

All values in an enumeration share a common, unique type defined as the Value type member of the enumeration (Value selected on the stable identifier path of the enumeration instance).

Values SHOULD NOT be added to an enumeration after its construction; doing so makes the enumeration thread-unsafe. If values are added to an enumeration from multiple threads (in a non-synchronized fashion) after construction, the behavior of the enumeration is undefined.

// Define a new enumeration with a type alias and work with the full set of enumerated values
object WeekDay extends Enumeration {
 type WeekDay = Value
 val Mon, Tue, Wed, Thu, Fri, Sat, Sun = Value
}
import WeekDay._
def isWorkingDay(d: WeekDay) = ! (d == Sat || d == Sun)
WeekDay.values filter isWorkingDay foreach println
// output:
// Mon
// Tue
// Wed
// Thu
// Fri

// Example of adding attributes to an enumeration by extending the Enumeration.Val class
object Planet extends Enumeration {
 protected case class PlanetVal(mass: Double, radius: Double) extends super.Val {
   def surfaceGravity: Double = Planet.G * mass / (radius * radius)
   def surfaceWeight(otherMass: Double): Double = otherMass * surfaceGravity
 }
 import scala.language.implicitConversions
 implicit def valueToPlanetVal(x: Value): PlanetVal = x.asInstanceOf[PlanetVal]
 val G: Double = 6.67300E-11
 val Mercury = PlanetVal(3.303e+23, 2.4397e6)
 val Venus   = PlanetVal(4.869e+24, 6.0518e6)
 val Earth   = PlanetVal(5.976e+24, 6.37814e6)
 val Mars    = PlanetVal(6.421e+23, 3.3972e6)
 val Jupiter = PlanetVal(1.9e+27, 7.1492e7)
 val Saturn  = PlanetVal(5.688e+26, 6.0268e7)
 val Uranus  = PlanetVal(8.686e+25, 2.5559e7)
 val Neptune = PlanetVal(1.024e+26, 2.4746e7)
}
println(Planet.values.filter(_.radius > 7.0e6))
// output:
// Planet.ValueSet(Jupiter, Saturn, Uranus, Neptune)

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trait Equals

An interface containing operations for equality. The only method not already present in class AnyRef is canEqual.

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final abstract class Float extends AnyVal

Float, a 32-bit IEEE-754 floating point number (equivalent to Java's float primitive type) is a subtype of scala.AnyVal. Instances of Float are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Float => scala.runtime.RichFloat which provides useful non-primitive operations.

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object Float

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object Function

A module defining utility methods for higher-order functional programming.

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trait Function0[@specialized +R] extends AnyRef

A function of 0 parameters.

In the following example, the definition of javaVersion is a shorthand for the anonymous class definition anonfun0:

object Main extends App {
  val javaVersion = () => sys.props("java.version")

  val anonfun0 = new Function0[String] {
    def apply(): String = sys.props("java.version")
  }
  assert(javaVersion() == anonfun0())
}

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object Function1

Source@implicitNotFound(msg = "No implicit view available from ${T1} => ${R}.")

trait Function1[@specialized -T1, @specialized +R] extends AnyRef

A function of 1 parameter.

In the following example, the definition of succ is a shorthand for the anonymous class definition anonfun1:

object Main extends App {
  val succ = (x: Int) => x + 1
  val anonfun1 = new Function1[Int, Int] {
    def apply(x: Int): Int = x + 1
  }
  assert(succ(0) == anonfun1(0))
}

Note that the difference between Function1 and scala.PartialFunction is that the latter can specify inputs which it will not handle.

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trait Function10[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, +R] extends AnyRef

A function of 10 parameters.

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trait Function11[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, +R] extends AnyRef

A function of 11 parameters.

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trait Function12[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, +R] extends AnyRef

A function of 12 parameters.

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trait Function13[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, +R] extends AnyRef

A function of 13 parameters.

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trait Function14[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, +R] extends AnyRef

A function of 14 parameters.

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trait Function15[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, +R] extends AnyRef

A function of 15 parameters.

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trait Function16[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, +R] extends AnyRef

A function of 16 parameters.

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trait Function17[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, +R] extends AnyRef

A function of 17 parameters.

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trait Function18[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, +R] extends AnyRef

A function of 18 parameters.

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trait Function19[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, -T19, +R] extends AnyRef

A function of 19 parameters.

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trait Function2[@specialized -T1, @specialized -T2, @specialized +R] extends AnyRef

A function of 2 parameters.

In the following example, the definition of max is a shorthand for the anonymous class definition anonfun2:

object Main extends App {
  val max = (x: Int, y: Int) => if (x < y) y else x

  val anonfun2 = new Function2[Int, Int, Int] {
    def apply(x: Int, y: Int): Int = if (x < y) y else x
  }
  assert(max(0, 1) == anonfun2(0, 1))
}

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trait Function20[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, -T19, -T20, +R] extends AnyRef

A function of 20 parameters.

