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This article mainly talks about "what are the naming parameters of Scala2.8". Interested friends may wish to take a look. The method introduced in this paper is simple, fast and practical. Now let the editor to take you to learn "what are the Scala2.8 naming parameters"!

Scala 2.8introduces a new feature-named parameters-which means that we can specify parameters by using parameter names. Naming parameters can avoid confusing the meaning of each parameter when the parameter type is the same, and enhance the readability of the code.

Def resize (width: Int, height: Int) = {...} resize (width = 120,42)

Also, Scala 2.8supports the use of default parameters in methods. In the current version of Scala, parameter defaults can only be achieved through method overloading. This leads to some code redundancy, which can be avoided by default parameters.

Def f (elems: List [Int], x: Int = 0, cond: Boolean = true) f (List (1)) f (Nil, cond = false)

The above example also demonstrates that named parameters allow selective use of default parameters: in the second call to the f method, f uses the default value of parameter x instead of the default value of cond.

Compiler generates copy method

A very useful use of named and default parameters is that the compiler automatically generates copy methods for case classes. This method takes a lightweight syntax to create a modified copy of the original instance. The copy method has the same type and parameters as the basic constructor of the copied case class, and each parameter uses the corresponding value in the basic constructor as the default value.

Case class A [T] (a: t, b: Int) {/ / def copy [T'] (a: t'= this.a, b: Int = this.b): a [T'] = new A [T'] (a, b)} val A1: a [Int] = A (1,2) val a2: a [String] = a1.copy (a = "someString")

Scala 2.8has not been officially released yet, but the above features are already implemented in its nightly build. If you want to experience it in advance, click here to download. To learn more about named parameters and default parameters, please read the corresponding SID.

All the new features of Scala 2.8are summarized as follows:

1. A redesigned collection framework.

The hierarchy of Scala's collection classes has been redesigned to be more standardized, easy to use, and efficient. There will be a white paper on the design. Most of the user's existing code will not be affected, but the implementation code for collection classes, such as sequence,set,map,etc. It needs to be modified to accommodate the new collection framework.

two。 Named parameters and default parameters.

The parameters of the method can be passed by the parameter name, and the parameters can have default values.

3.Package object

Not only can the package be defined in the source file, but you can also use some special object to define the package, and the members listed in the object belong to that package. For example, you can add some type aliases (type alias) to such an object, which will be visible as members of the package. This feature is used in the new collection framework to ensure backward compatibility. For example, the list class is located in scala.collection.immutable package in the new library, compared to the previous one in scala package. Code that previously used List is still available in the new collection framework by defining the following package object:

Package object scala {type List [+ A] = scala.collection.immutable.List [A] val List = scala.collection.immutable.List.}

4. Enhanced Scala Swing library

Some new features will be added to the scala.swing package and will be better documented.

5. Continuations support

A compiler plug-in will support continuations as an adjunct feature of Scala. This is done through a type-guided continuation pass-through transformation. Continuation is very helpful for the construction of advanced controls such as asynchronous IActiono UI event handling and data flow concurrency.

6. Type specialization.

Introduce an annotation:@specialize for type parameters. The annotation can generate special code for the basic types in generic code, which avoids expensive boxing and unboxing operations and significantly improves the speed of code execution.

7. Improved REPL.

Some new features have been added to Scala's interactive command console, and special command completion functions will be available.

8.Packrat parsing combiner (parser combinators)

Scala's parsing combiner library will be upgraded to packrat parsing. This helps a lot in efficiency and allows more syntax formats to be expressed as parsers (parser).

Most of the classes and methods marked deprecated in previous versions will be removed in 2.8and Java1.4 is no longer supported. The format of the class file changes, so Scala2.8 is not compatible with previous versions of binaries. But it maintains source code compatibility to a large extent.

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