object Predef extends LowPriorityImplicits
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.
- Source
- Predef.scala
- Grouped
- Alphabetic
- By Inheritance
- Predef
- LowPriorityImplicits
- LowPriorityImplicits2
- AnyRef
- Any
- Hide All
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- Public
- Protected
Utility Methods
- def ???: Nothing
???
can be used for marking methods that remain to be implemented.???
can be used for marking methods that remain to be implemented.- Exceptions thrown
NotImplementedError
when???
is invoked.
- def classOf[T]: Class[T]
Retrieve the runtime representation of a class type.
Retrieve the runtime representation of a class type.
classOf[T]
is equivalent to the class literalT.class
in Java.- returns
The runtime Class representation of type
T
.
val listClass = classOf[List[_]] // listClass is java.lang.Class[List[_]] = class scala.collection.immutable.List val mapIntString = classOf[Map[Int,String]] // mapIntString is java.lang.Class[Map[Int,String]] = interface scala.collection.immutable.Map
Example: - def identity[A](x: A): A
A method that returns its input value.
A method that returns its input value.
- A
type of the input value x.
- x
the value of type
A
to be returned.- returns
the value
x
.
- Annotations
- @inline()
- def implicitly[T](implicit e: T): T
Summon an implicit value of type
T
.Summon an implicit value of type
T
. Usually, the argument is not passed explicitly.- T
the type of the value to be summoned
- returns
the implicit value of type
T
- Annotations
- @inline()
- def locally[T](x: T): T
Used to mark code blocks as being expressions, instead of being taken as part of anonymous classes and the like.
Used to mark code blocks as being expressions, instead of being taken as part of anonymous classes and the like. This is just a different name for identity.
- Annotations
- @inline()
Separating code blocks from
new
:val x = new AnyRef { val y = ... println(y) } // the { ... } block is seen as the body of an anonymous class val x = new AnyRef { val y = ... println(y) } // an empty line is a brittle "fix" val x = new AnyRef locally { val y = ... println(y) } // locally guards the block and helps communicate intent
Example: - def valueOf[T](implicit vt: ValueOf[T]): T
Retrieve the single value of a type with a unique inhabitant.
Retrieve the single value of a type with a unique inhabitant.
- Annotations
- @inline()
object Foo val foo = valueOf[Foo.type] // foo is Foo.type = Foo val bar = valueOf[23] // bar is 23.type = 23
Example:
Assertions
These methods support program verification and runtime correctness.
- final def assert(assertion: Boolean, message: => Any): Unit
Tests an expression, throwing an
AssertionError
if false. - def assert(assertion: Boolean): Unit
Tests an expression, throwing an
AssertionError
if false. - final def assume(assumption: Boolean, message: => Any): Unit
Tests an expression, throwing an
AssertionError
if false.Tests an expression, throwing an
AssertionError
if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if-Xelide-below
is greater thanASSERTION
.- assumption
the expression to test
- message
a String to include in the failure message
- def assume(assumption: Boolean): Unit
Tests an expression, throwing an
AssertionError
if false.Tests an expression, throwing an
AssertionError
if false. This method differs from assert only in the intent expressed: assert contains a predicate which needs to be proven, while assume contains an axiom for a static checker. Calls to this method will not be generated if-Xelide-below
is greater thanASSERTION
.- assumption
the expression to test
- final def require(requirement: Boolean, message: => Any): Unit
Tests an expression, throwing an
IllegalArgumentException
if false.Tests an expression, throwing an
IllegalArgumentException
if false. This method is similar toassert
, but blames the caller of the method for violating the condition.- requirement
the expression to test
- message
a String to include in the failure message
- Annotations
- @inline()
- def require(requirement: Boolean): Unit
Tests an expression, throwing an
IllegalArgumentException
if false.Tests an expression, throwing an
IllegalArgumentException
if false. This method is similar toassert
, but blames the caller of the method for violating the condition.- requirement
the expression to test
Console Output
These methods provide output via the console.
- def print(x: Any): Unit
Prints an object to
out
using itstoString
method.Prints an object to
out
using itstoString
method.- x
the object to print; may be null.
- def printf(text: String, xs: Any*): Unit
Prints its arguments as a formatted string to the default output, based on a string pattern (in a fashion similar to printf in C).
Prints its arguments as a formatted string to the default output, based on a string pattern (in a fashion similar to printf in C).
The interpretation of the formatting patterns is described in java.util.Formatter.
Consider using the f interpolator as more type safe and idiomatic.
- text
the pattern for formatting the arguments.
- xs
the arguments used to instantiate the pattern.
- Exceptions thrown
java.lang.IllegalArgumentException
if there was a problem with the format string or arguments- See also
- def println(x: Any): Unit
Prints out an object to the default output, followed by a newline character.
Prints out an object to the default output, followed by a newline character.
- x
the object to print.
- def println(): Unit
Prints a newline character on the default output.
Aliases
These aliases bring selected immutable types into scope without any imports.
