The Scala standard library consists of the package scala with a
number of classes and modules. Some of these classes are described in
the following.
The root of this hierarchy is formed by class Any.
Every class in a Scala execution environment inherits directly or
indirectly from this class. Class Any has two direct
subclasses: AnyRef and AnyVal`.
The subclass AnyRef represents all values which are represented
as objects in the underlying host system. Classes written in other languages
inherit from scala.AnyRef.
The predefined subclasses of class AnyVal describe
values which are not implemented as objects in the underlying host
system.
User-defined Scala classes which do not explicitly inherit from
AnyVal inherit directly or indirectly from AnyRef. They can
not inherit from both AnyRef and AnyVal.
Classes AnyRef and AnyVal are required to provide only
the members declared in class Any, but implementations may add
host-specific methods to these classes (for instance, an
implementation may identify class AnyRef with its own root
class for objects).
The signatures of these root classes are described by the following definitions.
package scala
/** The universal root class */
abstract class Any {
/** Defined equality; abstract here */
def equals(that: Any): Boolean
/** Semantic equality between values */
final def == (that: Any): Boolean =
if (null eq this) null eq that else this equals that
/** Semantic inequality between values */
final def != (that: Any): Boolean = !(this == that)
/** Hash code; abstract here */
def hashCode: Int = $\ldots$
/** Textual representation; abstract here */
def toString: String = $\ldots$
/** Type test; needs to be inlined to work as given */
def isInstanceOf[a]: Boolean
/** Type cast; needs to be inlined to work as given */ */
def asInstanceOf[A]: A = this match {
case x: A => x
case _ => if (this eq null) this
else throw new ClassCastException()
}
}
/** The root class of all value types */
final class AnyVal extends Any
/** The root class of all reference types */
class AnyRef extends Any {
def equals(that: Any): Boolean = this eq that
final def eq(that: AnyRef): Boolean = $\ldots$ // reference equality
final def ne(that: AnyRef): Boolean = !(this eq that)
def hashCode: Int = $\ldots$ // hashCode computed from allocation address
def toString: String = $\ldots$ // toString computed from hashCode and class name
def synchronized[T](body: => T): T // execute `body` in while locking `this`.
}
```scala
The type test `$x$.isInstanceOf[$T$]` is equivalent to a typed
pattern match
```scala
$x$ match {
case _: $T'$ => true
case _ => false
}
where the type $T'$ is the same as $T$ except if $T$ is
of the form $D$ or $D[\mathit{tps}]$ where $D$ is a type member of some outer class $C$.
In this case $T'$ is $C$#$D$ (or $C$#$D[tps]$, respectively), whereas $T$ itself would expand to $C$.this.$D[tps]$.
In other words, an isInstanceOf test does not check that types have the same enclosing instance.
The test $x$.asInstanceOf[$T$] is treated specially if $T$ is a
numeric value type. In this case the cast will
be translated to an application of a conversion method
x.to$T$. For non-numeric values $x$ the operation will raise a
ClassCastException.
Value classes are classes whose instances are not represented as
objects by the underlying host system. All value classes inherit from
class AnyVal. Scala implementations need to provide the
value classes Unit, Boolean, Double, Float,
Long, Int, Char, Short, and Byte
(but are free to provide others as well).
The signatures of these classes are defined in the following.
Classes Double, Float,
Long, Int, Char, Short, and Byte
are together called numeric value types. Classes Byte,
Short, or Char are called subrange types.
Subrange types, as well as Int and Long are called integer types, whereas Float and Double are called floating point types.
Numeric value types are ranked in the following partial order:
Byte - Short
\
Int - Long - Float - Double
/
Char
Byte and Short are the lowest-ranked types in this order,
whereas Double is the highest-ranked. Ranking does not
imply a conformance relationship; for
instance Int is not a subtype of Long. However, object
Predef defines views
from every numeric value type to all higher-ranked numeric value types.
Therefore, lower-ranked types are implicitly converted to higher-ranked types
when required by the context.
Given two numeric value types $S$ and $T$, the operation type of
$S$ and $T$ is defined as follows: If both $S$ and $T$ are subrange
types then the operation type of $S$ and $T$ is Int. Otherwise
the operation type of $S$ and $T$ is the larger of the two types wrt
ranking. Given two numeric values $v$ and $w$ the operation type of
$v$ and $w$ is the operation type of their run-time types.