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trait Function21[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, -T19, -T20, -T21, +R] extends AnyRef

A function of 21 parameters.

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trait Function22[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, -T10, -T11, -T12, -T13, -T14, -T15, -T16, -T17, -T18, -T19, -T20, -T21, -T22, +R] extends AnyRef

A function of 22 parameters.

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trait Function3[-T1, -T2, -T3, +R] extends AnyRef

A function of 3 parameters.

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trait Function4[-T1, -T2, -T3, -T4, +R] extends AnyRef

A function of 4 parameters.

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trait Function5[-T1, -T2, -T3, -T4, -T5, +R] extends AnyRef

A function of 5 parameters.

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trait Function6[-T1, -T2, -T3, -T4, -T5, -T6, +R] extends AnyRef

A function of 6 parameters.

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trait Function7[-T1, -T2, -T3, -T4, -T5, -T6, -T7, +R] extends AnyRef

A function of 7 parameters.

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trait Function8[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, +R] extends AnyRef

A function of 8 parameters.

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trait Function9[-T1, -T2, -T3, -T4, -T5, -T6, -T7, -T8, -T9, +R] extends AnyRef

A function of 9 parameters.

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object IArray

An immutable array. An IArray[T] has the same representation as an Array[T], but it cannot be updated. Unlike regular arrays, immutable arrays are covariant.

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final abstract class Int extends AnyVal

Int, a 32-bit signed integer (equivalent to Java's int primitive type) is a subtype of scala.AnyVal. Instances of Int are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Int => scala.runtime.RichInt which provides useful non-primitive operations.

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object Int

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final abstract class Long extends AnyVal

Long, a 64-bit signed integer (equivalent to Java's long primitive type) is a subtype of scala.AnyVal. Instances of Long are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Long => scala.runtime.RichLong which provides useful non-primitive operations.

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object Long

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final class MatchError(obj: Any) extends RuntimeException

This class implements errors which are thrown whenever an object doesn't match any pattern of a pattern matching expression.

open trait Matchable

The base trait of types that can be safely pattern matched against.

See https://dotty.epfl.ch/docs/reference/other-new-features/matchable.html.

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sealed trait NonEmptyTuple extends Tuple

Tuple of arbitrary non-zero arity

Source@SerialVersionUID(5066590221178148012L)

case object None extends Option[Nothing]

This case object represents non-existent values.

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final class NotImplementedError(msg: String) extends Error

Throwing this exception can be a temporary replacement for a method body that remains to be implemented. For instance, the exception is thrown by Predef.???.

final abstract open class Nothing

Nothing is - together with scala.Null - at the bottom of Scala's type hierarchy.

Nothing is a subtype of every other type (including scala.Null); there exist no instances of this type. Although type Nothing is uninhabited, it is nevertheless useful in several ways. For instance, the Scala library defines a value scala.collection.immutable.Nil of type List[Nothing]. Because lists are covariant in Scala, this makes scala.collection.immutable.Nil an instance of List[T], for any element of type T.

Another usage for Nothing is the return type for methods which never return normally. One example is method error in scala.sys, which always throws an exception.

final abstract open class Null

Null is - together with scala.Nothing - at the bottom of the Scala type hierarchy.

Null is the type of the null literal. It is a subtype of every type except those of value classes. Value classes are subclasses of AnyVal, which includes primitive types such as Int, Boolean, and user-defined value classes.

Since Null is not a subtype of value types, null is not a member of any such type. For instance, it is not possible to assign null to a variable of type scala.Int.

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object Option

Source@SerialVersionUID(-114498752079829388L)

sealed abstract class Option[+A] extends IterableOnce[A] with Product with Serializable

Represents optional values. Instances of Option are either an instance of scala.Some or the object None.

The most idiomatic way to use an scala.Option instance is to treat it as a collection or monad and use map,flatMap, filter, or foreach:

val name: Option[String] = request getParameter "name"
val upper = name map { _.trim } filter { _.length != 0 } map { _.toUpperCase }
println(upper getOrElse "")

Note that this is equivalent to

val upper = for {
  name <- request getParameter "name"
  trimmed <- Some(name.trim)
  upper <- Some(trimmed.toUpperCase) if trimmed.length != 0
} yield upper
println(upper getOrElse "")

Because of how for comprehension works, if None is returned from request.getParameter, the entire expression results in None

This allows for sophisticated chaining of scala.Option values without having to check for the existence of a value.

