Changelog

This changelog was no longer maintained after version 2.8.0.

A pull request updating this chapter to list the most significant changes made in more recent Scala versions would be highly welcome.

Many language changes, especially larger ones, are documented in SIP (Scala Improvement Process) proposals. Most proposals that were accepted and implemented have not merged into the main spec. Pull requests that merge SIPs into the main spec are also highly welcome.

To find out what has changed in Scala 2 since 2.8.0, you can consult the following sources:

Changes in Version 2.8.0

Trailing commas in expression, argument, type or pattern sequences are no longer supported.

Changed visibility rules for nested packages (where done?)

Changed visibility rules so that packages are no longer treated specially.

Added section on weak conformance. Relaxed type rules for conditionals, match expressions, try expressions to compute their result type using least upper bound wrt weak conformance. Relaxed type rule for local type inference so that argument types need only weekly conform to inferred formal parameter types. Added section on numeric widening to support weak conformance.

Tightened rules to avoid accidental overrides.

Removed class literals.

Added section on context bounds.

Clarified differences between isInstanceOf and pattern matches.

Allowed implicit modifier on function literals with a single parameter.

Changes in Version 2.7.2

(10-Nov-2008)

Precedence of Assignment Operators

The precedence of assignment operators has been brought in line with. From now on +=, has the same precedence as =.

Wildcards as function parameters

A formal parameter to an anonymous function may now be a wildcard represented by an underscore.

 _ => 7   // The function that ignores its argument
          // and always returns 7.

Unicode alternative for left arrow

The Unicode glyph ‘´\leftarrow´’ ´\u2190´ is now treated as a reserved identifier, equivalent to the ASCII symbol ‘<-’.

Changes in Version 2.7.1

(09-April-2008)

Change in Scoping Rules for Wildcard Placeholders in Types

A wildcard in a type now binds to the closest enclosing type application. For example List[List[_]] is now equivalent to this existential type:

List[List[t] forSome { type t }]

In version 2.7.0, the type expanded instead to:

List[List[t]] forSome { type t }

The new convention corresponds exactly to the way wildcards in Java are interpreted.

No Contractiveness Requirement for Implicits

The contractiveness requirement for implicit method definitions has been dropped. Instead, it is checked for each implicit expansion individually that the expansion does not result in a cycle or a tree of infinitely growing types.

Changes in Version 2.7.0

(07-Feb-2008)

Java Generics

Scala now supports Java generic types by default:

This translation works if -target:jvm-1.5 is specified, which is the new default. For any other target, Java generics are not recognized. To ensure upgradability of Scala codebases, extraneous type parameters for Java classes under -target:jvm-1.4 are simply ignored. For instance, when compiling with -target:jvm-1.4, a Scala type such as ArrayList[String] is simply treated as the unparameterized type ArrayList.

Changes to Case Classes

The Scala compiler generates a companion extractor object for every case class now. For instance, given the case class:

  case class X(elem: String)

the following companion object is generated:

  object X {
    def unapply(x: X): Some[String] = Some(x.elem)
    def apply(s: String): X = new X(s)
  }

If the object exists already, only the apply and unapply methods are added to it.

Three restrictions on case classes have been removed.

  1. Case classes can now inherit from other case classes.

  2. Case classes may now be abstract.

  3. Case classes may now come with companion objects.

Changes in Version 2.6.1

(30-Nov-2007)

Mutable variables introduced by pattern binding

Mutable variables can now be introduced by a pattern matching definition, just like values can. Examples:

  var (x, y) = if (positive) (1, 2) else (-1, -3)
  var hd :: tl = mylist

Self-types

Self types can now be introduced without defining an alias name for this. Example:

  class C {
    type T <: Trait
    trait Trait { this: T => ... }
  }

Changes in Version 2.6

(27-July-2007)