Any numeric value type $T$ supports the following methods.
==), not-equals (!=),
less-than (<), greater-than (>), less-than-or-equals
(<=), greater-than-or-equals (>=), which each exist in 7
overloaded alternatives. Each alternative takes a parameter of some
numeric value type. Its result type is type Boolean. The
operation is evaluated by converting the receiver and its argument to
their operation type and performing the given comparison operation of
that type.+), subtraction (-),
multiplication (*), division (/), and remainder
(%), which each exist in 7 overloaded alternatives. Each
alternative takes a parameter of some numeric value type $U$. Its
result type is the operation type of $T$ and $U$. The operation is
evaluated by converting the receiver and its argument to their
operation type and performing the given arithmetic operation of that
type.+) and negation
(-), with result type $T$. The first of these returns the
receiver unchanged, whereas the second returns its negation.toByte, toShort, toChar,
toInt, toLong, toFloat, toDouble which
convert the receiver object to the target type, using the rules of
Java's numeric type cast operation. The conversion might truncate the
numeric value (as when going from Long to Int or from
Int to Byte) or it might lose precision (as when going
from Double to Float or when converting between
Long and Float). Integer numeric value types support in addition the following operations:
Bit manipulation methods bitwise-and (&), bitwise-or
{|}, and bitwise-exclusive-or (^), which each exist in 5
overloaded alternatives. Each alternative takes a parameter of some
integer numeric value type. Its result type is the operation type of
$T$ and $U$. The operation is evaluated by converting the receiver and
its argument to their operation type and performing the given bitwise
operation of that type.
A parameterless bit-negation method (~). Its result type is
the reciver type $T$ or Int, whichever is larger.
The operation is evaluated by converting the receiver to the result
type and negating every bit in its value.
Bit-shift methods left-shift (<<), arithmetic right-shift
(>>), and unsigned right-shift (>>>). Each of these
methods has two overloaded alternatives, which take a parameter $n$
of type Int, respectively Long. The result type of the
operation is the receiver type $T$, or Int, whichever is larger.
The operation is evaluated by converting the receiver to the result
type and performing the specified shift by $n$ bits.
Numeric value types also implement operations equals,
hashCode, and toString from class Any.
The equals method tests whether the argument is a numeric value
type. If this is true, it will perform the == operation which
is appropriate for that type. That is, the equals method of a
numeric value type can be thought of being defined as follows:
def equals(other: Any): Boolean = other match {
case that: Byte => this == that
case that: Short => this == that
case that: Char => this == that
case that: Int => this == that
case that: Long => this == that
case that: Float => this == that
case that: Double => this == that
case _ => false
}
The hashCode method returns an integer hashcode that maps equal
numeric values to equal results. It is guaranteed to be the identity for
for type Int and for all subrange types.
The toString method displays its receiver as an integer or
floating point number.
This is the signature of the numeric value type Int:
package scala
abstract sealed class Int extends AnyVal {
def == (that: Double): Boolean // double equality
def == (that: Float): Boolean // float equality
def == (that: Long): Boolean // long equality
def == (that: Int): Boolean // int equality
def == (that: Short): Boolean // int equality
def == (that: Byte): Boolean // int equality
def == (that: Char): Boolean // int equality
/* analogous for !=, <, >, <=, >= */
def + (that: Double): Double // double addition
def + (that: Float): Double // float addition
def + (that: Long): Long // long addition
def + (that: Int): Int // int addition
def + (that: Short): Int // int addition
def + (that: Byte): Int // int addition
def + (that: Char): Int // int addition
/* analogous for -, *, /, % */
def & (that: Long): Long // long bitwise and
def & (that: Int): Int // int bitwise and
def & (that: Short): Int // int bitwise and
def & (that: Byte): Int // int bitwise and
def & (that: Char): Int // int bitwise and
/* analogous for |, ^ */
def << (cnt: Int): Int // int left shift
def << (cnt: Long): Int // long left shift
/* analogous for >>, >>> */
def unary_+ : Int // int identity
def unary_- : Int // int negation
def unary_~ : Int // int bitwise negation
def toByte: Byte // convert to Byte
def toShort: Short // convert to Short
def toChar: Char // convert to Char
def toInt: Int // convert to Int
def toLong: Long // convert to Long
def toFloat: Float // convert to Float
def toDouble: Double // convert to Double
}
BooleanClass Boolean has only two values: true and
false. It implements operations as given in the following
class definition.