These are useful methods that exist for both scala.Some and None. - isDefined — True if not empty - isEmpty — True if empty - nonEmpty — True if not empty - orElse — Evaluate and return alternate optional value if empty - getOrElse — Evaluate and return alternate value if empty - get — Return value, throw exception if empty - fold — Apply function on optional value, return default if empty - map — Apply a function on the optional value - flatMap — Same as map but function must return an optional value - foreach — Apply a procedure on option value - collect — Apply partial pattern match on optional value - filter — An optional value satisfies predicate - filterNot — An optional value doesn't satisfy predicate - exists — Apply predicate on optional value, or false if empty - forall — Apply predicate on optional value, or true if empty - contains — Checks if value equals optional value, or false if empty - zip — Combine two optional values to make a paired optional value - unzip — Split an optional pair to two optional values - unzip3 — Split an optional triple to three optional values - toList — Unary list of optional value, otherwise the empty list

A less-idiomatic way to use scala.Option values is via pattern matching:

val nameMaybe = request getParameter "name"
nameMaybe match {
  case Some(name) =>
    println(name.trim.toUppercase)
  case None =>
    println("No name value")
}

Interacting with code that can occasionally return null can be safely wrapped in scala.Option to become None and scala.Some otherwise.

val abc = new java.util.HashMap[Int, String]
abc.put(1, "A")
bMaybe = Option(abc.get(2))
bMaybe match {
 case Some(b) =>
   println(s"Found $b")
 case None =>
   println("Not found")
}

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trait PartialFunction[-A, +B] extends A => B

A partial function of type PartialFunction[A, B] is a unary function where the domain does not necessarily include all values of type A. The function isDefinedAt allows to test dynamically if a value is in the domain of the function.

Even if isDefinedAt returns true for an a: A, calling apply(a) may still throw an exception, so the following code is legal:

val f: PartialFunction[Int, Any] = { case x => x / 0 }   // ArithmeticException: / by zero

It is the responsibility of the caller to call isDefinedAt before calling apply, because if isDefinedAt is false, it is not guaranteed apply will throw an exception to indicate an error condition. If an exception is not thrown, evaluation may result in an arbitrary value.

The usual way to respect this contract is to call applyOrElse, which is expected to be more efficient than calling both isDefinedAt and apply.

The main distinction between PartialFunction and scala.Function1 is that the user of a PartialFunction may choose to do something different with input that is declared to be outside its domain. For example:

val sample = 1 to 10
def isEven(n: Int) = n % 2 == 0
val eveningNews: PartialFunction[Int, String] = {
  case x if isEven(x) => s"$x is even"
}

// The method collect is described as "filter + map"
// because it uses a PartialFunction to select elements
// to which the function is applied.
val evenNumbers = sample.collect(eveningNews)

val oddlyEnough: PartialFunction[Int, String] = {
  case x if !isEven(x) => s"$x is odd"
}

// The method orElse allows chaining another PartialFunction
// to handle input outside the declared domain.
val numbers = sample.map(eveningNews orElse oddlyEnough)

// same as
val numbers = sample.map(n => eveningNews.applyOrElse(n, oddlyEnough))

val half: PartialFunction[Int, Int] = {
  case x if isEven(x) => x / 2
}

// Calculating the domain of a composition can be expensive.
val oddByHalf = half.andThen(oddlyEnough)

// Invokes `half.apply` on even elements!
val oddBalls = sample.filter(oddByHalf.isDefinedAt)

// Better than filter(oddByHalf.isDefinedAt).map(oddByHalf)
val oddBalls = sample.collect(oddByHalf)

// Providing "default" values.
val oddsAndEnds = sample.map(n => oddByHalf.applyOrElse(n, (i: Int) => s"[$i]"))

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object PartialFunction

A few handy operations which leverage the extra bit of information available in partial functions. Examples:

import PartialFunction._

def strangeConditional(other: Any): Boolean = cond(other) {
  case x: String if x == "abc" || x == "def"  => true
  case x: Int => true
}
def onlyInt(v: Any): Option[Int] = condOpt(v) { case x: Int => x }

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trait PolyFunction

Marker trait for polymorphic function types.

This is the only trait that can be refined with a polymorphic method, as long as that method is called apply, e.g.: PolyFunction { def apply[T_1, ..., T_M](x_1: P_1, ..., x_N: P_N): R } This type will be erased to FunctionN.

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object Predef

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 and scala.collection.immutable.Set.

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.

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trait Product extends Equals

Base trait for all products, which in the standard library include at least scala.Product1 through scala.Product22 and therefore also their subclasses scala.Tuple1 through scala.Tuple22. In addition, all case classes implement Product with synthetically generated methods.

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object Product1

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trait Product1[@specialized(Int, Long, Double) +T1] extends Product

Product1 is a Cartesian product of 1 component.

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object Product10

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trait Product10[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10] extends Product

Product10 is a Cartesian product of 10 components.

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object Product11

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trait Product11[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11] extends Product

Product11 is a Cartesian product of 11 components.

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object Product12

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trait Product12[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12] extends Product

Product12 is a Cartesian product of 12 components.

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object Product13

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trait Product13[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13] extends Product

Product13 is a Cartesian product of 13 components.

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object Product14

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trait Product14[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14] extends Product

Product14 is a Cartesian product of 14 components.