Existential types

It is now possible to define existential types. An existential type has the form T forSome {Q} where Q is a sequence of value and/or type declarations. Given the class definitions

class Ref[T]
abstract class Outer { type T }

one may for example write the following existential types

Ref[T] forSome { type T <: java.lang.Number }
Ref[x.T] forSome { val x: Outer }

Lazy values

It is now possible to define lazy value declarations using the new modifier lazy. A lazy value definition evaluates its right hand side ´e´ the first time the value is accessed. Example:

import compat.Platform._
val t0 = currentTime
lazy val t1 = currentTime
val t2 = currentTime

println("t0 <= t2: " + (t0 <= t2))  //true
println("t1 <= t2: " + (t1 <= t2))  //false (lazy evaluation of t1)

Structural types

It is now possible to declare structural types using type refinements. For example:

class File(name: String) {
  def getName(): String = name
  def open() { /*..*/ }
  def close() { println("close file") }
}
def test(f: { def getName(): String }) { println(f.getName) }

test(new File("test.txt"))
test(new java.io.File("test.txt"))

There’s also a shorthand form for creating values of structural types. For instance,

new { def getName() = "aaron" }

is a shorthand for

new AnyRef{ def getName() = "aaron" }

Changes in Version 2.5

(02-May-2007)

Type constructor polymorphism

Implemented by Adriaan Moors

Type parameters and abstract type members can now also abstract over type constructors.

This allows a more precise Iterable interface:

trait Iterable[+T] {
  type MyType[+T] <: Iterable[T] // MyType is a type constructor

  def filter(p: T => Boolean): MyType[T] = ...
  def map[S](f: T => S): MyType[S] = ...
}

abstract class List[+T] extends Iterable[T] {
  type MyType[+T] = List[T]
}

This definition of Iterable makes explicit that mapping a function over a certain structure (e.g., a List) will yield the same structure (containing different elements).

Early object initialization

Early object initialization makes it possible to initialize some fields of an object before any parent constructors are called. This is particularly useful for traits, which do not have normal constructor parameters. Example:

trait Greeting {
  val name: String
  val msg = "How are you, "+name
}
class C extends {
  val name = "Bob"
} with Greeting {
  println(msg)
}

In the code above, the field is initialized before the constructor of is called. Therefore, field msg in class is properly initialized to .

For-comprehensions, revised

The syntax of for-comprehensions has changed. In the new syntax, generators do not start with a val anymore, but filters start with an if (and are called guards). A semicolon in front of a guard is optional. For example:

for (val x <- List(1, 2, 3); x % 2 == 0) println(x)

is now written

for (x <- List(1, 2, 3) if x % 2 == 0) println(x)

The old syntax is still available but will be deprecated in the future.

Implicit anonymous functions

It is now possible to define anonymous functions using underscores in parameter position. For instance, the expressions in the left column are each function values which expand to the anonymous functions on their right.

_ + 1                  x => x + 1
_ * _                  (x1, x2) => x1 * x2
(_: int) * 2           (x: int) => (x: int) * 2
if (_) x else y        z => if (z) x else y
_.map(f)               x => x.map(f)
_.map(_ + 1)           x => x.map(y => y + 1)

As a special case, a partially unapplied method is now designated m _   instead of the previous notation  &m.

The new notation will displace the special syntax forms .m() for abstracting over method receivers and &m for treating an unapplied method as a function value. For the time being, the old syntax forms are still available, but they will be deprecated in the future.

Pattern matching anonymous functions, refined

It is now possible to use case clauses to define a function value directly for functions of arities greater than one. Previously, only unary functions could be defined that way. Example:

def scalarProduct(xs: Array[Double], ys: Array[Double]) =
  (xs zip ys).foldLeft(0.0) {
    case (a, (b, c)) => a + b * c
  }

Changes in Version 2.4

(09-Mar-2007)

Object-local private and protected

The private and protected modifiers now accept a [this] qualifier. A definition ´M´ which is labelled private[this] is private, and in addition can be accessed only from within the current object. That is, the only legal prefixes for ´M´ are this or ´C´.this. Analogously, a definition ´M´ which is labelled protected[this] is protected, and in addition can be accessed only from within the current object.

