Object Predef.Predef

object Predef

The Predef object provides definitions that are accessible in all Scala compilation units without explicit qualification.

Commonly Used Types

Predef provides type aliases for types which are commonly used, such as the immutable collection types scala.collection.immutable.Map and scala.collection.immutable.Set.

Console Output

For basic console output, Predef provides convenience methods print and println, which are aliases of the methods in the object scala.Console.

Assertions

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

Variants of assert intended for use with static analysis tools are also provided: assume, require and ensuring. require and ensuring are intended for use as a means of design-by-contract style specification of pre- and post-conditions on functions, with the intention that these specifications could be consumed by a static analysis tool. For instance,

def addNaturals(nats: List[Int]): Int = {
  require(nats forall (_ >= 0), "List contains negative numbers")
  nats.foldLeft(0)(_ + _)
} ensuring(_ >= 0)

The declaration of addNaturals states that the list of integers passed should only contain natural numbers (i.e. non-negative), and that the result returned will also be natural. require is distinct from assert in that if the condition fails, then the caller of the function is to blame rather than a logical error having been made within addNaturals itself. ensuring is a form of assert that declares the guarantee the function is providing with regards to its return value.

Implicit Conversions

A number of commonly applied implicit conversions are also defined here, and in the parent type scala.LowPriorityImplicits. Implicit conversions are provided for the "widening" of numeric values, for instance, converting a Short value to a Long value as required, and to add additional higher-order functions to Array values. These are described in more detail in the documentation of scala.Array.

Utility Methods

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def ???: Nothing

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

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def classOf[T]: Class[T]

Retrieve the runtime representation of a class type. classOf[T] is equivalent to the class literal T.class in Java.

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

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def identity[A](x: A): A

A method that returns its input value.

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def implicitly[T](implicit e: T): T

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

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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. This is just a different name for identity.

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

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def valueOf[T](implicit vt: ValueOf[T]): T

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

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

Assertions

These methods support program verification and runtime correctness.

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def assert(assertion: Boolean): Unit

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

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final def assert(assertion: Boolean, message: => Any): Unit

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

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def assume(assumption: Boolean): Unit

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 than ASSERTION.

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final def assume(assumption: Boolean, message: => Any): Unit

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 than ASSERTION.

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def require(requirement: Boolean): Unit

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

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final def require(requirement: Boolean, message: => Any): Unit

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

Console Output

These methods provide output via the console.

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def print(x: Any): Unit

Prints an object to out using its toString method.

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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).

The interpretation of the formatting patterns is described in java.util.Formatter.

Consider using the f interpolator as more type safe and idiomatic.

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def println(): Unit

Prints a newline character on the default output.

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def println(x: Any): Unit

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

Aliases

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

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val ->: Tuple2.type

Allows destructuring tuples with the same syntax as constructing them.

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

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type Class[T] = Class[T]

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type Function[-A, +B] = A => B

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

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val Map: Map.type

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type Set[A] = Set[A]

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val Set: Set.type

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type String = String

The String type in Scala has all the methods of the underlying java.lang.String, of which it is just an alias.

In addition, extension methods in scala.collection.StringOps are added implicitly through the conversion augmentString.

String Conversions

Conversions from String to StringOps or WrappedString.

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implicit def augmentString(x: String): StringOps

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implicit def wrapString(s: String): WrappedString

Implicit Classes

These implicit classes add useful extension methods to every type.

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final implicit class ArrowAssoc[A](self: A) extends AnyVal

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final implicit def ArrowAssoc[A](self: A): ArrowAssoc[A]

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final implicit class Ensuring[A](self: A) extends AnyVal

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final implicit def Ensuring[A](self: A): Ensuring[A]

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final implicit class StringFormat[A](self: A) extends AnyVal

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final implicit def StringFormat[A](self: A): StringFormat[A]

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final implicit class any2stringadd[A](self: A) extends AnyVal

Injects String concatenation operator + to any classes.

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final implicit def any2stringadd[A](self: A): any2stringadd[A]

Injects String concatenation operator + to any classes.

CharSequence Wrappers

Wrappers that implements CharSequence and were implicit classes.

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final class ArrayCharSequence(arrayOfChars: Array[Char]) extends CharSequence

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def ArrayCharSequence(arrayOfChars: Array[Char]): ArrayCharSequence

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final class SeqCharSequence(sequenceOfChars: IndexedSeq[Char]) extends CharSequence

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def SeqCharSequence(sequenceOfChars: IndexedSeq[Char]): SeqCharSequence

Java to Scala

Implicit conversion from Java primitive wrapper types to Scala equivalents.

