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Scala 3/scala/Predef

Predef

scala.Predef

objectPredef

ThePredef 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 typesscala.collection.immutable.Map andscala.collection.immutable.Set.

Console Output

For basic console output,Predef provides convenience methodsprint andprintln, which are aliases of the methods in the objectscala.Console.

Assertions

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

Variants ofassert intended for use with static analysis tools are also provided:assume,require andensuring.require andensuring 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 ofaddNaturals 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 fromassert in that if the condition fails, then the caller of the function is to blame rather than a logical error having been made withinaddNaturals itself.ensuring is a form ofassert 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 typescala.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 ofscala.Array.

Attributes

Source: Predef.scala
Graph
Supertypes: classObject
traitMatchable
classAny
Self type: Predef.type

Members list

Grouped members

Utility Methods

def???:Nothing

??? can be used for marking methods that remain to be implemented.

Attributes

Throws: NotImplementedErrorwhen??? is invoked.
Source: Predef.scala

defclassOf[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.

Attributes

Returns

The runtimeClass representation of typeT.

Example

val listClass = classOf[List[_]]// listClass is java.lang.Class[List[_]] = class scala.collection.immutable.Listval mapIntString = classOf[Map[Int,String]]// mapIntString is java.lang.Class[Map[Int,String]] = interface scala.collection.immutable.Map

Source

Predef.scala

defidentity[A](x:A):A

A method that returns its input value.

Type parameters

A: type of the input value x.

Value parameters

x: the value of typeA to be returned.

Attributes

Returns: the valuex.
Source: Predef.scala

defimplicitly[T](implicite:T):T

Summon an implicit value of typeT.

Summon an implicit value of typeT. Usually, the argument is not passed explicitly.

Type parameters

T: the type of the value to be summoned

Attributes

Returns: the implicit value of typeT
Source: Predef.scala

deflocally[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 foridentity.

Attributes

Example

Separating code blocks fromnew:

val x = new AnyRef{  val y = ...  println(y)}// the { ... } block is seen as the body of an anonymous classval x = new AnyRef{  val y = ...  println(y)}// an empty line is a brittle "fix"val x = new AnyReflocally {  val y = ...  println(y)}// locally guards the block and helps communicate intent

Source

Predef.scala

defvalueOf[T](implicitvt:ValueOf[T]):T

Retrieve the single value of a type with a unique inhabitant.

Attributes

Example

object Fooval foo = valueOf[Foo.type]// foo is Foo.type = Fooval bar = valueOf[23]// bar is 23.type = 23

Source

Predef.scala

Assertions

These methods support program verification and runtime correctness.

defassert(assertion:Boolean):Unit

Tests an expression, throwing anAssertionError if false.

Tests an expression, throwing anAssertionError if false. Calls to this method will not be generated if-Xelide-below is greater thanASSERTION.

Value parameters

assertion: the expression to test

Attributes

See also: elidable
Source: Predef.scala

finaldefassert(assertion:Boolean,message:=>Any):Unit

Tests an expression, throwing anAssertionError if false.

Tests an expression, throwing anAssertionError if false. Calls to this method will not be generated if-Xelide-below is greater thanASSERTION.

Value parameters

assertion: the expression to test
message: a String to include in the failure message

Attributes

See also: elidable
Source: Predef.scala

defassume(assumption:Boolean):Unit

Tests an expression, throwing anAssertionError if false.

Tests an expression, throwing anAssertionError 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.

Value parameters

assumption: the expression to test

Attributes

See also: elidable
Source: Predef.scala

finaldefassume(assumption:Boolean,message:=>Any):Unit

Tests an expression, throwing anAssertionError if false.

Value parameters

assumption: the expression to test
message: a String to include in the failure message

Attributes

See also: elidable
Source: Predef.scala

defrequire(requirement:Boolean):Unit

Tests an expression, throwing anIllegalArgumentException if false.

Tests an expression, throwing anIllegalArgumentException if false. This method is similar toassert, but blames the caller of the method for violating the condition.

Value parameters

requirement: the expression to test

Attributes

Source: Predef.scala

finaldefrequire(requirement:Boolean,message:=>Any):Unit

Tests an expression, throwing anIllegalArgumentException if false.

Tests an expression, throwing anIllegalArgumentException if false. This method is similar toassert, but blames the caller of the method for violating the condition.

Value parameters

message: a String to include in the failure message
requirement: the expression to test

Attributes

Source: Predef.scala

Console Output

These methods provide output via the console.

defprint(x:Any):Unit

Prints an object toout using itstoString method.

