Predef
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$, method fallbackCanBuildFrom
(Since version 2.10.0) Use scala.reflect.ClassTag instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple2 instead
(Since version 2.11.0) Use built-in tuple syntax or Tuple3 instead
??? can be used for marking methods that remain to be implemented.
??? can be used for marking methods that remain to be implemented.
NotImplementedError
Tests an expression, throwing an AssertionError if false.
Tests an expression, throwing an AssertionError if false.
Tests an expression, throwing an AssertionError if false.
Tests an expression, throwing an AssertionError if false.
This method differs from assert only in the intent expressed:
assert contains a predicate which needs to be proven, while
assume contains an axiom for a static checker. Calls to this method
will not be generated if -Xelide-below is at least ASSERTION.
the expression to test
a String to include in the failure message
Tests an expression, throwing an AssertionError if false.
Tests an expression, throwing an AssertionError if false.
This method differs from assert only in the intent expressed:
assert contains a predicate which needs to be proven, while
assume contains an axiom for a static checker. Calls to this method
will not be generated if -Xelide-below is at least ASSERTION.
the expression to test
We prefer the java.lang.* boxed types to these wrappers in any potential conflicts.
We prefer the java.lang.* boxed types to these wrappers in any potential conflicts. Conflicts do exist because the wrappers need to implement ScalaNumber in order to have a symmetric equals method, but that implies implementing java.lang.Number as well.
Note - these are inlined because they are value classes, but the call to xxxWrapper is not eliminated even though it does nothing. Even inlined, every call site does a no-op retrieval of Predef's MODULE$ 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
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
(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 2.10.0) Use scala.reflect.classTag[T] instead
(Since version 2.11.0) Use implicitly[T <:< U] or identity instead.
(Since version 2.9.0) Use sys.error(message) instead
(Since version 2.11.0) Use Throwable directly
(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 2.11.0) Use SeqCharSequence
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.assertstatements can be elided at runtime by providing the command line argument-Xdisable-assertionsto thescalacommand.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 it's 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.