Predef
Related Doc:
package scala
An instance of A <:< B witnesses that A is a subtype of B.
An instance of A <:< B witnesses that A is a subtype of B.
Requiring an implicit argument of the type A <:< B encodes
the generalized constraint A <: B.
we need a new type constructor <:< and evidence conforms,
as reusing Function1 and identity leads to ambiguities in
case of type errors (any2stringadd is inferred)
To constrain any abstract type T that's in scope in a method's
argument list (not just the method's own type parameters) simply
add an implicit argument of type T <:< U, where U is the required
upper bound; or for lower-bounds, use: L <:< T, where L is the
required lower bound.
In part contributed by Jason Zaugg.
An instance of A =:= B witnesses that the types A and B are equal.
An instance of A =:= B witnesses that the types A and B are equal.
<:< for expressing subtyping constraints
A type for which there is always an implicit value.
A type for which there is always an implicit value.
scala.Array$, method fallbackCanBuildFrom
(Since version 2.10.0) Use scala.reflect.ClassTag instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple2 instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple3 instead
Test two objects for inequality.
Test two objects for inequality.
true if !(this == that), false otherwise.
Equivalent to x.hashCode except for boxed numeric types and null.
Equivalent to x.hashCode except for boxed numeric types and null.
For numerics, it returns a hash value which is consistent
with value equality: if two value type instances compare
as true, then ## will produce the same hash value for each
of them.
For null returns a hashcode where null.hashCode throws a
NullPointerException.
a hash value consistent with ==
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
The expression x == that is equivalent to if (x eq null) that eq null else x.equals(that).
the object to compare against this object for equality.
true if the receiver object is equivalent to the argument; false otherwise.
??? can be used for marking methods that remain to be implemented.
??? can be used for marking methods that remain to be implemented.
Cast the receiver object to be of type T0.
Cast the receiver object to be of type T0.
Note that the success of a cast at runtime is modulo Scala's erasure semantics.
Therefore the expression 1.asInstanceOf[String] will throw a ClassCastException at
runtime, while the expression List(1).asInstanceOf[List[String]] will not.
In the latter example, because the type argument is erased as part of compilation it is
not possible to check whether the contents of the list are of the requested type.
the receiver object.
ClassCastException if the receiver object is not an instance of the erasure of type T0.
Tests an expression, throwing an AssertionError if false.
Tests an expression, throwing an AssertionError if false.
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 at least ASSERTION.
the expression to test
a String to include in the failure message
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 at least ASSERTION.
the expression to test
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!
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.
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
Create a copy of the receiver object.
Tests whether the argument (arg0) is a reference to the receiver object (this).
Tests whether the argument (arg0) is a reference to the receiver object (this).
The eq method implements an equivalence relation on
non-null instances of AnyRef, and has three additional properties:
x and y of type AnyRef, multiple invocations of
x.eq(y) consistently returns true or consistently returns false.x of type AnyRef, x.eq(null) and null.eq(x) returns false.null.eq(null) returns true. When overriding the equals or hashCode methods, it is important to ensure that their behavior is
consistent with reference equality. Therefore, if two objects are references to each other (o1 eq o2), they
should be equal to each other (o1 == o2) and they should hash to the same value (o1.hashCode == o2.hashCode).
true if the argument is a reference to the receiver object; false otherwise.
The equality method for reference types.
Called by the garbage collector on the receiver object when there are no more references to the object.
Called by the garbage collector on the receiver object when there are no more references to the object.
The details of when and if the finalize method is invoked, as
well as the interaction between finalize and non-local returns
and exceptions, are all platform dependent.
A representation that corresponds to the dynamic class of the receiver object.
A representation that corresponds to the dynamic class of the receiver object.
The nature of the representation is platform dependent.
a representation that corresponds to the dynamic class of the receiver object.
not specified by SLS as a member of AnyRef
The hashCode method for reference types.
Test whether the dynamic type of the receiver object is T0.
Test whether the dynamic type of the receiver object is T0.
Note that the result of the test is modulo Scala's erasure semantics.
Therefore the expression 1.isInstanceOf[String] will return false, while the
expression List(1).isInstanceOf[List[String]] will return true.
In the latter example, because the type argument is erased as part of compilation it is
not possible to check whether the contents of the list are of the specified type.
true if the receiver object is an instance of erasure of type T0; false otherwise.
Equivalent to !(this eq that).
Equivalent to !(this eq that).
true if the argument is not a reference to the receiver object; false otherwise.
Wakes up a single thread that is waiting on the receiver object's monitor.
Wakes up a single thread that is waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
Wakes up all threads that are waiting on the receiver object's monitor.
Wakes up all threads that are waiting on the receiver object's monitor.
not specified by SLS as a member of AnyRef
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.
the expression to test
a String to include in the failure message
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.
the expression to test
Creates a String representation of this object.
Creates a String representation of this object. The default representation is platform dependent. On the java platform it is the concatenation of the class name, "@", and the object's hashcode in hexadecimal.
a String representation of the object.
(Since version 2.10.0) Use scala.reflect.ClassTag instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple2 instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple3 instead
(Since version 2.11.0) Use ArrowAssoc
(Since version 2.11.0) Use Ensuring
(Since version 2.11.0) Use StringFormat
(Since version 2.11.0) Use ArrayCharSequence
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.10.0) Use scala.reflect.classTag[T] instead
(Since version 2.11.0) Use implicitly[T <:< U] or identity instead.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.9.0) Use sys.error(message) instead
(Since version 2.11.0) Use Throwable directly
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.11.0) Use the method in scala.io.StdIn
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.11.0) Use SeqCharSequence
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
(Since version 2.12.0-M2) For binary compatibility only. Release new partest and remove in M3.
The
Predefobject 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, scala.collection.immutable.Set, and the scala.collection.immutable.List constructors (scala.collection.immutable.:: and scala.collection.immutable.Nil).
Console I/O
Predef provides a number of simple functions for console I/O, such as
print,println,readLine,readInt, etc. These functions are all aliases of the functions provided by scala.Console.Assertions
A set of
assertfunctions are provided for use as a way to document and dynamically check invariants in code. Invocations ofassertcan be elided at compile time by providing the command line option-Xdisable-assertions, which raises-Xelide-belowaboveelidable.ASSERTION, to thescalaccommand.Variants of
assertintended for use with static analysis tools are also provided:assume,requireandensuring.requireandensuringare 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,The declaration of
addNaturalsstates that the list of integers passed should only contain natural numbers (i.e. non-negative), and that the result returned will also be natural.requireis distinct fromassertin that if the condition fails, then the caller of the function is to blame rather than a logical error having been made withinaddNaturalsitself.ensuresis a form ofassertthat 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.