Value parameters

x: the object to print; may be null.

Attributes

Source: Predef.scala

defprintf(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).

The interpretation of the formatting patterns is described injava.util.Formatter.

Consider using thef interpolator as more type safe and idiomatic.

Value parameters

text: the pattern for formatting the arguments.
xs: the arguments used to instantiate the pattern.

Attributes

Throws: java.lang.IllegalArgumentException if there was a problem with the format string or arguments
See also: StringContext.f
Source: Predef.scala

defprintln():Unit

Prints a newline character on the default output.

Attributes

Source: Predef.scala

defprintln(x:Any):Unit

Prints out an object to the default output, followed by a newline character.

Value parameters

x: the object to print.

Attributes

Source: Predef.scala

Aliases

These aliases bring selected immutable types into scope without any imports.

val->:Tuple2.type

Allows destructuring tuples with the same syntax as constructing them.

Attributes

Example

val tup = "foobar" -> 3val c = tup match { case str -> i => str.charAt(i)}

Source

Predef.scala

typeClass[T] =Class[T]

Attributes

Source: Predef.scala

typeFunction[-A,+B] =A=>B

Attributes

Source: Predef.scala

typeMap[K,+V] =Map[K,V]

Attributes

Source: Predef.scala

valMap:Map.type

Attributes

Source: Predef.scala

typeSet[A] =Set[A]

Attributes

Source: Predef.scala

valSet:Set.type

Attributes

Source: Predef.scala

typeString =String

TheString type in Scala has all the methods of the underlyingjava.lang.String, of which it is just an alias.

In addition, extension methods inscala.collection.StringOps are added implicitly through the conversionaugmentString.

Attributes

Source: Predef.scala

String Conversions

Conversions from String to StringOps or WrappedString.

implicitdefaugmentString(x:String):StringOps

Attributes

Source: Predef.scala

implicitdefwrapString(s:String):WrappedString

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

Implicit Classes

These implicit classes add useful extension methods to every type.

final implicitclassArrowAssoc[A](self:A) extendsAnyVal

Attributes

Source: Predef.scala
Supertypes: classAnyVal
traitMatchable
classAny

final implicitdefArrowAssoc[A](self:A):ArrowAssoc[A]

Attributes

Source: Predef.scala

final implicitclassEnsuring[A](self:A) extendsAnyVal

Attributes

Source: Predef.scala
Supertypes: classAnyVal
traitMatchable
classAny

final implicitdefEnsuring[A](self:A):Ensuring[A]

Attributes

Source: Predef.scala

final implicitclassStringFormat[A](self:A) extendsAnyVal

Attributes

Source: Predef.scala
Supertypes: classAnyVal
traitMatchable
classAny

final implicitdefStringFormat[A](self:A):StringFormat[A]

Attributes

Source: Predef.scala

final implicitclassany2stringadd[A](self:A) extendsAnyVal

Injects String concatenation operator+ to any classes.

Attributes

Deprecated: [Since version 2.13.0]Implicit injection of + is deprecated. Convert to String to call +
Source: Predef.scala
Supertypes: classAnyVal
traitMatchable
classAny

final implicitdefany2stringadd[A](self:A):any2stringadd[A]

Injects String concatenation operator+ to any classes.

Attributes

Deprecated: [Since version 2.13.0]Implicit injection of + is deprecated. Convert to String to call +
Source: Predef.scala

CharSequence Wrappers

Wrappers that implements CharSequence and were implicit classes.

finalclassArrayCharSequence(arrayOfChars:Array[Char]) extendsCharSequence

Attributes

Source: Predef.scala
Supertypes: traitCharSequence
classObject
traitMatchable
classAny

defArrayCharSequence(arrayOfChars:Array[Char]):ArrayCharSequence

Attributes

Source: Predef.scala

finalclassSeqCharSequence(sequenceOfChars:IndexedSeq[Char]) extendsCharSequence

Attributes

Source: Predef.scala
Supertypes: traitCharSequence
classObject
traitMatchable
classAny

defSeqCharSequence(sequenceOfChars:IndexedSeq[Char]):SeqCharSequence

Attributes

Source: Predef.scala

Java to Scala

Implicit conversion from Java primitive wrapper types to Scala equivalents.

