This package object contains primitives for concurrent and parallel programming.
A more detailed guide to Futures and Promises, including discussion and examples can be found at http://docs.scala-lang.org/overviews/core/futures.html.
When working with Futures, you will often find that importing the whole concurrent package is convenient:
import scala.concurrent._
When using things like Future
s, it is often required to have an implicit ExecutionContext
in scope. The general advice for these implicits are as follows.
If the code in question is a class or method definition, and no ExecutionContext
is available, request one from the caller by adding an implicit parameter list:
def myMethod(myParam: MyType)(implicit ec: ExecutionContext) = … //Or class MyClass(myParam: MyType)(implicit ec: ExecutionContext) { … }
This allows the caller of the method, or creator of the instance of the class, to decide which ExecutionContext
should be used.
For typical REPL usage and experimentation, importing the global ExecutionContext
is often desired.
import scala.concurrent.ExcutionContext.Implicits.global
Operations often require a duration to be specified. A duration DSL is available to make defining these easier:
import scala.concurrent.duration._ val d: Duration = 10.seconds
Basic use of futures is easy with the factory method on Future, which executes a provided function asynchronously, handing you back a future result of that function without blocking the current thread. In order to create the Future you will need either an implicit or explicit ExecutionContext to be provided:
import scala.concurrent._ import ExecutionContext.Implicits.global // implicit execution context val firstZebra: Future[Int] = Future { val source = scala.io.Source.fromFile("/etc/dictionaries-common/words") source.toSeq.indexOfSlice("zebra") }
Although blocking is possible in order to await results (with a mandatory timeout duration):
import scala.concurrent.duration._ Await.result(firstZebra, 10.seconds)
and although this is sometimes necessary to do, in particular for testing purposes, blocking in general is discouraged when working with Futures and concurrency in order to avoid potential deadlocks and improve performance. Instead, use callbacks or combinators to remain in the future domain:
val animalRange: Future[Int] = for { aardvark <- firstAardvark zebra <- firstZebra } yield zebra - aardvark animalRange.onSuccess { case x if x > 500000 => println("It's a long way from Aardvark to Zebra") }
The package object scala.math
contains methods for performing basic numeric operations such as elementary exponential, logarithmic, root and trigonometric functions.
All methods forward to java.lang.Math unless otherwise noted.
java.lang.Math
The package object scala.sys
contains methods for reading and altering core aspects of the virtual machine as well as the world outside of it.
2.9
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 To
s in a type with From
s.
In part contributed by Jason Zaugg.
a type which is proved a subtype of To
a type which is proved a supertype of From
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. }
=:= for expressing equality constraints
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 To
s in a type with From
s.
a type which is proved equal to To
a type which is proved equal to From
An in-place variant of scala.collection.mutable.ArrayBuffer#transpose
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. }
<:< for expressing subtyping constraints
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 def
s 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.
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:
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...
val
parameter that is the underlying runtime representation.can define def
s, but no val
s, var
s, or nested traits
s, class
es or object
s.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.
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.
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
.
2.1
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
.
1.0
Scala Language Specification, for in-depth information on the transformations the Scala compiler makes on Arrays (Sections 6.6 and 6.15 respectively.)
"Scala 2.8 Arrays" the Scala Improvement Document detailing arrays since Scala 2.8.
"The Scala 2.8 Collections' API" section on Array
by Martin Odersky for more information.
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.
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.
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.
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.
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.
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 Val(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): Val = x.asInstanceOf[Val] val G: Double = 6.67300E-11 val Mercury = Val(3.303e+23, 2.4397e6) val Venus = Val(4.869e+24, 6.0518e6) val Earth = Val(5.976e+24, 6.37814e6) val Mars = Val(6.421e+23, 3.3972e6) val Jupiter = Val(1.9e+27, 7.1492e7) val Saturn = Val(5.688e+26, 6.0268e7) val Uranus = Val(8.686e+25, 2.5559e7) val Neptune = Val(1.024e+26, 2.4746e7) } println(Planet.values.filter(_.radius > 7.0e6)) // output: // Planet.ValueSet(Jupiter, Saturn, Uranus, Neptune)
An interface containing operations for equality. The only method not already present in class AnyRef
is canEqual
.
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.
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()) }
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.
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)) }
(Changed in version 2.13.0) scala.IndexedSeq is now scala.collection.immutable.IndexedSeq instead of scala.collection.IndexedSeq
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.
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.
This class implements errors which are thrown whenever an object doesn't match any pattern of a pattern matching expression.
2.0
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.???
.
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.
Null
is - together with scala.Nothing - at the bottom of the Scala type hierarchy.
Null
is a subtype of all reference types; its only instance is the null
reference. 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.
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
.