Tuples, revised

The syntax for tuples has been changed from ´\{…\}´ to ´(…)´. For any sequence of types ´T_1 , … , T_n´,

´(T_1 , … , T_n)´ is a shorthand for Tuple´n´[´T_1 , … , T_n´].

Analogously, for any sequence of expressions or patterns ´x_1 , … , x_n´,

´(x_1 , … , x_n)´ is a shorthand for Tuple´n´(´x_1 , … , x_n´).

Access modifiers for primary constructors

The primary constructor of a class can now be marked private or protected. If such an access modifier is given, it comes between the name of the class and its value parameters. Example:

class C[T] private (x: T) { ... }

Annotations

The support for attributes has been extended and its syntax changed. Attributes are now called annotations. The syntax has been changed to follow Java’s conventions, e.g. @attribute instead of [attribute]. The old syntax is still available but will be deprecated in the future.

Annotations are now serialized so that they can be read by compile-time or run-time tools. Class has two sub-traits which are used to indicate how annotations are retained. Instances of an annotation class inheriting from trait will be stored in the generated class files. Instances of an annotation class inheriting from trait will be visible to the Scala type-checker in every compilation unit where the annotated symbol is accessed.

Decidable subtyping

The implementation of subtyping has been changed to prevent infinite recursions. Termination of subtyping is now ensured by a new restriction of class graphs to be finitary.

Case classes cannot be abstract

It is now explicitly ruled out that case classes can be abstract. The specification was silent on this point before, but did not explain how abstract case classes were treated. The Scala compiler allowed the idiom.

New syntax for self aliases and self types

It is now possible to give an explicit alias name and/or type for the self reference this. For instance, in

class C { self: D =>
  ...
}

the name self is introduced as an alias for this within C and the self type of C is assumed to be D. This construct is introduced now in order to replace eventually both the qualified this construct and the clause in Scala.

Assignment Operators

It is now possible to combine operators with assignments. Example:

var x: int = 0
x += 1

Changes in Version 2.3.2

(23-Jan-2007)

Extractors

It is now possible to define patterns independently of case classes, using unapply methods in extractor objects. Here is an example:

object Twice {
  def apply(x:Int): int = x*2
  def unapply(z:Int): Option[int] = if (z%2==0) Some(z/2) else None
}
val x = Twice(21)
x match { case Twice(n) => Console.println(n) } // prints 21

In the example, Twice is an extractor object with two methods:

In the second-to-last line, Twice’s method is used to construct a number x. In the last line, x is tested against the pattern Twice(n). This pattern succeeds for even numbers and assigns to the variable n one half of the number that was tested. The pattern match makes use of the unapply method of object Twice. More details on extractors can be found in the paper “Matching Objects with Patterns” by Emir, Odersky and Williams.

Tuples

A new lightweight syntax for tuples has been introduced. For any sequence of types ´T_1 , … , T_n´,

´{T_1 , … , T_n }´ is a shorthand for Tuple´n´[´T_1 , … , T_n´].

Analogously, for any sequence of expressions or patterns ´x_1, … , x_n´,

´{x_1 , … , x_n }´ is a shorthand for Tuple´n´(´x_1 , … , x_n´).

Infix operators of greater arities

It is now possible to use methods which have more than one parameter as infix operators. In this case, all method arguments are written as a normal parameter list in parentheses. Example:

class C {
  def +(x: int, y: String) = ...
}
val c = new C
c + (1, "abc")

Deprecated attribute

A new standard attribute deprecated is available. If a member definition is marked with this attribute, any reference to the member will cause a “deprecated” warning message to be emitted.

Changes in Version 2.3

(23-Nov-2006)

Procedures

A simplified syntax for methods returning unit has been introduced. Scala now allows the following shorthands:

def f(params) for def f(params): unit
def f(params) { ... } for def f(params): unit = { ... }

Type Patterns

The syntax of types in patterns has been refined. Scala now distinguishes between type variables (starting with a lower case letter) and types as type arguments in patterns. Type variables are bound in the pattern. Other type arguments are, as in previous versions, erased. The Scala compiler will now issue an “unchecked” warning at places where type erasure might compromise type-safety.

