Scala中的惰性vals和隐式参数

Question

我一直试图弄清楚隐式参数如何在Scala中工作。据我可以告诉隐式参数分辨率是这样的：

1. 显式传递一个对象的方法。
2. 在范围中定义的隐式定义。作为一个隐含参数

然而类的

伴侣的对象，当我开始结合瓦尔斯懒此玩弄我有点supprise的。似乎懒惰的vals只使用最后的解析规则。下面是一些示例代码来说明：

class Bar(val name:String) 
object Bar { implicit def bar = new Bar("some default bar") } 

class Foo { 
    lazy val list = initialize 
    def initialize(implicit f:Bar) = { 
    println("initialize called with Bar: '" + f.name + "' ...") 
    List[Int]() 
    } 
} 

trait NonDefaultBar extends Foo { 
    implicit def f = new Bar("mixed in implicit bar") 
    def mixedInInit = initialize 
    lazy val mixedInList = list 
} 

object Test { 
    def test = { 
     println("Case 1: with implicitp parameter from companion object") 
     val foo1 = new Foo 
     foo1.list 
     foo1.initialize 

     println("Case 2: with mixedin implicit parameter overriding the default one...") 
     val foo2 = new Foo with NonDefaultBar 
     foo2.mixedInList 

     val foo3 = new Foo with NonDefaultBar 
     foo3.mixedInInit 

     println("Case 3: with local implicit parameter overriding the default one...") 
     implicit def nonDefaultBar = new Bar("locally scoped implicit bar") 
     val foo4 = new Foo 
     foo4.list 
     foo4.initialize 
    } 
} 

调用Test.test给出了下面的输出：

Case 1: with implicitp parameter from companion object 
initialize called with Bar: 'some default bar' ... 
initialize called with Bar: 'some default bar' ... 
Case 2: with mixedin implicit parameter overriding the default one... 
initialize called with Bar: 'some default bar' ... 
initialize called with Bar: 'mixed in implicit bar'... 
Case 3: with local implicit parameter overriding the default one... 
initialize called with Bar: 'some default bar' ... 
initialize called with Bar: 'locally scoped implicit bar' ... 

为什么编译器抓不住，有案例2调用mixedInList当混合隐吧。在情况3中，它在访问列表时也忽略了本地定义的隐式条。

是否有任何方法使用隐式参数与惰性vals不使用隐式定义的伴随对象？

Answer 1

这是因为在编译器编译Foo类时没有其他implicit Bar。在Java反编译的代码如下所示：

public class Foo 
    implements ScalaObject 
{ 
    private List<Object> list; 
    public volatile int bitmap$0; 

    public List<Object> list() 
    { 
    if (
     (this.bitmap$0 & 0x1) == 0); 
    synchronized (this) 
    { 
     if (
     (this.bitmap$0 & 0x1) == 0) { 
     this.list = initialize(Bar..MODULE$.bar()); this.bitmap$0 |= 1; } return this.list; 
    } 
    } 
    public List<Object> initialize(Bar f) { Predef..MODULE$.println(new StringBuilder().append("initialize called with Bar: '").append(f.name()).append("' ...").toString()); 
    return Nil..MODULE$; 
    } 
} 

的lazy val仅仅是检查变量是否已经设置的方法，要么返回它，或设置它，然后返回它。所以你的mixin根本没有被考虑到。如果你想这样做，你必须自己照顾初始化。

Answer 2

虽然scala不支持带隐式参数的惰性vals，但您可以使用选项自定义它。因此，解决办法是更换：

lazy val list = initialize 

通过

private var _list: Option[List[Int]] = None 
def list(implicit f: Bar) = 
    _list.getOrElse{ 
    _list = Some(initialize) 
    _list.get 
    } 

然后运行Test.test显示预期的结果：

Case 1: with implicitp parameter from companion object 
initialize called with Bar: 'some default bar' ... 
initialize called with Bar: 'some default bar' ... 
Case 2: with mixedin implicit parameter overriding the default one... 
initialize called with Bar: 'mixed in implicit bar' ... 
initialize called with Bar: 'mixed in implicit bar' ... 
Case 3: with local implicit parameter overriding the default one... 
initialize called with Bar: 'locally scoped implicit bar' ... 
initialize called with Bar: 'locally scoped implicit bar' ... 

请注意，如果你有mutable options，你可以取代你的懒惰val只有两行来获得相同的结果。

private val _list: MutableOpt[List[Int]] = MutableOpt.from(None) 
def list(implicit f: Bar) = _list.getOrSet(initialize) 

就个人而言，我希望有一天斯卡拉将让我们写这使用一个行：

lazy val list(implicit f: Bar) = initialize 

这将是非常有意义的：在您要访问的变量列表的价值的时候，你需要一个范围的Bar，虽然只有第一个计算会很重要。