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") }
1.1
Many of the methods in here are duplicative with those in the Traversable hierarchy, but they are duplicated for a reason: the implicit conversion tends to leave one with an Iterable in situations where one could have retained an Option.
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 _ => 1/0 }
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 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 val isEven: PartialFunction[Int, String] = { case x if x % 2 == 0 => x+" is even" } // the method collect can use isDefinedAt to select which members to collect val evenNumbers = sample collect isEven val isOdd: PartialFunction[Int, String] = { case x if x % 2 == 1 => x+" is odd" } // the method orElse allows chaining another partial function to handle // input outside the declared domain val numbers = sample map (isEven orElse isOdd)
1.0
Optional Functions, PartialFunctions and extractor objects can be converted to each other as shown in the following table. | How to convert ... | to a PartialFunction | to an optional Function | to an extractor | | :---: | --- | --- | --- | | from a PartialFunction | Predef.identity | lift | Predef.identity | | from optional Function | Function.UnliftOps#unlift or Function.unlift | Predef.identity | Function.UnliftOps#unlift | | from an extractor | { case extractor(x) => x }
| extractor.unapply _
| Predef.identity |
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.
2.3
Product1 is a Cartesian product of 1 component.
2.3
Product10 is a Cartesian product of 10 components.
2.3
Product11 is a Cartesian product of 11 components.
2.3
Product12 is a Cartesian product of 12 components.
2.3
Product13 is a Cartesian product of 13 components.
2.3
Product14 is a Cartesian product of 14 components.
2.3
Product15 is a Cartesian product of 15 components.
2.3
Product16 is a Cartesian product of 16 components.
2.3
Product17 is a Cartesian product of 17 components.
2.3
Product18 is a Cartesian product of 18 components.
2.3
Product19 is a Cartesian product of 19 components.
2.3
Product2 is a Cartesian product of 2 components.
2.3
Product20 is a Cartesian product of 20 components.
2.3
Product21 is a Cartesian product of 21 components.
2.3
Product22 is a Cartesian product of 22 components.
2.3
Product3 is a Cartesian product of 3 components.
2.3
Product4 is a Cartesian product of 4 components.
2.3
Product5 is a Cartesian product of 5 components.
2.3
Product6 is a Cartesian product of 6 components.
2.3
Product7 is a Cartesian product of 7 components.
2.3
Product8 is a Cartesian product of 8 components.
2.3
Product9 is a Cartesian product of 9 components.
2.3
(Changed in version 2.13.0) scala.Seq is now scala.collection.immutable.Seq instead of scala.collection.Seq
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.
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.
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
Class Some[A]
represents existing values of type A
.
1.0
A common supertype for companions of specializable types. Should not be extended in user code.
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.
The parts that make up the interpolated string, without the expressions that get inserted by interpolation.
2.10.0
This class provides a simple way to get unique objects for equal strings. Since symbols are interned, they can be compared using reference equality. Instances of Symbol
can be created easily with Scala's built-in quote mechanism.
For instance, the Scala term 'mysym
will invoke the constructor of the Symbol
class in the following way: Symbol("mysym")
.
1.7
A tuple of 1 elements; the canonical representation of a scala.Product1.
Element 1 of this Tuple1
A tuple of 10 elements; the canonical representation of a scala.Product10.
Element 1 of this Tuple10
Element 2 of this Tuple10
Element 3 of this Tuple10
Element 4 of this Tuple10
Element 5 of this Tuple10
Element 6 of this Tuple10
Element 7 of this Tuple10
Element 8 of this Tuple10
Element 9 of this Tuple10
Element 10 of this Tuple10
A tuple of 11 elements; the canonical representation of a scala.Product11.
Element 1 of this Tuple11
Element 2 of this Tuple11
Element 3 of this Tuple11
Element 4 of this Tuple11
Element 5 of this Tuple11
Element 6 of this Tuple11
Element 7 of this Tuple11
Element 8 of this Tuple11
Element 9 of this Tuple11
Element 10 of this Tuple11
Element 11 of this Tuple11
A tuple of 12 elements; the canonical representation of a scala.Product12.
Element 1 of this Tuple12
Element 2 of this Tuple12
Element 3 of this Tuple12
Element 4 of this Tuple12
Element 5 of this Tuple12
Element 6 of this Tuple12
Element 7 of this Tuple12
Element 8 of this Tuple12
Element 9 of this Tuple12
Element 10 of this Tuple12
Element 11 of this Tuple12
Element 12 of this Tuple12
A tuple of 13 elements; the canonical representation of a scala.Product13.
