Shapeless：隐含在函数体中

Question

我有以下代码片段：

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

这里的implicitly是什么意思？

Answer 1

表示："summon the implicit instance that you have in scope for the type CsvEncoder[String :: Int :: Boolean :: HNil]"。下面这个简单的例子，在 Scala REPL 会话中，应该很清楚：

$ 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

如您所见，分配给 anotherStr 的值是 str 的值，这是范围内类型 String 的唯一隐式值。请注意，如果您在作用域中有多个相同类型的隐式值，编译将失败并显示错误："ambiguous implicit values"。确实：

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>

Shapeless：隐含在函数体中

Shapeless: Implicitly in the function body

scala

shapeless