implicitdefBoolean2boolean(x:Boolean):Boolean

Attributes

Source: Predef.scala

implicitdefByte2byte(x:Byte):Byte

Attributes

Source: Predef.scala

implicitdefCharacter2char(x:Character):Char

Attributes

Source: Predef.scala

implicitdefDouble2double(x:Double):Double

Attributes

Source: Predef.scala

implicitdefFloat2float(x:Float):Float

Attributes

Source: Predef.scala

implicitdefInteger2int(x:Integer):Int

Attributes

Source: Predef.scala

implicitdefLong2long(x:Long):Long

Attributes

Source: Predef.scala

implicitdefShort2short(x:Short):Short

Attributes

Source: Predef.scala

Scala to Java

Implicit conversion from Scala AnyVals to Java primitive wrapper types equivalents.

implicitdefboolean2Boolean(x:Boolean):Boolean

Attributes

Source: Predef.scala

implicitdefbyte2Byte(x:Byte):Byte

Attributes

Source: Predef.scala

implicitdefchar2Character(x:Char):Character

Attributes

Source: Predef.scala

implicitdefdouble2Double(x:Double):Double

Attributes

Source: Predef.scala

implicitdeffloat2Float(x:Float):Float

Attributes

Source: Predef.scala

implicitdefint2Integer(x:Int):Integer

Attributes

Source: Predef.scala

implicitdeflong2Long(x:Long):Long

Attributes

Source: Predef.scala

implicitdefshort2Short(x:Short):Short

Attributes

Source: Predef.scala

Array to ArraySeq

Conversions from Arrays to ArraySeqs.

implicitdefgenericWrapArray[T](xs:Array[T]):ArraySeq[T]

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapBooleanArray(xs:Array[Boolean]):ofBoolean

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapByteArray(xs:Array[Byte]):ofByte

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapCharArray(xs:Array[Char]):ofChar

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapDoubleArray(xs:Array[Double]):ofDouble

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapFloatArray(xs:Array[Float]):ofFloat

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapIntArray(xs:Array[Int]):ofInt

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapLongArray(xs:Array[Long]):ofLong

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapRefArray[T <:AnyRef](xs:Array[T]):ofRef[T]

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapShortArray(xs:Array[Short]):ofShort

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefwrapUnitArray(xs:Array[Unit]):ofUnit

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

Type members

Types

typeManifest[T] =Manifest[T]

Attributes

Source: Predef.scala

typeOptManifest[T] =OptManifest[T]

Attributes

Source: Predef.scala

infixtypeis[A <:AnyKind,B] =B {typeSelf =A; }

A type supporting Self-based type classes.

A is TC

expands to

TC { type Self = A }

which is what is needed for a context bound[A: TC].

Attributes

Source: Predef.scala

Value members

Concrete methods

defmanifest[T](implicitm:Manifest[T]):Manifest[T]

Attributes

Source: Predef.scala

defoptManifest[T](implicitm:OptManifest[T]):OptManifest[T]

Attributes

Source: Predef.scala

transparent inlinedefsummon[T](usingx:T): x.type

Summon a given value of typeT. Usually, the argument is not passed explicitly.

Type parameters

T: the type of the value to be summoned

Attributes

Returns: the given value typed: the provided type parameter
Source: Predef.scala

Concrete fields

valManifest:Manifest.type

Attributes

Source: Predef.scala

valNoManifest:NoManifest.type

Attributes

Source: Predef.scala

Extensions

extension[T](x:T |Null)

inlinedefnn: x.type &T

Strips away the nullability from a value. Note that.nn performs a checked cast, so if invoked on anull value it will throw anNullPointerException.

Attributes

Example

val s1: String | Null = "hello"val s2: String = s1.nnval s3: String | Null = nullval s4: String = s3.nn // throw NullPointerException

Source

Predef.scala

extension(inlinex:AnyRef |Null)

infix inlinedefeq(inliney:AnyRef |Null):Boolean

Enables an expression of typeT|Null, whereT is a subtype ofAnyRef, to be checked fornull usingeq rather than only==. This is needed becauseNull no longer haseq orne methods, only== and!= inherited fromAny.

Attributes

Source: Predef.scala

infix inlinedefne(inliney:AnyRef |Null):Boolean

Enables an expression of typeT|Null, whereT is a subtype ofAnyRef, to be checked fornull usingne rather than only!=. This is needed becauseNull no longer haseq orne methods, only== and!= inherited fromAny.