Standard Types

The recommended names for the two bottom classes in Scala’s type hierarchy have changed as follows:

All      ==>     Nothing
AllRef   ==>     Null

The old names are still available as type aliases.

Changes in Version 2.1.8

(23-Aug-2006)

Visibility Qualifier for protected

Protected members can now have a visibility qualifier, e.g. protected[<qualifier>]. In particular, one can now simulate package protected access as in Java writing

  protected[P] def X ...

where would name the package containing X.

Relaxation of Private Access

Private members of a class can now be referenced from the companion module of the class and vice versa.

Implicit Lookup

The lookup method for implicit definitions has been generalized. When searching for an implicit definition matching a type ´T´, now are considered

  1. all identifiers accessible without prefix, and

  2. all members of companion modules of classes associated with ´T´.

(The second clause is more general than before). Here, a class is associated with a type ´T´ if it is referenced by some part of ´T´, or if it is a base class of some part of ´T´. For instance, to find implicit members corresponding to the type

  HashSet[List[Int], String]

one would now look in the companion modules (aka static parts) of HashSet, List, Int, and String. Before, it was just the static part of .

Tightened Pattern Match

A typed pattern match with a singleton type p.type now tests whether the selector value is reference-equal to p. Example:

  val p = List(1, 2, 3)
  val q = List(1, 2)
  val r = q
  r match {
    case _: p.type => Console.println("p")
    case _: q.type => Console.println("q")
  }

This will match the second case and hence will print “q”. Before, the singleton types were erased to List, and therefore the first case would have matched, which is non-sensical.

Changes in Version 2.1.7

(19-Jul-2006)

Multi-Line string literals

It is now possible to write multi-line string-literals enclosed in triple quotes. Example:

"""this is a
   multi-line
   string literal"""

No escape substitutions except for unicode escapes are performed in such string literals.

Closure Syntax

The syntax of closures has been slightly restricted. The form

  x: T => E

is valid only when enclosed in braces, i.e.  { x: T => E }. The following is illegal, because it might be read as the value x typed with the type T => E:

  val f = x: T => E

Legal alternatives are:

  val f = { x: T => E }
  val f = (x: T) => E

Changes in Version 2.1.5

(24-May-2006)

Class Literals

There is a new syntax for class literals: For any class type ´C´, classOf[´C´] designates the run-time representation of ´C´.

Changes in Version 2.0

(12-Mar-2006)

Scala in its second version is different in some details from the first version of the language. There have been several additions and some old idioms are no longer supported. This appendix summarizes the main changes.

New Keywords

The following three words are now reserved; they cannot be used as identifiers:

implicit    match     requires

Newlines as Statement Separators

Newlines can now be used as statement separators in place of semicolons.

Syntax Restrictions

There are some other situations where old constructs no longer work:

Pattern matching expressions

The match keyword now appears only as infix operator between a selector expression and a number of cases, as in:

  expr match {
    case Some(x) => ...
    case None => ...
  }

Variants such as expr.match {...} or just match {...} are no longer supported.

“With” in extends clauses

The idiom

class C with M { ... }

is no longer supported. A with connective is only allowed following an extends clause. For instance, the line above would have to be written

class C extends AnyRef with M { ... } .

However, assuming M is a trait, it is also legal to write

class C extends M { ... }

The latter expression is treated as equivalent to

class C extends S with M { ... }

where S is the superclass of M.

Regular Expression Patterns

The only form of regular expression pattern that is currently supported is a sequence pattern, which might end in a sequence wildcard . Example:

case List(1, 2, _*) => ... // will match all lists starting with 1, 2, ...

It is at current not clear whether this is a permanent restriction. We are evaluating the possibility of re-introducing full regular expression patterns in Scala.

