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.
Attributes
- Source
- Predef.scala
- Graph
-
- Supertypes
- Self type
-
Predef.type
Members list
Grouped members
Utility Methods
???
can be used for marking methods that remain to be implemented.
???
can be used for marking methods that remain to be implemented.
Attributes
- Throws
-
NotImplementedError when
???
is invoked. - Source
- Predef.scala
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.
Attributes
- 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
A method that returns its input value.
A method that returns its input value.
Type parameters
- A
-
type of the input value x.
Value parameters
- x
-
the value of type
A
to be returned.
Attributes
- Returns
-
the value
x
. - Source
- Predef.scala
Summon an implicit value of type T
.
Summon an implicit value of type T
. Usually, the argument is not passed explicitly.
Type parameters
- T
-
the type of the value to be summoned
Attributes
- Returns
-
the implicit value of type
T
- Source
- Predef.scala
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.
Attributes
- 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
Retrieve the single value of a type with a unique inhabitant.
Retrieve the single value of a type with a unique inhabitant.
Attributes
- 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.
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 parameters
- assertion
-
the expression to test
Attributes
- See also
- Source
- Predef.scala
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 parameters
- assertion
-
the expression to test
- message
-
a String to include in the failure message
Attributes
- See also
- Source
- Predef.scala
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 parameters
- assumption
-
the expression to test
Attributes
- See also
- Source
- Predef.scala
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 parameters
- assumption
-
the expression to test
- message
-
a String to include in the failure message
Attributes
- See also
- Source
- Predef.scala
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 parameters
- requirement
-
the expression to test
Attributes
- Source
- Predef.scala
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 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.
Prints an object to out
using its toString
method.
Prints an object to out
using its toString
method.
Value parameters
- x
-
the object to print; may be null.
Attributes
- Source
- Predef.scala
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 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
- Source
- Predef.scala
Prints a newline character on the default output.
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 parameters
- x
-
the object to print.
Attributes
- Source
- Predef.scala
Aliases
These aliases bring selected immutable types into scope without any imports.
Allows destructuring tuples with the same syntax as constructing them.
Allows destructuring tuples with the same syntax as constructing them.
Attributes
- Example
-
val tup = "foobar" -> 3 val c = tup match { case str -> i => str.charAt(i) }
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
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.
In addition, extension methods in scala.collection.StringOps are added implicitly through the conversion augmentString.
Attributes
- Source
- Predef.scala
String Conversions
Conversions from String to StringOps or WrappedString.
Attributes
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Implicit Classes
These implicit classes add useful extension methods to every type.
Attributes
- Source
- Predef.scala
- Supertypes
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
- Supertypes
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
- Supertypes
Attributes
- Source
- Predef.scala
Injects String concatenation operator +
to any classes.
Injects String concatenation operator +
to any classes.
Attributes
- Deprecated
- true
- Source
- Predef.scala
- Supertypes
Injects String concatenation operator +
to any classes.
Injects String concatenation operator +
to any classes.
Attributes
- Deprecated
- true
- Source
- Predef.scala
CharSequence Wrappers
Wrappers that implements CharSequence and were implicit classes.
Attributes
- Source
- Predef.scala
- Supertypes
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
- Supertypes
Attributes
- Source
- Predef.scala
Java to Scala
Implicit conversion from Java primitive wrapper types to Scala equivalents.
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Scala to Java
Implicit conversion from Scala AnyVals to Java primitive wrapper types equivalents.
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
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- Predef.scala
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- Predef.scala
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- Predef.scala
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- Source
- Predef.scala
Array to ArraySeq
Conversions from Arrays to ArraySeqs.
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Type members
Types
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
A type supporting Self-based type classes.
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
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Summon a given value of type T
. Usually, the argument is not passed explicitly.
Summon a given value of type T
. 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
Extensions
Extensions
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
.
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
.
Attributes
- 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
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
.
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
.
Attributes
- Source
- Predef.scala
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
.
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
.
Attributes
- Source
- Predef.scala
Experimental extensions
Attributes
- Experimental
- true
- Source
- Predef.scala
Implicits
Implicits
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)
.
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
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- Predef.scala
Attributes
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- Predef.scala
Attributes
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- Predef.scala
Attributes
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- Predef.scala
Attributes
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- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Attributes
- Source
- Predef.scala
Inherited implicits
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
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
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Attributes
- Inherited from:
- LowPriorityImplicits (hidden)
- Source
- Predef.scala
Deprecated and Inherited implicits
Attributes
- Deprecated
-
[Since version 2.13.0]
- Inherited from:
- LowPriorityImplicits2 (hidden)
- Source
- Predef.scala