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 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:
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 toArrayList[_ <: Number]
. This is itself a shorthand for the existential typeArrayList[T] forSome { type T <: Number }
.A raw type in Java such as
ArrayList
is translated toArrayList[_]
, which is a shorthand forArrayList[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 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.
Case classes can now inherit from other case classes.
Case classes may now be
abstract
.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]) =
(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.
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:
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 ofapply
.unapply
methods return option types:Some(...)
for a match that suceeds,None
for a match that fails. Pattern variables are returned as the elements ofSome
. 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 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)
\(\mbox{for}\) def f(params): unit
def f(params) { ... }
\(\mbox{for}\) 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 Acess
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
all identifiers accessible without prefix, and
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:
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.
Member resolution and super accesses are now both defined in terms of a class linearization.
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.