- type Class[T] = java.lang.Class[T]
- type Function[-A, +B] = (A) => B
- type Map[K, +V] = collection.immutable.Map[K, V]
- type Set[A] = collection.immutable.Set[A]
- type String = java.lang.String
The
String
type in Scala has all the methods of the underlyingjava.lang.String
, of which it is just an alias.The
String
type in Scala has all the methods of the underlyingjava.lang.String
, of which it is just an alias. (See the documentation corresponding to your Java version, for example https://docs.oracle.com/javase/8/docs/api/java/lang/String.html.) In addition, extension methods in scala.collection.StringOps are added implicitly through the conversion augmentString.
- val ->: Tuple2.type
Allows destructuring tuples with the same syntax as constructing them.
Allows destructuring tuples with the same syntax as constructing them.
val tup = "foobar" -> 3 val c = tup match { case str -> i => str.charAt(i) }
Example: - val Map: collection.immutable.Map.type
- val Set: collection.immutable.Set.type
String Conversions
Conversions from String to StringOps or WrappedString.
Implicit Classes
These implicit classes add useful extension methods to every type.
- implicit final class ArrowAssoc[A] extends AnyVal
- implicit final class Ensuring[A] extends AnyVal
- implicit final class StringFormat[A] extends AnyVal
- implicit final class any2stringadd[A] extends AnyVal
Injects String concatenation operator
+
to any classes.Injects String concatenation operator
+
to any classes.- Annotations
- @deprecated
- Deprecated
(Since version 2.13.0) Implicit injection of + is deprecated. Convert to String to call +
CharSequence Wrappers
Wrappers that implements CharSequence and were implicit classes.
- final class ArrayCharSequence extends CharSequence
- final class SeqCharSequence extends CharSequence
- def ArrayCharSequence(arrayOfChars: Array[Char]): ArrayCharSequence
- def SeqCharSequence(sequenceOfChars: collection.IndexedSeq[Char]): SeqCharSequence
Java to Scala
Implicit conversion from Java primitive wrapper types to Scala equivalents.
- implicit def Boolean2boolean(x: java.lang.Boolean): Boolean
- implicit def Byte2byte(x: java.lang.Byte): Byte
- implicit def Character2char(x: Character): Char
- implicit def Double2double(x: java.lang.Double): Double
- implicit def Float2float(x: java.lang.Float): Float
- implicit def Integer2int(x: Integer): Int
- implicit def Long2long(x: java.lang.Long): Long
- implicit def Short2short(x: java.lang.Short): Short
Scala to Java
Implicit conversion from Scala AnyVals to Java primitive wrapper types equivalents.
- implicit def boolean2Boolean(x: Boolean): java.lang.Boolean
- implicit def byte2Byte(x: Byte): java.lang.Byte
- implicit def char2Character(x: Char): Character
- implicit def double2Double(x: Double): java.lang.Double
- implicit def float2Float(x: Float): java.lang.Float
- implicit def int2Integer(x: Int): Integer
- implicit def long2Long(x: Long): java.lang.Long
- implicit def short2Short(x: Short): java.lang.Short
Array to ArraySeq
Conversions from Arrays to ArraySeqs.
- implicit def genericWrapArray[T](xs: Array[T]): ArraySeq[T]
- Definition Classes
- LowPriorityImplicits
- implicit def wrapBooleanArray(xs: Array[Boolean]): ofBoolean
- Definition Classes
- LowPriorityImplicits
- implicit def wrapByteArray(xs: Array[Byte]): ofByte
- Definition Classes
- LowPriorityImplicits
- implicit def wrapCharArray(xs: Array[Char]): ofChar
- Definition Classes
- LowPriorityImplicits
- implicit def wrapDoubleArray(xs: Array[Double]): ofDouble
- Definition Classes
- LowPriorityImplicits
- implicit def wrapFloatArray(xs: Array[Float]): ofFloat
- Definition Classes
- LowPriorityImplicits
- implicit def wrapIntArray(xs: Array[Int]): ofInt
- Definition Classes
- LowPriorityImplicits
- implicit def wrapLongArray(xs: Array[Long]): ofLong
- Definition Classes
- LowPriorityImplicits
- implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): ofRef[T]
- Definition Classes
- LowPriorityImplicits
- implicit def wrapShortArray(xs: Array[Short]): ofShort
- Definition Classes
- LowPriorityImplicits
- implicit def wrapUnitArray(xs: Array[Unit]): ofUnit
- Definition Classes
- LowPriorityImplicits
Ungrouped
- type Manifest[T] = reflect.Manifest[T]
- Annotations
- @implicitNotFound(msg = "No Manifest available for ${T}.")