Element 1 of this Tuple13
Element 2 of this Tuple13
Element 3 of this Tuple13
Element 4 of this Tuple13
Element 5 of this Tuple13
Element 6 of this Tuple13
Element 7 of this Tuple13
Element 8 of this Tuple13
Element 9 of this Tuple13
Element 10 of this Tuple13
Element 11 of this Tuple13
Element 12 of this Tuple13
Element 13 of this Tuple13
A tuple of 14 elements; the canonical representation of a scala.Product14.
Element 1 of this Tuple14
Element 2 of this Tuple14
Element 3 of this Tuple14
Element 4 of this Tuple14
Element 5 of this Tuple14
Element 6 of this Tuple14
Element 7 of this Tuple14
Element 8 of this Tuple14
Element 9 of this Tuple14
Element 10 of this Tuple14
Element 11 of this Tuple14
Element 12 of this Tuple14
Element 13 of this Tuple14
Element 14 of this Tuple14
A tuple of 15 elements; the canonical representation of a scala.Product15.
Element 1 of this Tuple15
Element 2 of this Tuple15
Element 3 of this Tuple15
Element 4 of this Tuple15
Element 5 of this Tuple15
Element 6 of this Tuple15
Element 7 of this Tuple15
Element 8 of this Tuple15
Element 9 of this Tuple15
Element 10 of this Tuple15
Element 11 of this Tuple15
Element 12 of this Tuple15
Element 13 of this Tuple15
Element 14 of this Tuple15
Element 15 of this Tuple15
A tuple of 16 elements; the canonical representation of a scala.Product16.
Element 1 of this Tuple16
Element 2 of this Tuple16
Element 3 of this Tuple16
Element 4 of this Tuple16
Element 5 of this Tuple16
Element 6 of this Tuple16
Element 7 of this Tuple16
Element 8 of this Tuple16
Element 9 of this Tuple16
Element 10 of this Tuple16
Element 11 of this Tuple16
Element 12 of this Tuple16
Element 13 of this Tuple16
Element 14 of this Tuple16
Element 15 of this Tuple16
Element 16 of this Tuple16
A tuple of 17 elements; the canonical representation of a scala.Product17.
Element 1 of this Tuple17
Element 2 of this Tuple17
Element 3 of this Tuple17
Element 4 of this Tuple17
Element 5 of this Tuple17
Element 6 of this Tuple17
Element 7 of this Tuple17
Element 8 of this Tuple17
Element 9 of this Tuple17
Element 10 of this Tuple17
Element 11 of this Tuple17
Element 12 of this Tuple17
Element 13 of this Tuple17
Element 14 of this Tuple17
Element 15 of this Tuple17
Element 16 of this Tuple17
Element 17 of this Tuple17
A tuple of 18 elements; the canonical representation of a scala.Product18.
Element 1 of this Tuple18
Element 2 of this Tuple18
Element 3 of this Tuple18
Element 4 of this Tuple18
Element 5 of this Tuple18
Element 6 of this Tuple18
Element 7 of this Tuple18
Element 8 of this Tuple18
Element 9 of this Tuple18
Element 10 of this Tuple18
Element 11 of this Tuple18
Element 12 of this Tuple18
Element 13 of this Tuple18
Element 14 of this Tuple18
Element 15 of this Tuple18
Element 16 of this Tuple18
Element 17 of this Tuple18
Element 18 of this Tuple18
A tuple of 19 elements; the canonical representation of a scala.Product19.
Element 1 of this Tuple19
Element 2 of this Tuple19
Element 3 of this Tuple19
Element 4 of this Tuple19
Element 5 of this Tuple19
Element 6 of this Tuple19
Element 7 of this Tuple19
Element 8 of this Tuple19
Element 9 of this Tuple19
Element 10 of this Tuple19
Element 11 of this Tuple19
Element 12 of this Tuple19
Element 13 of this Tuple19
Element 14 of this Tuple19
Element 15 of this Tuple19
Element 16 of this Tuple19
Element 17 of this Tuple19
Element 18 of this Tuple19
Element 19 of this Tuple19
A tuple of 2 elements; the canonical representation of a scala.Product2.
Element 1 of this Tuple2
Element 2 of this Tuple2
A tuple of 20 elements; the canonical representation of a scala.Product20.
Element 1 of this Tuple20
Element 2 of this Tuple20
Element 3 of this Tuple20
Element 4 of this Tuple20
Element 5 of this Tuple20
Element 6 of this Tuple20
Element 7 of this Tuple20
Element 8 of this Tuple20
Element 9 of this Tuple20
Element 10 of this Tuple20
Element 11 of this Tuple20
Element 12 of this Tuple20
Element 13 of this Tuple20
Element 14 of this Tuple20
Element 15 of this Tuple20
Element 16 of this Tuple20
Element 17 of this Tuple20
Element 18 of this Tuple20
Element 19 of this Tuple20
Element 20 of this Tuple20
A tuple of 21 elements; the canonical representation of a scala.Product21.