package scala
abstract sealed class Boolean extends AnyVal {
def && (p: => Boolean): Boolean = // boolean and
if (this) p else false
def || (p: => Boolean): Boolean = // boolean or
if (this) true else p
def & (x: Boolean): Boolean = // boolean strict and
if (this) x else false
def | (x: Boolean): Boolean = // boolean strict or
if (this) true else x
def == (x: Boolean): Boolean = // boolean equality
if (this) x else x.unary_!
def != (x: Boolean): Boolean = // boolean inequality
if (this) x.unary_! else x
def unary_!: Boolean = // boolean negation
if (this) false else true
}
The class also implements operations equals, hashCode,
and toString from class Any.
The equals method returns true if the argument is the
same boolean value as the receiver, false otherwise. The
hashCode method returns a fixed, implementation-specific hash-code when invoked on true,
and a different, fixed, implementation-specific hash-code when invoked on false. The toString method
returns the receiver converted to a string, i.e. either "true" or "false".
UnitClass Unit has only one value: (). It implements only
the three methods equals, hashCode, and toString
from class Any.
The equals method returns true if the argument is the
unit value (), false otherwise. The
hashCode method returns a fixed, implementation-specific hash-code,
The toString method returns "()".
This section presents some standard Scala reference classes which are treated in a special way in Scala compiler -- either Scala provides syntactic sugar for them, or the Scala compiler generates special code for their operations. Other classes in the standard Scala library are documented in the Scala library documentation by HTML pages.
StringScala's String class is usually derived from the standard String
class of the underlying host system (and may be identified with
it). For Scala clients the class is taken to support in each case a
method
def + (that: Any): String
which concatenates its left operand with the textual representation of its right operand.
Tuple classesScala defines tuple classes Tuple$n$ for $n = 2 , \ldots , 22$.
These are defined as follows.
package scala
case class Tuple$n$[+T_1, ..., +T_n](_1: T_1, ..., _$n$: T_$n$) {
def toString = "(" ++ _1 ++ "," ++ $\ldots$ ++ "," ++ _$n$ ++ ")"
}
The implicitly imported Predef object defines
the names Pair as an alias of Tuple2 and Triple
as an alias for Tuple3.
Function ClassesScala defines function classes Function$n$ for $n = 1 , \ldots , 22$.
These are defined as follows.
package scala
trait Function$n$[-T_1, ..., -T_$n$, +R] {
def apply(x_1: T_1, ..., x_$n$: T_$n$): R
def toString = "<function>"
}
The PartialFunction subclass of Function1 represents functions that (indirectly) specify their domain.
Use the isDefined method to query whether the partial function is defined for a given input (i.e., whether the input is part of the function's domain).
class PartialFunction[-A, +B] extends Function1[A, B] {
def isDefinedAt(x: A): Boolean
}
The implicitly imported Predef object defines the name
Function as an alias of Function1.
ArrayAll operations on arrays desugar to the corresponding operations of the underlying platform. Therefore, the following class definition is given for informational purposes only:
final class Array[T](_length: Int)
extends java.io.Serializable with java.lang.Cloneable {
def length: Int = $\ldots$
def apply(i: Int): T = $\ldots$
def update(i: Int, x: T): Unit = $\ldots$
override def clone(): Array[T] = $\ldots$
}
If $T$ is not a type parameter or abstract type, the type Array[T]
is represented as the array type |T|[] in the
underlying host system, where |T| is the erasure of T.
If $T$ is a type parameter or abstract type, a different representation might be
used (it is Object on the Java platform).
length returns the length of the array, apply means subscripting,
and update means element update.
Because of the syntactic sugar for apply and update operations,
we have the following correspondences between Scala and Java/C# code for
operations on an array xs:
| Scala | Java/C# |
xs.length |
xs.length |
xs(i) |
xs[i] |
xs(i) = e |
xs[i] = e |
Two implicit conversions exist in Predef that are frequently applied to arrays:
a conversion to scala.collection.mutable.ArrayOps and a conversion to
scala.collection.mutable.WrappedArray (a subtype of scala.collection.Seq).
