无形：隐函数体

Question

我有下面的代码片段：

val reprEncoder: CsvEncoder[String :: Int :: Boolean :: HNil] = 
    implicitly 

是什么意思implicitly这里？

Answer 1

它的意思是：“召唤你的范围内的暗示实例为CsvEncoder[String :: Int :: Boolean :: HNil]”。下面简单的例子，在Scala的REPL会话，应该清楚：

$ scala 
Welcome to Scala 2.12.3 (Java HotSpot(TM) 64-Bit Server VM, Java 1.8.0_144). 
Type in expressions for evaluation. Or try :help. 

scala> implicit val str: String = "hello" 
str: String = hello 

scala> val anotherStr: String = implicitly 
anotherStr: String = hello 

正如你可以看到分配给anotherStr值是，str这是在范围String类型的唯一的内含价值。请注意，如果在范围编译中有多个相同类型的隐式值，将会失败并显示错误：“模糊隐式值”。确实如下：

scala> implicit val str: String = "hello" 
str: String = hello 

scala> implicit val str2: String = "world" 
str2: String = world 

scala> val anotherStr: String = implicitly 
<console>:16: error: ambiguous implicit values: 
both value StringCanBuildFrom in object Predef of type => 
scala.collection.generic.CanBuildFrom[String,Char,String] 
and method $conforms in object Predef of type [A]=> A <:< A 
match expected type T 
     val anotherStr: String = implicitly 
           ^

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