- type OptManifest[T] = reflect.OptManifest[T]
- implicit def $conforms[A]: (A) => A
An implicit of type
A => A
is available for allA
because it can always be implemented using the identity function.An implicit of type
A => A
is available for allA
because it can always be implemented using the identity function. This also means that an implicit of typeA => B
is always available whenA <: B
, because(A => A) <: (A => B)
. - val Manifest: reflect.Manifest.type
- val NoManifest: reflect.NoManifest.type
- implicit def booleanArrayOps(xs: Array[Boolean]): ArrayOps[Boolean]
- Annotations
- @inline()
- implicit def booleanWrapper(x: Boolean): RichBoolean
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def byteArrayOps(xs: Array[Byte]): ArrayOps[Byte]
- Annotations
- @inline()
- implicit def byteWrapper(x: Byte): RichByte
We prefer the java.lang.* boxed types to these wrappers in any potential conflicts.
We prefer the java.lang.* boxed types to these wrappers in any potential conflicts. Conflicts do exist because the wrappers need to implement ScalaNumber in order to have a symmetric equals method, but that implies implementing java.lang.Number as well.
Note - these are inlined because they are value classes, but the call to xxxWrapper is not eliminated even though it does nothing. Even inlined, every call site does a no-op retrieval of Predef's MODULE$ because maybe loading Predef has side effects!
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def charArrayOps(xs: Array[Char]): ArrayOps[Char]
- Annotations
- @inline()
- implicit def charWrapper(c: Char): RichChar
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def doubleArrayOps(xs: Array[Double]): ArrayOps[Double]
- Annotations
- @inline()
- implicit def doubleWrapper(x: Double): RichDouble
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def floatArrayOps(xs: Array[Float]): ArrayOps[Float]
- Annotations
- @inline()
- implicit def floatWrapper(x: Float): RichFloat
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def genericArrayOps[T](xs: Array[T]): ArrayOps[T]
- Annotations
- @inline()
- implicit def intArrayOps(xs: Array[Int]): ArrayOps[Int]
- Annotations
- @inline()
- implicit def intWrapper(x: Int): RichInt
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def longArrayOps(xs: Array[Long]): ArrayOps[Long]
- Annotations
- @inline()
- implicit def longWrapper(x: Long): RichLong
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- def manifest[T](implicit m: Manifest[T]): Manifest[T]
- def optManifest[T](implicit m: OptManifest[T]): OptManifest[T]
- implicit def refArrayOps[T <: AnyRef](xs: Array[T]): ArrayOps[T]
- Annotations
- @inline()
- implicit def shortArrayOps(xs: Array[Short]): ArrayOps[Short]
- Annotations
- @inline()
- implicit def shortWrapper(x: Short): RichShort
- Definition Classes
- LowPriorityImplicits
- Annotations
- @inline()
- implicit def tuple2ToZippedOps[T1, T2](x: (T1, T2)): Ops[T1, T2]
- implicit def tuple3ToZippedOps[T1, T2, T3](x: (T1, T2, T3)): Ops[T1, T2, T3]
- implicit def unitArrayOps(xs: Array[Unit]): ArrayOps[Unit]
- Annotations
- @inline()
- implicit def copyArrayToImmutableIndexedSeq[T](xs: Array[T]): IndexedSeq[T]
- Definition Classes
- LowPriorityImplicits2
- Annotations
- @deprecated
- Deprecated
(Since version 2.13.0) Implicit conversions from Array to immutable.IndexedSeq are implemented by copying; Use the more efficient non-copying ArraySeq.unsafeWrapArray or an explicit toIndexedSeq call
This is the documentation for the Scala standard library.
Package structure
The scala package contains core types like
Int
,Float
,Array
orOption
which are accessible in all Scala compilation units without explicit qualification or imports.Notable packages include:
scala.collection
and its sub-packages contain Scala's collections frameworkscala.collection.immutable
- Immutable, sequential data-structures such asVector
,List
,Range
,HashMap
orHashSet
scala.collection.mutable
- Mutable, sequential data-structures such asArrayBuffer
,StringBuilder
,HashMap
orHashSet
scala.collection.concurrent
- Mutable, concurrent data-structures such asTrieMap
scala.concurrent
- Primitives for concurrent programming such asFutures
andPromises
scala.io
- Input and output operationsscala.math
- Basic math functions and additional numeric types likeBigInt
andBigDecimal
scala.sys
- Interaction with other processes and the operating systemscala.util.matching
- Regular expressionsOther packages exist. See the complete list on the right.
Additional parts of the standard library are shipped as separate libraries. These include:
scala.reflect
- Scala's reflection API (scala-reflect.jar)scala.xml
- XML parsing, manipulation, and serialization (scala-xml.jar)scala.collection.parallel
- Parallel collections (scala-parallel-collections.jar)scala.util.parsing
- Parser combinators (scala-parser-combinators.jar)scala.swing
- A convenient wrapper around Java's GUI framework called Swing (scala-swing.jar)Automatic imports
Identifiers in the scala package and the
scala.Predef
object are always in scope by default.Some of these identifiers are type aliases provided as shortcuts to commonly used classes. For example,
List
is an alias forscala.collection.immutable.List
.Other aliases refer to classes provided by the underlying platform. For example, on the JVM,
String
is an alias forjava.lang.String
.