Scala 3/scala/Predef

Predef

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
class Object
trait Matchable
class Any

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.

Throws
NotImplementedError

when ??? is invoked.

Source
Predef.scala
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 literal T.class in Java.

Returns

The runtime Class representation of type T.

Example

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
Source
Predef.scala
def identity[A](x: A): A

A method that returns its input value.

A method that returns its input value.

Type Params
A

type of the input value x.

Value Params
x

the value of type A to be returned.

Returns

the value x.

Source
Predef.scala
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.

Type Params
T

the type of the value to be summoned

Returns

the implicit value of type T

Source
Predef.scala
def locally[T](@deprecatedName("x") 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.

Example

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
Source
Predef.scala
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.

Example

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

Assertions

These methods support program verification and runtime correctness.

def assert(assertion: Boolean): Unit

Tests an expression, throwing an AssertionError if false.

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

Value Params
assertion

the expression to test

See also
Source
Predef.scala
final def assert(assertion: Boolean, message: => Any): Unit

Tests an expression, throwing an AssertionError if false.

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

Value Params
assertion

the expression to test

message

a String to include in the failure message

See also
Source
Predef.scala
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 than ASSERTION.

Value Params
assumption

the expression to test

See also
Source
Predef.scala
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 than ASSERTION.

Value Params
assumption

the expression to test

message

a String to include in the failure message

See also
Source
Predef.scala
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 to assert, but blames the caller of the method for violating the condition.

Value Params
requirement

the expression to test

Source
Predef.scala
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 to assert, but blames the caller of the method for violating the condition.

Value Params
message

a String to include in the failure message

requirement

the expression to test

Source
Predef.scala

Console Output

These methods provide output via the console.

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.

Value Params
text

the pattern for formatting the arguments.

xs

the arguments used to instantiate the pattern.

Throws
java.lang.IllegalArgumentException

if there was a problem with the format string or arguments

See also
Source
Predef.scala
def println(): Unit

Prints a newline character on the default output.

Prints a newline character on the default output.

Source
Predef.scala
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.

Value Params
x

the object to print.

Source
Predef.scala

Aliases

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

val ->: Tuple2

Allows destructuring tuples with the same syntax as constructing them.

Allows destructuring tuples with the same syntax as constructing them.

Example

val tup = "foobar" -> 3
val c = tup match {
 case str -> i => str.charAt(i)
}
Source
Predef.scala
type Class[T] = Class[T]
type Function[-A, +B] = A => B
type Map[K, +V] = Map[K, V]
val Map: Map
type Set[A] = Set[A]
val Set: Set
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.

The String type in Scala has all the methods of the underlying java.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.

Source
Predef.scala

String Conversions

Conversions from String to StringOps or WrappedString.

Implicit Classes

These implicit classes add useful extension methods to every type.

final class ArrowAssoc[A](self: A) extends AnyVal
final class Ensuring[A](self: A) extends AnyVal
final class StringFormat[A](self: A) extends AnyVal
@deprecated("Implicit injection of + is deprecated. Convert to String to call +", "2.13.0") @deprecated("Implicit injection of + is deprecated. Convert to String to call +", "2.13.0")
final class any2stringadd[A](self: A) extends AnyVal

Injects String concatenation operator + to any classes.

Injects String concatenation operator + to any classes.

Deprecated
Source
Predef.scala

CharSequence Wrappers

Wrappers that implements CharSequence and were implicit classes.

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

Java to Scala

Implicit conversion from Java primitive wrapper types to Scala equivalents.

implicit def Byte2byte(x: Byte): Byte
implicit def Character2char(x: Character): Char
implicit def Double2double(x: Double): Double
implicit def Float2float(x: Float): Float
implicit def Integer2int(x: Integer): Int
implicit def Long2long(x: Long): Long
implicit def Short2short(x: Short): Short

Scala to Java

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

implicit def byte2Byte(x: Byte): Byte
implicit def char2Character(x: Char): Character
implicit def double2Double(x: Double): Double
implicit def float2Float(x: Float): Float
implicit def int2Integer(x: Int): Integer
implicit def long2Long(x: Long): Long
implicit def short2Short(x: Short): Short

Array to ArraySeq

Conversions from Arrays to ArraySeqs.

implicit def genericWrapArray[T](xs: Array[T]): ArraySeq[T]
implicit def wrapByteArray(xs: Array[Byte]): ofByte
implicit def wrapCharArray(xs: Array[Char]): ofChar
implicit def wrapFloatArray(xs: Array[Float]): ofFloat
implicit def wrapIntArray(xs: Array[Int]): ofInt
implicit def wrapLongArray(xs: Array[Long]): ofLong
implicit def wrapRefArray[T <: AnyRef](xs: Array[T]): ofRef[T]
implicit def wrapShortArray(xs: Array[Short]): ofShort
implicit def wrapUnitArray(xs: Array[Unit]): ofUnit

Type members

Types

@implicitNotFound(msg = "No Manifest available for ${T}.")
type Manifest[T] = Manifest[T]

Value members

Concrete methods

def manifest[T](implicit m: Manifest[T]): Manifest[T]
def optManifest[T](implicit m: OptManifest[T]): OptManifest[T]
inline def summon[T](using inline x: T): x

Summon a given value of type T.

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

Type Params
T

the type of the value to be summoned

Returns

the given value typed: the provided type parameter

Source
Predef.scala

Concrete fields

Extensions

Extensions

extension (x: T | Null)
inline def nn[T]: x & T

Strips away the nullability from a value.

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.

Example

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

Implicits

Implicits

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.

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).

Source
Predef.scala
implicit def genericArrayOps[T](xs: Array[T]): ArrayOps[T]
implicit def intArrayOps(xs: Array[Int]): ArrayOps[Int]
implicit def refArrayOps[T <: AnyRef](xs: Array[T]): ArrayOps[T]
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]

Inherited implicits

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!

Inherited from
LowPriorityImplicits
Source
Predef.scala
implicit def charWrapper(c: Char): RichChar
implicit def intWrapper(x: Int): RichInt
implicit def longWrapper(x: Long): RichLong

Deprecated and Inherited implicits

@deprecated("Implicit conversions from Array to immutable.IndexedSeq are implemented by copying; Use the more efficient non-copying ArraySeq.unsafeWrapArray or an explicit toIndexedSeq call", "2.13.0")
implicit def copyArrayToImmutableIndexedSeq[T](xs: Array[T]): IndexedSeq[T]
Deprecated
Inherited from
LowPriorityImplicits2
Source
Predef.scala