伴随对象中的隐式视图(Implicit views in companion object)

伴随对象中的隐式视图(Implicit views in companion object)

我正在阅读Joshua D. Suereth的Scala In Depth，并且在scala中遇到了关于隐式视图的以下代码：

object test { trait Foo trait Bar object Foo { implicit def fooToBar(f : Foo) = new Bar{ } } }

然后定义一个需要Bar作为参数的方法：

def bar(x : Bar) = println("bar")

为什么以下工作：

val f = new Foo{} bar(f) // print "bar"

但

bar(new Foo{})

会导致编译器给出类型不匹配错误：

error: type mismatch; found : java.lang.Object with test.Foo required: test.Bar bar(new Foo {}) ^

I was reading Scala In Depth by Joshua D. Suereth, and came across the following code about implicit views in scala:

object test { trait Foo trait Bar object Foo { implicit def fooToBar(f : Foo) = new Bar{ } } }

Then define a method that requires a Bar as argument:

def bar(x : Bar) = println("bar")

Why the following works:

val f = new Foo{} bar(f) // print "bar"

but

bar(new Foo{})

would cause the compiler to give type mismatch error:

error: type mismatch; found : java.lang.Object with test.Foo required: test.Bar bar(new Foo {}) ^

最满意答案

这是关于你在做什么的事情：

new Foo {} // Anonymous subclass of Object with trait Foo new Foo () // Foo new Foo // Foo

当你做bar(new Foo {})这样的事情时，编译器还不知道你在做什么 - 它试图找到一个接受new Foo {}的bar方法，但它还不知道究竟是什么键入new Foo {} ，因为它取决于什么是bar 。

如果声明val f = new Foo{} ，则f的类型变得固定，这有助于编译器找出它应该对bar做什么。

Here's something about what you are doing:

new Foo {} // Anonymous subclass of Object with trait Foo new Foo () // Foo new Foo // Foo

When you do something like bar(new Foo {}), the compiler doesn't know yet what you are doing -- it tries to find a bar method that will accept new Foo {}, but it doesn't know yet exactly what type new Foo {} is, because it depends on what bar is.

If you declare val f = new Foo{}, f's type becomes fixed, which then helps the compiler find out what it should do about bar.