Selftype Annotations

The recommended syntax of selftype annotations has changed.

class C: T extends B { ... }

becomes

class C requires T extends B { ... }

That is, selftypes are now indicated by the new requires keyword. The old syntax is still available but is considered deprecated.

For-comprehensions

For-comprehensions now admit value and pattern definitions. Example:

for {
  val x <- List.range(1, 100)
  val y <- List.range(1, x)
  val z = x + y
  isPrime(z)
} yield Pair(x, y)

Note the definition  val z = x + y as the third item in the for-comprehension.

Conversions

The rules for implicit conversions of methods to functions have been tightened. Previously, a parameterized method used as a value was always implicitly converted to a function. This could lead to unexpected results when method arguments where forgotten. Consider for instance the statement below:

show(x.toString)

where show is defined as follows:

def show(x: String) = Console.println(x) .

Most likely, the programmer forgot to supply an empty argument list () to toString. The previous Scala version would treat this code as a partially applied method, and expand it to:

show(() => x.toString())

As a result, the address of a closure would be printed instead of the value of s.

Scala version 2.0 will apply a conversion from partially applied method to function value only if the expected type of the expression is indeed a function type. For instance, the conversion would not be applied in the code above because the expected type of show’s parameter is String, not a function type.

The new convention disallows some previously legal code. Example:

def sum(f: int => double)(a: int, b: int): double =
  if (a > b) 0 else f(a) + sum(f)(a + 1, b)

val sumInts  =  sum(x => x)  // error: missing arguments

The partial application of sum in the last line of the code above will not be converted to a function type. Instead, the compiler will produce an error message which states that arguments for method sum are missing. The problem can be fixed by providing an expected type for the partial application, for instance by annotating the definition of sumInts with its type:

val sumInts: (int, int) => double  =  sum(x => x)  // OK

On the other hand, Scala version 2.0 now automatically applies methods with empty parameter lists to () argument lists when necessary. For instance, the show expression above will now be expanded to

show(x.toString()) .

Scala version 2.0 also relaxes the rules of overriding with respect to empty parameter lists. The revised definition of matching members makes it now possible to override a method with an explicit, but empty parameter list () with a parameterless method, and vice versa. For instance, the following class definition is now legal:

class C {
  override def toString: String = ...
}

Previously this definition would have been rejected, because the toString method as inherited from java.lang.Object takes an empty parameter list.

Class Parameters

A class parameter may now be prefixed by val or var.

Private Qualifiers

Previously, Scala had three levels of visibility: private, protected and public. There was no way to restrict accesses to members of the current package, as in Java.

Scala 2 now defines access qualifiers that let one express this level of visibility, among others. In the definition

private[C] def f(...)

access to f is restricted to all code within the class or package C (which must contain the definition of f).

Changes in the Mixin Model

The model which details mixin composition of classes has changed significantly. The main differences are:

  1. We now distinguish between traits that are used as mixin classes and normal classes. The syntax of traits has been generalized from version 1.0, in that traits are now allowed to have mutable fields. However, as in version 1.0, traits still may not have constructor parameters.

  2. Member resolution and super accesses are now both defined in terms of a class linearization.

  3. Scala’s notion of method overloading has been generalized; in particular, it is now possible to have overloaded variants of the same method in a subclass and in a superclass, or in several different mixins. This makes method overloading in Scala conceptually the same as in Java.

Implicit Parameters

Views in Scala 1.0 have been replaced by the more general concept of implicit parameters.

Flexible Typing of Pattern Matching

The new version of Scala implements more flexible typing rules when it comes to pattern matching over heterogeneous class hierarchies. A heterogeneous class hierarchy is one where subclasses inherit a common superclass with different parameter types. With the new rules in Scala version 2.0 one can perform pattern matches over such hierarchies with more precise typings that keep track of the information gained by comparing the types of a selector and a matching pattern. This gives Scala capabilities analogous to guarded algebraic data types.