無形：隱函數體

Question

我有下面的代碼片段：

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

是什麼意思implicitly這裏？

Answer 1

它的意思是：「召喚你的範圍內的暗示實例爲CsvEncoder[String :: Int :: Boolean :: HNil]」。下面簡單的例子，在Scala的REPL會話，應該清楚：

$ 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給anotherStr值是，str這是在範圍String類型的唯一的內含價值。請注意，如果在範圍編譯中有多個相同類型的隱式值，將會失敗並顯示錯誤：「模糊隱式值」。確實如下：

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 
           ^

scala>