Element 1 of this Tuple21
Element 2 of this Tuple21
Element 3 of this Tuple21
Element 4 of this Tuple21
Element 5 of this Tuple21
Element 6 of this Tuple21
Element 7 of this Tuple21
Element 8 of this Tuple21
Element 9 of this Tuple21
Element 10 of this Tuple21
Element 11 of this Tuple21
Element 12 of this Tuple21
Element 13 of this Tuple21
Element 14 of this Tuple21
Element 15 of this Tuple21
Element 16 of this Tuple21
Element 17 of this Tuple21
Element 18 of this Tuple21
Element 19 of this Tuple21
Element 20 of this Tuple21
Element 21 of this Tuple21
A tuple of 22 elements; the canonical representation of a scala.Product22.
Element 1 of this Tuple22
Element 2 of this Tuple22
Element 3 of this Tuple22
Element 4 of this Tuple22
Element 5 of this Tuple22
Element 6 of this Tuple22
Element 7 of this Tuple22
Element 8 of this Tuple22
Element 9 of this Tuple22
Element 10 of this Tuple22
Element 11 of this Tuple22
Element 12 of this Tuple22
Element 13 of this Tuple22
Element 14 of this Tuple22
Element 15 of this Tuple22
Element 16 of this Tuple22
Element 17 of this Tuple22
Element 18 of this Tuple22
Element 19 of this Tuple22
Element 20 of this Tuple22
Element 21 of this Tuple22
Element 22 of this Tuple22
A tuple of 3 elements; the canonical representation of a scala.Product3.
Element 1 of this Tuple3
Element 2 of this Tuple3
Element 3 of this Tuple3
A tuple of 4 elements; the canonical representation of a scala.Product4.
Element 1 of this Tuple4
Element 2 of this Tuple4
Element 3 of this Tuple4
Element 4 of this Tuple4
A tuple of 5 elements; the canonical representation of a scala.Product5.
Element 1 of this Tuple5
Element 2 of this Tuple5
Element 3 of this Tuple5
Element 4 of this Tuple5
Element 5 of this Tuple5
A tuple of 6 elements; the canonical representation of a scala.Product6.
Element 1 of this Tuple6
Element 2 of this Tuple6
Element 3 of this Tuple6
Element 4 of this Tuple6
Element 5 of this Tuple6
Element 6 of this Tuple6
A tuple of 7 elements; the canonical representation of a scala.Product7.
Element 1 of this Tuple7
Element 2 of this Tuple7
Element 3 of this Tuple7
Element 4 of this Tuple7
Element 5 of this Tuple7
Element 6 of this Tuple7
Element 7 of this Tuple7
A tuple of 8 elements; the canonical representation of a scala.Product8.
Element 1 of this Tuple8
Element 2 of this Tuple8
Element 3 of this Tuple8
Element 4 of this Tuple8
Element 5 of this Tuple8
Element 6 of this Tuple8
Element 7 of this Tuple8
Element 8 of this Tuple8
A tuple of 9 elements; the canonical representation of a scala.Product9.
Element 1 of this Tuple9
Element 2 of this Tuple9
Element 3 of this Tuple9
Element 4 of this Tuple9
Element 5 of this Tuple9
Element 6 of this Tuple9
Element 7 of this Tuple9
Element 8 of this Tuple9
Element 9 of this Tuple9
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.
2.7
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
.
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]
.
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 2.14. (A deprecated element might be kept longer to ease migration, but developers should not rely on this.)
2.3
The official documentation on binary compatibility.
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 final
ized.
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 // ^
2.10
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) ^
2.8.1
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 final
ized.
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 // ^
2.10
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
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).
2.6
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
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] ..
2.8
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() }
2.1
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 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 } }
2.4
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)
.
1.0
Implements functionality for printing Scala values on the terminal. For reading values use StdIn. Also defines constants for marking up text on ANSI terminals.
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.
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() }
$ scala PrimeTest |
1234567891 |
yes |
$ scala PrimeTest |
56474 |
NO! |
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") } }
$ scala FunctionalPrimeTest |
First primes: Vector(2, 3, 5, 7, 11, 13, 17, 19, 23, 29, 31, 37, 41, 43, 47) |
1.0
A module defining utility methods for higher-order functional programming.
1.0
This case object represents non-existent values.
1.0
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 }
2.8
The Predef
object provides definitions that are accessible in all Scala compilation units without explicit qualification.
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).
For basic console output, Predef
provides convenience methods print and println, which are aliases of the methods in the object scala.Console.
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.
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.
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
© 2002-2019 EPFL, with contributions from Lightbend.
Licensed under the Apache License, Version 2.0.
https://www.scala-lang.org/api/2.13.0/scala/index.html
Core Scala types. They are always available without an explicit import.