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object Product15

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trait Product15[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15] extends Product

Product15 is a Cartesian product of 15 components.

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object Product16

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trait Product16[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16] extends Product

Product16 is a Cartesian product of 16 components.

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object Product17

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trait Product17[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17] extends Product

Product17 is a Cartesian product of 17 components.

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object Product18

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trait Product18[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18] extends Product

Product18 is a Cartesian product of 18 components.

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object Product19

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trait Product19[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19] extends Product

Product19 is a Cartesian product of 19 components.

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object Product2

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trait Product2[@specialized(Int, Long, Double) +T1, @specialized(Int, Long, Double) +T2] extends Product

Product2 is a Cartesian product of 2 components.

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object Product20

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trait Product20[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20] extends Product

Product20 is a Cartesian product of 20 components.

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object Product21

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trait Product21[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20, +T21] extends Product

Product21 is a Cartesian product of 21 components.

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object Product22

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trait Product22[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20, +T21, +T22] extends Product

Product22 is a Cartesian product of 22 components.

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object Product3

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trait Product3[+T1, +T2, +T3] extends Product

Product3 is a Cartesian product of 3 components.

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object Product4

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trait Product4[+T1, +T2, +T3, +T4] extends Product

Product4 is a Cartesian product of 4 components.

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object Product5

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trait Product5[+T1, +T2, +T3, +T4, +T5] extends Product

Product5 is a Cartesian product of 5 components.

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object Product6

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trait Product6[+T1, +T2, +T3, +T4, +T5, +T6] extends Product

Product6 is a Cartesian product of 6 components.

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object Product7

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trait Product7[+T1, +T2, +T3, +T4, +T5, +T6, +T7] extends Product

Product7 is a Cartesian product of 7 components.

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object Product8

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trait Product8[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8] extends Product

Product8 is a Cartesian product of 8 components.

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object Product9

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trait Product9[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9] extends Product

Product9 is a Cartesian product of 9 components.

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case class ScalaReflectionException(msg: String) extends Exception

An exception that indicates an error during Scala reflection

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trait Selectable

A marker trait for objects that support structural selection via selectDynamic and applyDynamic

Implementation classes should define, or make available as extension methods, the following two method signatures:

def selectDynamic(name: String): Any def applyDynamic(name: String)(args: Any*): Any =

selectDynamic is invoked for simple selections v.m, whereas applyDynamic is invoked for selections with arguments v.m(...). If there's only one kind of selection, the method supporting the other may be omitted. The applyDynamic can also have a second parameter list of java.lang.Class arguments, i.e. it may alternatively have the signature

def applyDynamic(name: String, paramClasses: Class[_]*)(args: Any*): Any

In this case the call will synthesize Class arguments for the erasure of all formal parameter types of the method in the structural type.

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object Selectable

Source@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class SerialVersionUID(value: Long) extends ConstantAnnotation

Annotation for specifying the serialVersionUID field of a (serializable) class.

On the JVM, a class with this annotation will receive a private, static, and final field called serialVersionUID with the provided value, which the JVM's serialization mechanism uses to determine serialization compatibility between different versions of a class.

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final abstract class Short extends AnyVal

Short, a 16-bit signed integer (equivalent to Java's short primitive type) is a subtype of scala.AnyVal. Instances of Short are not represented by an object in the underlying runtime system.

There is an implicit conversion from scala.Short => scala.runtime.RichShort which provides useful non-primitive operations.

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object Short

final open trait Singleton

Singleton is used by the compiler as a supertype for singleton types. This includes literal types, as they are also singleton types.

scala> object A { val x = 42 }
defined object A

scala> implicitly[A.type <:< Singleton]
res12: A.type <:< Singleton = generalized constraint

scala> implicitly[A.x.type <:< Singleton]
res13: A.x.type <:< Singleton = generalized constraint

scala> implicitly[42 <:< Singleton]
res14: 42 <:< Singleton = generalized constraint

scala> implicitly[Int <:< Singleton]
^
error: Cannot prove that Int <:< Singleton.

Singleton has a special meaning when it appears as an upper bound on a formal type parameter. Normally, type inference in Scala widens singleton types to the underlying non-singleton type. When a type parameter has an explicit upper bound of Singleton, the compiler infers a singleton type.

scala> def check42[T](x: T)(implicit ev: T =:= 42): T = x
check42: [T](x: T)(implicit ev: T =:= 42)T

scala> val x1 = check42(42)
^
error: Cannot prove that Int =:= 42.

scala> def singleCheck42[T <: Singleton](x: T)(implicit ev: T =:= 42): T = x
singleCheck42: [T <: Singleton](x: T)(implicit ev: T =:= 42)T

scala> val x2 = singleCheck42(42)
x2: Int = 42

See also SIP-23 about Literal-based Singleton Types.