Both types make many of the standard operations found in the Scala
collections API available. The conversion to ArrayOps is temporary, as all operations
defined on ArrayOps return a value of type Array, while the conversion to WrappedArray
is permanent as all operations return a value of type WrappedArray.
The conversion to ArrayOps takes priority over the conversion to WrappedArray.
Because of the tension between parametrized types in Scala and the ad-hoc implementation of arrays in the host-languages, some subtle points need to be taken into account when dealing with arrays. These are explained in the following.
Unlike arrays in Java or C#, arrays in Scala are not
co-variant; That is, $S <: T$ does not imply
Array[$S$] $<:$ Array[$T$] in Scala.
However, it is possible to cast an array
of $S$ to an array of $T$ if such a cast is permitted in the host
environment.
For instance Array[String] does not conform to
Array[Object], even though String conforms to Object.
However, it is possible to cast an expression of type
Array[String] to Array[Object], and this
cast will succeed without raising a ClassCastException. Example:
val xs = new Array[String](2)
// val ys: Array[Object] = xs // **** error: incompatible types
val ys: Array[Object] = xs.asInstanceOf[Array[Object]] // OK
The instantiation of an array with a polymorphic element type $T$ requires
information about type $T$ at runtime.
This information is synthesized by adding a context bound
of scala.reflect.ClassTag to type $T$.
An example is the
following implementation of method mkArray, which creates
an array of an arbitrary type $T$, given a sequence of $T$`s which
defines its elements:
import reflect.ClassTag
def mkArray[T : ClassTag](elems: Seq[T]): Array[T] = {
val result = new Array[T](elems.length)
var i = 0
for (elem <- elems) {
result(i) = elem
i += 1
}
result
}
If type $T$ is a type for which the host platform offers a specialized array representation, this representation is used.
On the Java Virtual Machine, an invocation of mkArray(List(1,2,3))
will return a primitive array of ints, written as int[] in Java.
Array's companion object provides various factory methods for the
instantiation of single- and multi-dimensional arrays, an extractor method
unapplySeq which enables pattern matching
over arrays and additional utility methods:
package scala
object Array {
/** copies array elements from `src` to `dest`. */
def copy(src: AnyRef, srcPos: Int,
dest: AnyRef, destPos: Int, length: Int): Unit = $\ldots$
/** Returns an array of length 0 */
def empty[T: ClassTag]: Array[T] =
/** Create an array with given elements. */
def apply[T: ClassTag](xs: T*): Array[T] = $\ldots$
/** Creates array with given dimensions */
def ofDim[T: ClassTag](n1: Int): Array[T] = $\ldots$
/** Creates a 2-dimensional array */
def ofDim[T: ClassTag](n1: Int, n2: Int): Array[Array[T]] = $\ldots$
$\ldots$
/** Concatenate all argument arrays into a single array. */
def concat[T: ClassTag](xss: Array[T]*): Array[T] = $\ldots$
/** Returns an array that contains the results of some element computation a number
* of times. */
def fill[T: ClassTag](n: Int)(elem: => T): Array[T] = $\ldots$
/** Returns a two-dimensional array that contains the results of some element
* computation a number of times. */
def fill[T: ClassTag](n1: Int, n2: Int)(elem: => T): Array[Array[T]] = $\ldots$
$\ldots$
/** Returns an array containing values of a given function over a range of integer
* values starting from 0. */
def tabulate[T: ClassTag](n: Int)(f: Int => T): Array[T] = $\ldots$
/** Returns a two-dimensional array containing values of a given function
* over ranges of integer values starting from `0`. */
def tabulate[T: ClassTag](n1: Int, n2: Int)(f: (Int, Int) => T): Array[Array[T]] = $\ldots$
$\ldots$
/** Returns an array containing a sequence of increasing integers in a range. */
def range(start: Int, end: Int): Array[Int] = $\ldots$
/** Returns an array containing equally spaced values in some integer interval. */
def range(start: Int, end: Int, step: Int): Array[Int] = $\ldots$
/** Returns an array containing repeated applications of a function to a start value. */
def iterate[T: ClassTag](start: T, len: Int)(f: T => T): Array[T] = $\ldots$
/** Enables pattern matching over arrays */
def unapplySeq[A](x: Array[A]): Option[IndexedSeq[A]] = Some(x)
}
package scala.xml
trait Node {
/** the label of this node */
def label: String
/** attribute axis */
def attribute: Map[String, String]
/** child axis (all children of this node) */
def child: Seq[Node]
/** descendant axis (all descendants of this node) */
def descendant: Seq[Node] = child.toList.flatMap {
x => x::x.descendant.asInstanceOf[List[Node]]
}
/** descendant axis (all descendants of this node) */
def descendant_or_self: Seq[Node] = this::child.toList.flatMap {
x => x::x.descendant.asInstanceOf[List[Node]]
}
override def equals(x: Any): Boolean = x match {
case that:Node =>
that.label == this.label &&
that.attribute.sameElements(this.attribute) &&
that.child.sameElements(this.child)
case _ => false
}
/** XPath style projection function. Returns all children of this node
* that are labeled with 'that'. The document order is preserved.
