path: root/spec/15-changelog.md

---
title: Changelog
layout: default
chapter: 15
---

# Changelog

Changes in Version 2.8.0
------------------------

#### Trailing commas

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

Changes in Version 2.8
----------------------

Changed visibility rules for nested packages (where done?)

Changed [visibility rules](02-identifiers-names-and-scopes.html)
so that packages are no longer treated specially.

Added section on [weak conformance](03-types.html#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](06-expressions.html#numeric-widening) to support
weak conformance.

Tightened rules to avoid accidental [overrides](05-classes-and-objects.html#overriding).

Removed class literals.

Added section on [context bounds](07-implicits.html#context-bounds-and-view-bounds).

Clarified differences between [`isInstanceOf` and pattern matches](12-the-scala-standard-library.html#root-classes).

Allowed [`implicit` modifier on function literals](06-expressions.html#anonymous-functions) with a single parameter.

Changes in Version 2.7.2
------------------------

_(10-Nov-2008)_

#### Precedence of Assignment Operators

The [precedence of assignment operators](06-expressions.html#prefix,-infix,-and-postfix-operations)
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](06-expressions.html#placeholder-syntax-for-anonymous-functions).

>      _ => 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](07-implicits.html#implicit-parameters)
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:

-   A generic type in Java such as `ArrayList<String>` is translated to
    a generic type in Scala: `ArrayList[String]`.

-   A wildcard type such as `ArrayList<? extends Number>` is translated
    to `ArrayList[_ <: Number]`. This is itself a shorthand for the
    existential type `ArrayList[T] forSome { type T <: Number }`.

-   A raw type in Java such as `ArrayList` is translated to
    `ArrayList[_]`, which is a shorthand for
    `ArrayList[T] forSome { type T }`.

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]
(05-classes-and-objects.html#case-classes) 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]
(04-basic-declarations-and-definitions.html#variable-declarations-and-definitions),
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`](05-classes-and-objects.html#templates). 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](03-types.html#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`](04-basic-declarations-and-definitions.html#value-declarations-and-definitions).
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]
(03-types.html#compound-types). 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](04-basic-declarations-and-definitions.html#type-parameters)
and abstract
[type members](04-basic-declarations-and-definitions.html#type-declarations-and-type-aliases) can now also abstract over [type constructors](03-types.html#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](05-classes-and-objects.html#early-definitions)
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](06-expressions.html#for-comprehensions-and-for-loops)
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]
(06-expressions.html#placeholder-syntax-for-anonymous-functions) 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](06-expressions.html#method-values)
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]
(08-pattern-matching.html#pattern-matching-anonymous-functions)
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]) =
      (0.0 /: (xs zip ys)) {
        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](05-classes-and-objects.html#modifiers).
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](06-expressions.html#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`]
(05-classes-and-objects.html#class-definitions).
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*](11-annotations.html). 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](05-classes-and-objects.html#inheritance-closure)
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](05-classes-and-objects.html#templates) `this`. For instance, in

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

the name `self` is introduced as an alias for `this` within `C` and the
[self type](05-classes-and-objects.html#class-definitions) 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]
(06-expressions.html#assignment-operators). 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](08-pattern-matching.html#extractor-patterns).
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:

-   The `apply` method is used to build even numbers.

-   The `unapply` method is used to decompose an even number; it is in a sense
    the reverse of `apply`. `unapply` methods return option types:
    `Some(...)` for a match that succeeds, `None` for a match that fails.
    Pattern variables are returned as the elements of `Some`.
    If there are several variables, they are grouped in a tuple.

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](06-expressions.html#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](06-expressions.html#infix-operations). 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`](11-annotations.html#deprecation-annotations)
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`]
(04-basic-declarations-and-definitions.html#procedures) has been introduced.
Scala now allows the following shorthands:

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

#### Type Patterns

The [syntax of types in patterns](08-pattern-matching.html#type-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>]`](05-classes-and-objects.html#protected).
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](05-classes-and-objects.html#private) can now be
referenced from the companion module of the class and vice versa.

#### Implicit Lookup

The lookup method for [implicit definitions](07-implicits.html#implicit-parameters)
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`](08-pattern-matching.html#type-patterns)
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]
(01-lexical-syntax.html#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](06-expressions.html#anonymous-functions)
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](06-expressions.html#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](01-lexical-syntax.html#identifiers):

    implicit    match     requires

#### Newlines as Statement Separators

[Newlines](http://www.scala-lang.org/files/archive/spec/2.11/)
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](05-classes-and-objects.html#traits),
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](06-expressions.html#for-comprehensions-and-for-loops)
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]
(06-expressions.html#method-conversions) 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_](05-classes-and-objects.html#class-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](05-classes-and-objects.html#class-definitions)
may now be prefixed by `val` or `var`.

#### Private Qualifiers

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

Scala 2 now defines [access qualifiers](05-classes-and-objects.html#modifiers)
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]
(05-classes-and-objects.html#templates) has changed significantly.
The main differences are:

1.  We now distinguish between <span>*traits*</span> 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 <span>*class linearization*</span>.

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](07-implicits.html#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]
(08-pattern-matching.html#pattern-matching-expressions).
A <span>*heterogeneous class hierarchy*</span> 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.