Source@SerialVersionUID(1234815782226070388L)

final case class Some[+A](value: A) extends Option[A]

Class Some[A] represents existing values of type A.

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trait Specializable

A common supertype for companions of specializable types. Should not be extended in user code.

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object Specializable

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case class StringContext(parts: String*)

This class provides the basic mechanism to do String Interpolation. String Interpolation allows users to embed variable references directly in *processed* string literals. Here's an example:

val name = "James"
println(s"Hello, $name")  // Hello, James

Any processed string literal is rewritten as an instantiation and method call against this class. For example:

s"Hello, $name"

is rewritten to be:

StringContext("Hello, ", "").s(name)

By default, this class provides the raw, s and f methods as available interpolators.

To provide your own string interpolator, create an implicit class which adds a method to StringContext. Here's an example:

implicit class JsonHelper(private val sc: StringContext) extends AnyVal {
  def json(args: Any*): JSONObject = ...
}
val x: JSONObject = json"{ a: $a }"

Here the JsonHelper extension class implicitly adds the json method to StringContext which can be used for json string literals.

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object StringContext

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final class Symbol extends Serializable

This class provides a simple way to get unique objects for equal strings. Since symbols are interned, they can be compared using reference equality.

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object Symbol

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sealed trait Tuple extends Product

Tuple of arbitrary arity

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object Tuple

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final case class Tuple1[@specialized(Int, Long, Double) +T1](_1: T1) extends Product1[T1]

A tuple of 1 elements; the canonical representation of a scala.Product1.

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final case class Tuple10[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10) extends Product10[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10]

A tuple of 10 elements; the canonical representation of a scala.Product10.

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final case class Tuple11[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11) extends Product11[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11]

A tuple of 11 elements; the canonical representation of a scala.Product11.

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final case class Tuple12[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12) extends Product12[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12]

A tuple of 12 elements; the canonical representation of a scala.Product12.

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final case class Tuple13[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13) extends Product13[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13]

A tuple of 13 elements; the canonical representation of a scala.Product13.

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final case class Tuple14[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14) extends Product14[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14]

A tuple of 14 elements; the canonical representation of a scala.Product14.

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final case class Tuple15[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15) extends Product15[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15]

A tuple of 15 elements; the canonical representation of a scala.Product15.

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final case class Tuple16[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16) extends Product16[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16]

A tuple of 16 elements; the canonical representation of a scala.Product16.

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final case class Tuple17[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17) extends Product17[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17]

A tuple of 17 elements; the canonical representation of a scala.Product17.

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final case class Tuple18[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17, _18: T18) extends Product18[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18]

A tuple of 18 elements; the canonical representation of a scala.Product18.

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final case class Tuple19[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17, _18: T18, _19: T19) extends Product19[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19]

A tuple of 19 elements; the canonical representation of a scala.Product19.

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final case class Tuple2[@specialized(Int, Long, Double, Char, Boolean) +T1, @specialized(Int, Long, Double, Char, Boolean) +T2](_1: T1, _2: T2) extends Product2[T1, T2]

A tuple of 2 elements; the canonical representation of a scala.Product2.

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final case class Tuple20[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17, _18: T18, _19: T19, _20: T20) extends Product20[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20]

A tuple of 20 elements; the canonical representation of a scala.Product20.

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final case class Tuple21[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20, +T21](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17, _18: T18, _19: T19, _20: T20, _21: T21) extends Product21[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21]

A tuple of 21 elements; the canonical representation of a scala.Product21.

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final case class Tuple22[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9, +T10, +T11, +T12, +T13, +T14, +T15, +T16, +T17, +T18, +T19, +T20, +T21, +T22](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9, _10: T10, _11: T11, _12: T12, _13: T13, _14: T14, _15: T15, _16: T16, _17: T17, _18: T18, _19: T19, _20: T20, _21: T21, _22: T22) extends Product22[T1, T2, T3, T4, T5, T6, T7, T8, T9, T10, T11, T12, T13, T14, T15, T16, T17, T18, T19, T20, T21, T22]

A tuple of 22 elements; the canonical representation of a scala.Product22.

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final case class Tuple3[+T1, +T2, +T3](_1: T1, _2: T2, _3: T3) extends Product3[T1, T2, T3]

A tuple of 3 elements; the canonical representation of a scala.Product3.

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final case class Tuple4[+T1, +T2, +T3, +T4](_1: T1, _2: T2, _3: T3, _4: T4) extends Product4[T1, T2, T3, T4]

A tuple of 4 elements; the canonical representation of a scala.Product4.

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final case class Tuple5[+T1, +T2, +T3, +T4, +T5](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5) extends Product5[T1, T2, T3, T4, T5]

A tuple of 5 elements; the canonical representation of a scala.Product5.

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final case class Tuple6[+T1, +T2, +T3, +T4, +T5, +T6](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6) extends Product6[T1, T2, T3, T4, T5, T6]

A tuple of 6 elements; the canonical representation of a scala.Product6.