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implicit def Boolean2boolean(x: Boolean): Boolean

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implicit def Byte2byte(x: Byte): Byte

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implicit def Character2char(x: Character): Char

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implicit def Double2double(x: Double): Double

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implicit def Float2float(x: Float): Float

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implicit def Integer2int(x: Integer): Int

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implicit def Long2long(x: Long): Long

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implicit def Short2short(x: Short): Short

Scala to Java

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

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implicit def boolean2Boolean(x: Boolean): Boolean

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implicit def byte2Byte(x: Byte): Byte

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implicit def char2Character(x: Char): Character

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implicit def double2Double(x: Double): Double

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implicit def float2Float(x: Float): Float

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implicit def int2Integer(x: Int): Integer

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implicit def long2Long(x: Long): Long

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implicit def short2Short(x: Short): Short

Array to ArraySeq

Conversions from Arrays to ArraySeqs.

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implicit def genericWrapArray[T](xs: Array[T]): ArraySeq[T]

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implicit def wrapBooleanArray(xs: Array[Boolean]): ofBoolean

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implicit def wrapByteArray(xs: Array[Byte]): ofByte

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implicit def wrapCharArray(xs: Array[Char]): ofChar

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implicit def wrapDoubleArray(xs: Array[Double]): ofDouble

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implicit def wrapFloatArray(xs: Array[Float]): ofFloat

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implicit def wrapIntArray(xs: Array[Int]): ofInt

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implicit def wrapLongArray(xs: Array[Long]): ofLong

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implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): ofRef[T]

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implicit def wrapShortArray(xs: Array[Short]): ofShort

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implicit def wrapUnitArray(xs: Array[Unit]): ofUnit

Types

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type Manifest[T] = Manifest[T]

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type OptManifest[T] = OptManifest[T]

Concrete methods

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def manifest[T](implicit m: Manifest[T]): Manifest[T]

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def optManifest[T](implicit m: OptManifest[T]): OptManifest[T]

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transparent inline def summon[T](using inline x: T): T

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

Concrete fields

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val Manifest: Manifest.type

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val NoManifest: NoManifest.type

Extensions

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inline def eq(inline y: AnyRef | Null): Boolean

Enables an expression of type T|Null, where T is a subtype of AnyRef, to be checked for null using eq rather than only ==. This is needed because Null no longer has eq or ne methods, only == and != inherited from Any.

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inline def ne(inline y: AnyRef | Null): Boolean

Enables an expression of type T|Null, where T is a subtype of AnyRef, to be checked for null using ne rather than only !=. This is needed because Null no longer has eq or ne methods, only == and != inherited from Any.

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inline def nn: T | Null & T

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

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

Implicits

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implicit def $conforms[A]: A => A

An implicit of type A => A is available for all A because it can always be implemented using the identity function. This also means that an implicit of type A => B is always available when A <: B, because (A => A) <: (A => B).

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implicit def booleanArrayOps(xs: Array[Boolean]): ArrayOps[Boolean]

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implicit def byteArrayOps(xs: Array[Byte]): ArrayOps[Byte]

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implicit def charArrayOps(xs: Array[Char]): ArrayOps[Char]

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implicit def doubleArrayOps(xs: Array[Double]): ArrayOps[Double]

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implicit def floatArrayOps(xs: Array[Float]): ArrayOps[Float]

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implicit def genericArrayOps[T](xs: Array[T]): ArrayOps[T]

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implicit def intArrayOps(xs: Array[Int]): ArrayOps[Int]

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implicit def longArrayOps(xs: Array[Long]): ArrayOps[Long]

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implicit def refArrayOps[T <: AnyRef](xs: Array[T]): ArrayOps[T]

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implicit def shortArrayOps(xs: Array[Short]): ArrayOps[Short]

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implicit def tuple2ToZippedOps[T1, T2](x: (T1, T2)): Ops[T1, T2]

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implicit def tuple3ToZippedOps[T1, T2, T3](x: (T1, T2, T3)): Ops[T1, T2, T3]

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implicit def unitArrayOps(xs: Array[Unit]): ArrayOps[Unit]

Inherited implicits

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implicit def booleanWrapper(x: Boolean): RichBoolean

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implicit def byteWrapper(x: Byte): RichByte

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!

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implicit def charWrapper(c: Char): RichChar

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implicit def doubleWrapper(x: Double): RichDouble

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implicit def floatWrapper(x: Float): RichFloat

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implicit def intWrapper(x: Int): RichInt

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implicit def longWrapper(x: Long): RichLong

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implicit def shortWrapper(x: Short): RichShort