无形状:隐含在函数体中

这意味着:“调用类型范围内的隐式实例 CsvEncoder[String :: Int :: Boolean :: HNil] “。在Scala REPL会话中,下面的简单示例应该清楚地说明这一点:

$ 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

正如您所看到的,分配给 anotherStr 这是 str 这是类型的唯一隐式值 String

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>