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final case class Tuple7[+T1, +T2, +T3, +T4, +T5, +T6, +T7](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7) extends Product7[T1, T2, T3, T4, T5, T6, T7]

A tuple of 7 elements; the canonical representation of a scala.Product7.

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final case class Tuple8[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8) extends Product8[T1, T2, T3, T4, T5, T6, T7, T8]

A tuple of 8 elements; the canonical representation of a scala.Product8.

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final case class Tuple9[+T1, +T2, +T3, +T4, +T5, +T6, +T7, +T8, +T9](_1: T1, _2: T2, _3: T3, _4: T4, _5: T5, _6: T6, _7: T7, _8: T8, _9: T9) extends Product9[T1, T2, T3, T4, T5, T6, T7, T8, T9]

A tuple of 9 elements; the canonical representation of a scala.Product9.

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final case class UninitializedFieldError(msg: String) extends RuntimeException

This class implements errors which are thrown whenever a field is used before it has been initialized.

Such runtime checks are not emitted by default. They can be enabled by the -Xcheckinit compiler option.

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final abstract class Unit extends AnyVal

Unit is a subtype of scala.AnyVal. There is only one value of type Unit, (), and it is not represented by any object in the underlying runtime system. A method with return type Unit is analogous to a Java method which is declared void.

Source@compileTimeOnly("`Unit` companion object is not allowed in source; instead, use `()` for the unit value")

object Unit

Source@implicitNotFound(msg = "No singleton value available for ${T}.")

final class ValueOf[T](val value: T) extends AnyVal

ValueOf[T] provides the unique value of the type T where T is a type which has a single inhabitant. Eligible types are singleton types of the form stablePath.type, Unit and singleton types corresponding to value literals.

Instances of ValueOf[T] are provided implicitly for all eligible types. Typically an instance would be required where a runtime value corresponding to a type level computation is needed.

For example, we might define a type Residue[M <: Int] corresponding to the group of integers modulo M. We could then mandate that residues can be summed only when they are parameterized by the same modulus,

case class Residue[M <: Int](n: Int) extends AnyVal {
 def +(rhs: Residue[M])(implicit m: ValueOf[M]): Residue[M] =
   Residue((this.n + rhs.n) % valueOf[M])
}

val fiveModTen = Residue[10](5)
val nineModTen = Residue[10](9)

fiveModTen + nineModTen    // OK == Residue[10](4)

val fourModEleven = Residue[11](4)

fiveModTen + fourModEleven // compiler error: type mismatch;
                          //   found   : Residue[11]
                          //   required: Residue[10]

Notice that here the modulus is encoded in the type of the values and so does not incur any additional per-value storage cost. When a runtime value of the modulus is required in the implementation of + it is provided at the call site via the implicit argument m of type ValueOf[M].

Source@getter @setter @beanGetter @beanSetter @field @deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class deprecated(message: String, since: String) extends ConstantAnnotation

An annotation that designates that a definition is deprecated. A deprecation warning is issued upon usage of the annotated definition.

Library authors should state the library's deprecation policy in their documentation to give developers guidance on how long a deprecated definition will be preserved.

Library authors should prepend the name of their library to the version number to help developers distinguish deprecations coming from different libraries:

@deprecated("this method will be removed", "FooLib 12.0")
def oldMethod(x: Int) = ...

The compiler will emit deprecation warnings grouped by library and version:

oldMethod(1)
oldMethod(2)
aDeprecatedMethodFromLibraryBar(3, 4)

// warning: there was one deprecation warning (since BarLib 3.2)
// warning: there were two deprecation warnings (since FooLib 12.0)
// warning: there were three deprecation warnings in total; re-run with -deprecation for details

@deprecated in the Scala language and its standard library

A deprecated element of the Scala language or a definition in the Scala standard library will be preserved at least for the current major version.

This means that an element deprecated in some 2.13.x release will be preserved in all 2.13.x releases, but may be removed in the future. (A deprecated element might be kept longer to ease migration, but developers should not rely on this.)

Source@getter @setter @beanGetter @beanSetter

final class deprecatedInheritance(message: String, since: String) extends ConstantAnnotation

An annotation that designates that inheriting from a class is deprecated.

This is usually done to warn about a non-final class being made final in a future version. Sub-classing such a class then generates a warning.

No warnings are generated if the subclass is in the same compilation unit.

Library authors should state the library's deprecation policy in their documentation to give developers guidance on when a type annotated with @deprecatedInheritance will be finalized.

Library authors should prepend the name of their library to the version number to help developers distinguish deprecations coming from different libraries:

@deprecatedInheritance("this class will be made final", "FooLib 12.0")
class Foo

val foo = new Foo     // no deprecation warning
class Bar extends Foo
// warning: inheritance from class Foo is deprecated (since FooLib 12.0): this class will be made final
// class Bar extends Foo
//                   ^

Source@param @deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class deprecatedName(name: String, since: String) extends StaticAnnotation

An annotation that designates that the name of a parameter is deprecated.

