SBinary 和 Scalacheck的一部分库,广泛使用了隐式参数。让人联想起了Haskell(一种函数式编程语言)的编程风格,我真心的希望在Scala中这种编程风格更加广泛。这是一种非常有用的技术。 作为开始如下多重入参(柯里化)的例子:
scala -> def foo(x: Int)(y: Int) = x+y
foo: (x: Int)(y: Int) Int
scala -> foo (1) (2)
ret0:Int = 3
scala> foo(1, 2);
:6: error: wrong number of arguments for method foo: (Int)(Int)Int
foo(1, 2);
^
scala> foo(1)
:6: error: missing arguments for method foo in object $iw;
follow this method with `_' if you want to treat it as a partially applied funct
ion
foo(1)
上面你完全可以用单一函数的入参列表来调用它,但是你必须用不同的语法来调用它(两个挎号) 有一种方式你可以申明最后一个参数为隐式(implicit),不需要显式的传递。语法如下:
scala> def speakImplicitly (implicit greeting : String) = println(greeting)
speakImplicitly: (implicit String)Unit
scala> speakImplicitly("Goodbye world")
Goodbye world
scala> speakImplicitly
:6: error: no implicit argument matching parameter type String was foud.
scala> implicit val hello = "Hello world"
hello: java.lang.String = Hello world
scala> speakImplicitly
Hello world
你可以正常调用,也可以不传参数,系统将从作用域中找到一个被标示为
implicit变量,如果找不到implicit变量,编译的时候将会报错。
隐式变量的查找是强制类型匹配的,下面是一些例子展现:
错误的类型:
scala> def speakImplicitly (implicit greeting : String) = println(greeting)
speakImplicitly: (implicit String)Unit
scala> implicit val aUnit = ();
aUnit: Unit = ()
scala> speakImplicitly
:7: error: no implicit argument matching parameter type String was found.
只有String类型的隐式变量才能够匹配。 错误的泛型:
scala> def speakImplicitly (implicit greeting : String) = println(greeting)
speakImplicitly: (implicit String)Unit
scala> implicit val hello : Any = "Hello world"
hello: Any = Hello world
scala> speakImplicitly
:7: error: no implicit argument matching parameter type String was found.
隐式参数匹配必须是静态类型,泛型不能够匹配。 多个相同类型的参数:
scala> def speakImplicitly (implicit greeting : String) = println(greeting)
speakImplicitly: (implicit String)Unit
scala> implicit val foo = "foo";
foo: java.lang.String = foo
scala> implicit val bar = "bar";
bar: java.lang.String = bar
scala> speakImplicitly
:9: error: ambiguous implicit values:
both value bar in object $iw of type => java.lang.String
and value foo in object $iw of type => java.lang.String
match expected type String
系统找到两个隐式参数是相同的类型,将无法识别。
子类可以匹配,
scala> def sayThings (implicit args : List[Any]) = args.foreach(println(_))
sayThings: (implicit List[Any])Unit
scala> implicit val nothingNiceToSay : List[Any] = Nil
nothingNiceToSay: List[Any] = List()
scala> sayThings
scala> implicit val hellos : List[String] = List("Hello world");
hellos: List[String] = List(Hello world)
scala> sayThings
Hello world
String作为Any的子类,所以系统可以匹配到