*/
def \(that: Symbol): NodeSeq = {
new NodeSeq({
that.name match {
case "_" => child.toList
case _ =>
var res:List[Node] = Nil
for (x <- child.elements if x.label == that.name) {
res = x::res
}
res.reverse
}
})
}
/** XPath style projection function. Returns all nodes labeled with the
* name 'that' from the 'descendant_or_self' axis. Document order is preserved.
*/
def \\(that: Symbol): NodeSeq = {
new NodeSeq(
that.name match {
case "_" => this.descendant_or_self
case _ => this.descendant_or_self.asInstanceOf[List[Node]].
filter(x => x.label == that.name)
})
}
/** hashcode for this XML node */
override def hashCode =
Utility.hashCode(label, attribute.toList.hashCode, child)
/** string representation of this node */
override def toString = Utility.toXML(this)
}
Predef ObjectThe Predef object defines standard functions and type aliases
for Scala programs. It is always implicitly imported, so that all its
defined members are available without qualification. Its definition
for the JVM environment conforms to the following signature:
package scala
object Predef {
// classOf ---------------------------------------------------------
/** Returns the runtime representation of a class type. */
def classOf[T]: Class[T] = null
// this is a dummy, classOf is handled by compiler.
// Standard type aliases ---------------------------------------------
type String = java.lang.String
type Class[T] = java.lang.Class[T]
// Miscellaneous -----------------------------------------------------
type Function[-A, +B] = Function1[A, B]
type Map[A, +B] = collection.immutable.Map[A, B]
type Set[A] = collection.immutable.Set[A]
val Map = collection.immutable.Map
val Set = collection.immutable.Set
// Manifest types, companions, and incantations for summoning ---------
type ClassManifest[T] = scala.reflect.ClassManifest[T]
type Manifest[T] = scala.reflect.Manifest[T]
type OptManifest[T] = scala.reflect.OptManifest[T]
val ClassManifest = scala.reflect.ClassManifest
val Manifest = scala.reflect.Manifest
val NoManifest = scala.reflect.NoManifest
def manifest[T](implicit m: Manifest[T]) = m
def classManifest[T](implicit m: ClassManifest[T]) = m
def optManifest[T](implicit m: OptManifest[T]) = m
// Minor variations on identity functions -----------------------------
def identity[A](x: A): A = x // @see `conforms` for the implicit version
def implicitly[T](implicit e: T) = e // for summoning implicit values from the nether world
@inline def locally[T](x: T): T = x // to communicate intent and avoid unmoored statements
// Asserts, Preconditions, Postconditions -----------------------------
def assert(assertion: Boolean) {
if (!assertion)
throw new java.lang.AssertionError("assertion failed")
}
def assert(assertion: Boolean, message: => Any) {
if (!assertion)
throw new java.lang.AssertionError("assertion failed: " + message)
}
def assume(assumption: Boolean) {
if (!assumption)
throw new IllegalArgumentException("assumption failed")
}
def assume(assumption: Boolean, message: => Any) {
if (!assumption)
throw new IllegalArgumentException(message.toString)
}
def require(requirement: Boolean) {
if (!requirement)
throw new IllegalArgumentException("requirement failed")
}
def require(requirement: Boolean, message: => Any) {
if (!requirement)
throw new IllegalArgumentException("requirement failed: "+ message)
}
// tupling ---------------------------------------------------------
type Pair[+A, +B] = Tuple2[A, B]
object Pair {
def apply[A, B](x: A, y: B) = Tuple2(x, y)
def unapply[A, B](x: Tuple2[A, B]): Option[Tuple2[A, B]] = Some(x)
}
type Triple[+A, +B, +C] = Tuple3[A, B, C]
object Triple {
def apply[A, B, C](x: A, y: B, z: C) = Tuple3(x, y, z)
def unapply[A, B, C](x: Tuple3[A, B, C]): Option[Tuple3[A, B, C]] = Some(x)
}
// Printing and reading -----------------------------------------------
def print(x: Any) = Console.print(x)
def println() = Console.println()