Using this name in a named argument generates a deprecation warning.

Library authors should state the library's deprecation policy in their documentation to give developers guidance on how long a deprecated name will be preserved.

Library authors should prepend the name of their library to the version number to help developers distinguish deprecations coming from different libraries:

def inc(x: Int, @deprecatedName("y", "FooLib 12.0") n: Int): Int = x + n
inc(1, y = 2)

will produce the following warning:

warning: the parameter name y is deprecated (since FooLib 12.0): use n instead
inc(1, y = 2)
         ^

Source@getter @setter @beanGetter @beanSetter @deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class deprecatedOverriding(message: String, since: String) extends ConstantAnnotation

An annotation that designates that overriding a member is deprecated.

Overriding such a member in a sub-class then generates a warning.

Library authors should state the library's deprecation policy in their documentation to give developers guidance on when a method annotated with @deprecatedOverriding will be finalized.

Library authors should prepend the name of their library to the version number to help developers distinguish deprecations coming from different libraries:

class Foo {
  @deprecatedOverriding("this method will be made final", "FooLib 12.0")
  def add(x: Int, y: Int) = x + y
}

class Bar extends Foo // no deprecation warning
class Baz extends Foo {
  override def add(x: Int, y: Int) = x - y
}
// warning: overriding method add in class Foo is deprecated (since FooLib 12.0): this method will be made final
// override def add(x: Int, y: Int) = x - y
//              ^

Source@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class inline extends StaticAnnotation

An annotation for methods that the optimizer should inline.

Note that by default, the Scala optimizer is disabled and no callsites are inlined. See -opt:help for information on how to enable the optimizer and inliner.

When inlining is enabled, the inliner will always try to inline methods or callsites annotated @inline (under the condition that inlining from the defining class is allowed, see -opt-inline-from:help). If inlining is not possible, for example because the method is not final, an optimizer warning will be issued. See -opt-warnings:help for details.

Examples:

@inline   final def f1(x: Int) = x
@noinline final def f2(x: Int) = x
         final def f3(x: Int) = x

def t1 = f1(1)              // inlined if possible
def t2 = f2(1)              // not inlined
def t3 = f3(1)              // may be inlined (the inliner heuristics can select the callsite)
def t4 = f1(1): @noinline   // not inlined (override at callsite)
def t5 = f2(1): @inline     // inlined if possible (override at callsite)
def t6 = f3(1): @inline     // inlined if possible
def t7 = f3(1): @noinline   // not inlined
}

Note: parentheses are required when annotating a callsite within a larger expression.

def t1 = f1(1) + f1(1): @noinline   // equivalent to (f1(1) + f1(1)): @noinline
def t2 = f1(1) + (f1(1): @noinline) // the second call to f1 is not inlined

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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 the Dynamic trait

• existentials enables writing existential types

• higherKinds enables writing higher-kinded types

• implicitConversions enables defining implicit methods and members

• postfixOps enables postfix operators (not recommended)

• reflectiveCalls enables using structural types

• experimental contains newer features that have not yet been tested in production

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object languageFeature

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class main extends Annotation

An annotation that designates a main function

Source@deprecatedInheritance("Scheduled for being final in the future", "2.13.0")

class native extends StaticAnnotation

Marker for native methods.

@native def f(x: Int, y: List[Long]): String = ...

A @native method is compiled to the platform's native method, while discarding the method's body (if any). The body will be type checked if present.

A method marked @native must be a member of a class, not a trait (since 2.12).

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final class noinline extends StaticAnnotation

An annotation for methods that the optimizer should not inline.

Note that by default, the Scala optimizer is disabled and no callsites are inlined. See -opt:help for information how to enable the optimizer and inliner.

When inlining is enabled, the inliner will never inline methods or callsites annotated @noinline.

Examples:

@inline   final def f1(x: Int) = x
@noinline final def f2(x: Int) = x
         final def f3(x: Int) = x

def t1 = f1(1)              // inlined if possible
def t2 = f2(1)              // not inlined
def t3 = f3(1)              // may be inlined (the inliner heuristics can select the callsite)
def t4 = f1(1): @noinline   // not inlined (override at callsite)
def t5 = f2(1): @inline     // inlined if possible (override at callsite)
def t6 = f3(1): @inline     // inlined if possible
def t7 = f3(1): @noinline   // not inlined
}

Note: parentheses are required when annotating a callsite within a larger expression.

def t1 = f1(1) + f1(1): @noinline   // equivalent to (f1(1) + f1(1)): @noinline
def t2 = f1(1) + (f1(1): @noinline) // the second call to f1 is not inlined

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final class specialized(group: SpecializedGroup) extends StaticAnnotation

Annotate type parameters on which code should be automatically specialized. For example:

class MyList[@specialized T] ...