Attributes

Source: Predef.scala

Experimental extensions

extension(opt:Option.type)

inlinedeffromNullable[T](t:T |Null):Option[T]

Attributes

Experimental: true
Source: Predef.scala

extension[T](x:T)

inlinedefruntimeChecked: x.type

Asserts that a term should be exempt from static checks that can be reliably checked at runtime.

Attributes

Example

val xs: Option[Int] = Option(1)xs.runtimeChecked match  case Some(x) => x // `Some(_)` can be checked at runtime, so no warning

val xs: List[Int] = List(1,2,3)val y :: ys = xs.runtimeChecked // `_ :: _` can be checked at runtime, so no warning

Experimental

true

Source

Predef.scala

Implicits

implicitdef$conforms[A]:A=>A

An implicit of typeA => A is available for allA because it can always be implemented using the identity function.

An implicit of typeA => 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).

Attributes

Source: Predef.scala

implicitdefbooleanArrayOps(xs:Array[Boolean]):ArrayOps[Boolean]

Attributes

Source: Predef.scala

implicitdefbyteArrayOps(xs:Array[Byte]):ArrayOps[Byte]

Attributes

Source: Predef.scala

implicitdefcharArrayOps(xs:Array[Char]):ArrayOps[Char]

Attributes

Source: Predef.scala

implicitdefdoubleArrayOps(xs:Array[Double]):ArrayOps[Double]

Attributes

Source: Predef.scala

implicitdeffloatArrayOps(xs:Array[Float]):ArrayOps[Float]

Attributes

Source: Predef.scala

implicitdefgenericArrayOps[T](xs:Array[T]):ArrayOps[T]

Attributes

Source: Predef.scala

implicitdefintArrayOps(xs:Array[Int]):ArrayOps[Int]

Attributes

Source: Predef.scala

implicitdeflongArrayOps(xs:Array[Long]):ArrayOps[Long]

Attributes

Source: Predef.scala

implicitdefrefArrayOps[T <:AnyRef](xs:Array[T]):ArrayOps[T]

Attributes

Source: Predef.scala

implicitdefshortArrayOps(xs:Array[Short]):ArrayOps[Short]

Attributes

Source: Predef.scala

implicitdeftuple2ToZippedOps[T1,T2](x: (T1,T2)):Ops[T1,T2]

Attributes

Source: Predef.scala

implicitdeftuple3ToZippedOps[T1,T2,T3](x: (T1,T2,T3)):Ops[T1,T2,T3]

Attributes

Source: Predef.scala

implicitdefunitArrayOps(xs:Array[Unit]):ArrayOps[Unit]

Attributes

Source: Predef.scala

Inherited implicits

implicitdefbooleanWrapper(x:Boolean):RichBoolean

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefbyteWrapper(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!

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefcharWrapper(c:Char):RichChar

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefdoubleWrapper(x:Double):RichDouble

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdeffloatWrapper(x:Float):RichFloat

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefintWrapper(x:Int):RichInt

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdeflongWrapper(x:Long):RichLong

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

implicitdefshortWrapper(x:Short):RichShort

Attributes

Inherited from:: LowPriorityImplicits (hidden)
Source: Predef.scala

Deprecated and Inherited implicits

implicitdefcopyArrayToImmutableIndexedSeq[T](xs:Array[T]):IndexedSeq[T]

Attributes

Deprecated: [Since version 2.13.0]implicit conversions from Array to immutable.IndexedSeq are implemented by copying; use `toIndexedSeq` explicitly if you want to copy, or use the more efficient non-copying ArraySeq.unsafeWrapArray
Inherited from:: LowPriorityImplicits2 (hidden)
Source: Predef.scala

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Predef

Commonly Used Types

Console Output

Assertions

Implicit Conversions

Attributes

Members list

Grouped members

Utility Methods

Attributes

Attributes

Type parameters

Value parameters

Attributes

Type parameters

Attributes

Attributes

Attributes

Assertions

Value parameters

Attributes

Value parameters

Attributes

Value parameters

Attributes

Value parameters

Attributes

Value parameters

Attributes

Value parameters

Attributes

Console Output

Value parameters

Attributes

Value parameters

Attributes

Attributes

Value parameters

Attributes

Aliases

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Attributes

Attributes

Attributes

Attributes

Attributes

Attributes

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String Conversions

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Attributes

Implicit Classes

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Attributes

Attributes

Attributes

Attributes

Attributes

Attributes

Attributes

CharSequence Wrappers

Attributes

Attributes

Attributes

Attributes

Java to Scala

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Scala to Java

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