def println(x: Any) = Console.println(x)
def printf(text: String, xs: Any*) = Console.printf(text.format(xs: _*))
def readLine(): String = Console.readLine()
def readLine(text: String, args: Any*) = Console.readLine(text, args)
def readBoolean() = Console.readBoolean()
def readByte() = Console.readByte()
def readShort() = Console.readShort()
def readChar() = Console.readChar()
def readInt() = Console.readInt()
def readLong() = Console.readLong()
def readFloat() = Console.readFloat()
def readDouble() = Console.readDouble()
def readf(format: String) = Console.readf(format)
def readf1(format: String) = Console.readf1(format)
def readf2(format: String) = Console.readf2(format)
def readf3(format: String) = Console.readf3(format)
// Implict conversions ------------------------------------------------
...
}
The Predef object also contains a number of implicit definitions, which are available by default (because Predef is implicitly imported).
Implicit definitions come in two priorities. High-priority implicits are defined in the Predef class itself whereas low priority implicits are defined in a class inherited by Predef. The rules of
static overloading resolution
stipulate that, all other things being equal, implicit resolution
prefers high-priority implicits over low-priority ones.
The available low-priority implicits include definitions falling into the following categories.
For every primitive type, a wrapper that takes values of that type
to instances of a runtime.Rich* class. For instance, values of type Int
can be implicitly converted to instances of class runtime.RichInt.
For every array type with elements of primitive type, a wrapper that
takes the arrays of that type to instances of a runtime.WrappedArray class. For instance, values of type Array[Float] can be implicitly converted to instances of class runtime.WrappedArray[Float].
There are also generic array wrappers that take elements
of type Array[T] for arbitrary T to WrappedArrays.
An implicit conversion from String to WrappedString.
The available high-priority implicits include definitions falling into the following categories.
An implicit wrapper that adds ensuring methods
with the following overloaded variants to type Any.
def ensuring(cond: Boolean): A = { assert(cond); x }
def ensuring(cond: Boolean, msg: Any): A = { assert(cond, msg); x }
def ensuring(cond: A => Boolean): A = { assert(cond(x)); x }
def ensuring(cond: A => Boolean, msg: Any): A = { assert(cond(x), msg); x }
An implicit wrapper that adds a -> method with the following implementation
to type Any.
def -> [B](y: B): (A, B) = (x, y)
For every array type with elements of primitive type, a wrapper that
takes the arrays of that type to instances of a runtime.ArrayOps
class. For instance, values of type Array[Float] can be implicitly
converted to instances of class runtime.ArrayOps[Float]. There are
also generic array wrappers that take elements of type Array[T] for
arbitrary T to ArrayOpss.
An implicit wrapper that adds + and formatted method with the following
implementations to type Any.
def +(other: String) = String.valueOf(self) + other
def formatted(fmtstr: String): String = fmtstr format self
Numeric primitive conversions that implement the transitive closure of the following mappings:
Byte -> Short
Short -> Int
Char -> Int
Int -> Long
Long -> Float
Float -> Double
Boxing and unboxing conversions between primitive types and their boxed versions:
Byte <-> java.lang.Byte
Short <-> java.lang.Short
Char <-> java.lang.Character
Int <-> java.lang.Integer
Long <-> java.lang.Long
Float <-> java.lang.Float
Double <-> java.lang.Double
Boolean <-> java.lang.Boolean
An implicit definition that generates instances of type T <:< T, for
any type T. Here, <:< is a class defined as follows.
sealed abstract class <:<[-From, +To] extends (From => To)
Implicit parameters of <:< types are typically used to implement type constraints.