Type T can be specialized on a subset of the primitive types by specifying a list of primitive types to specialize at:

class MyList[@specialized(Int, Double, Boolean) T] ..

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final class throws[T <: Throwable](cause: String) extends StaticAnnotation

Annotation for specifying the exceptions thrown by a method. For example:

class Reader(fname: String) {
 private val in = new BufferedReader(new FileReader(fname))
 @throws[IOException]("if the file doesn't exist")
 def read() = in.read()
}

Source@field

final class transient extends StaticAnnotation

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final class unchecked extends Annotation

An annotation to designate that the annotated entity should not be considered for additional compiler checks. Specific applications include annotating the subject of a match expression to suppress exhaustiveness and reachability warnings, and annotating a type argument in a match case to suppress unchecked warnings.

Such suppression should be used with caution, without which one may encounter scala.MatchError or java.lang.ClassCastException at runtime. In most cases one can and should address the warning instead of suppressing it.

object Test extends App {
  // This would normally warn "match is not exhaustive"
  // because `None` is not covered.
  def f(x: Option[String]) = (x: @unchecked) match { case Some(y) => y }
  // This would normally warn "type pattern is unchecked"
  // but here will blindly cast the head element to String.
  def g(xs: Any) = xs match { case x: List[String @unchecked] => x.head }
}

Source@experimental

object unsafeExceptions

Source@field

final class volatile extends StaticAnnotation

Types

type & = [X0, X1] =>> X0 & X1

The intersection of two types.

See https://dotty.epfl.ch/docs/reference/new-types/intersection-types.html.

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type ::[+A] = ::[A]

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type AbstractMethodError = AbstractMethodError

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type ArrayIndexOutOfBoundsException = ArrayIndexOutOfBoundsException

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type BigDecimal = BigDecimal

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type BigInt = BigInt

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type ClassCastException = ClassCastException

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type Cloneable = Cloneable

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type Either[+A, +B] = Either[A, B]

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type EmptyTuple = EmptyTuple.type

A tuple of 0 elements

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type Equiv[T] = Equiv[T]

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type Error = Error

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type Exception = Exception

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type Fractional[T] = Fractional[T]

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opaque type IArray[+T]

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type IllegalArgumentException = IllegalArgumentException

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type IndexOutOfBoundsException = IndexOutOfBoundsException

Source@migration("scala.IndexedSeq is now scala.collection.immutable.IndexedSeq instead of scala.collection.IndexedSeq", "2.13.0")

type IndexedSeq[+A] = IndexedSeq[A]

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type Integral[T] = Integral[T]

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type InterruptedException = InterruptedException

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type Iterable[+A] = Iterable[A]

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type IterableOnce[+A] = IterableOnce[A]

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type Iterator[+A] = Iterator[A]

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type LazyList[+A] = LazyList[A]

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type Left[+A, +B] = Left[A, B]

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type List[+A] = List[A]

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type NoSuchElementException = NoSuchElementException

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type NullPointerException = NullPointerException

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type NumberFormatException = NumberFormatException

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type Numeric[T] = Numeric[T]

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type Ordered[T] = Ordered[T]

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type Ordering[T] = Ordering[T]

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type PartialOrdering[T] = PartialOrdering[T]

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type PartiallyOrdered[T] = PartiallyOrdered[T]

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type Range = Range

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type Right[+A, +B] = Right[A, B]

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type RuntimeException = RuntimeException

Source@migration("scala.Seq is now scala.collection.immutable.Seq instead of scala.collection.Seq", "2.13.0")

type Seq[+A] = Seq[A]

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type Serializable = Serializable

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type StringBuilder = StringBuilder

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type StringIndexOutOfBoundsException = StringIndexOutOfBoundsException

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type Throwable = Throwable

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type UnsupportedOperationException = UnsupportedOperationException

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type Vector[+A] = Vector[A]

type | = [X0, X1] =>> X0 | X1

The union of two types.

See https://dotty.epfl.ch/docs/reference/new-types/union-types.html.

Concrete fields

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val +:: +:.type

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val :+: :+.type

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val ::: ::.type

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val AnyRef: Specializable

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val BigDecimal: BigDecimal.type

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val BigInt: BigInt.type

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val Either: Either.type

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val Equiv: Equiv.type

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val Fractional: Fractional.type

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val IndexedSeq: IndexedSeq.type

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val Integral: Integral.type

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val Iterable: Iterable.type

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val Iterator: Iterator.type

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val LazyList: LazyList.type

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val Left: Left.type

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val List: List.type

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val Nil: Nil.type

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val Numeric: Numeric.type

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val Ordered: Ordered.type

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val Ordering: Ordering.type

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val Range: Range.type

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val Right: Right.type

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val Seq: Seq.type

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val StringBuilder: StringBuilder.type

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val Vector: Vector.type