Predef
The Predef object provides definitions that are accessible in all Scala
compilation units without explicit qualification.
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
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
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
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
Ato be returned.
- Returns
the value
x.
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
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
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 Params
- assertion
the expression to test
- See also
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
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
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
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
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
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 Params
- x
the object to print; may be null.
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.
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
Aliases
These aliases bring selected immutable types into scope without any imports.
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.
String Conversions
Conversions from String to StringOps or WrappedString.
Implicit Classes
These implicit classes add useful extension methods to every type.
Injects String concatenation operator + to any classes.
Injects String concatenation operator + to any classes.
- Deprecated
CharSequence Wrappers
Wrappers that implements CharSequence and were implicit classes.
Java to Scala
Implicit conversion from Java primitive wrapper types to Scala equivalents.
Scala to Java
Implicit conversion from Scala AnyVals to Java primitive wrapper types equivalents.
Array to ArraySeq
Conversions from Arrays to ArraySeqs.
Type members
Types
Value members
Extensions
Extensions
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
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).
Inherited implicits
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
Deprecated and Inherited implicits
- Deprecated
- Inherited from
- LowPriorityImplicits2