导入时无法找到隐式参数（来自对象）

Question

为什么下面的代码找不到从MyProducers导入的隐含条目？根据我的理解，这应该可以正常工作，因为隐含在范围之内。

Error:(16, 34) could not find implicit value for evidence parameter of type MyProducers.ListProducer[Int] 
    val stuffInt:Int = getHead[Int]() 
Error:(16, 34) not enough arguments for method getHead: (implicit evidence$2: MyProducers.ListProducer[Int])Int. 
Unspecified value parameter evidence$2. 
    val stuffInt:Int = getHead[Int]() 
Error:(18, 43) could not find implicit value for evidence parameter of type MyProducers.ListProducer[String] 
    val stuffString:String = getHead[String]() 
Error:(18, 43) not enough arguments for method getHead: (implicit evidence$2: MyProducers.ListProducer[String])String. 
Unspecified value parameter evidence$2. 
    val stuffString:String = getHead[String]() 

代码：

object Resolver { 

    import MyProducers._ 
    import MyProducers._ 

    def getList[T:ListProducer]():List[T]= implicitly[ListProducer[T]].produceList 


    def getHead[T:ListProducer]():T= getList[T]().head 

    val stuffInt:Int = getHead[Int]() 

    val stuffString:String = getHead[String]() 

    val im=ip // this compiles fine, so implicits are in scope 
    val sm=sp 
} 

object MyProducers{ 
    trait ListProducer[T]{ 
    def produceList:List[T] 
    } 
    object IntProducer extends ListProducer[Int]{ 
    override def produceList = List(22, 42) 
    } 
    implicit val ip=IntProducer 

    object StringProducer extends ListProducer[String]{ 
    override def produceList = List("stuff", "Shiraly") 
    } 
    implicit val sp=StringProducer 
} 

有什么奇怪，这个代码编译好：

object Resolver { 

    import MyProducers._ 
    import MyProducers._ 
    trait ListProducer[T]{ 
    def produceList:List[T] 
    } 
    object IntProducer extends ListProducer[Int]{ 
    override def produceList = List(22, 42) 
    } 
    implicit val ip=IntProducer 

    object StringProducer extends ListProducer[String]{ 
    override def produceList = List("stuff", "Shiraly") 
    } 
    implicit val sp=StringProducer 


    def getList[T:ListProducer]():List[T]= implicitly[ListProducer[T]].produceList 


    def getHead[T:ListProducer]():T= getList[T]().head 

    val stuffInt:Int = getHead[Int]() 

    val stuffString:String = getHead[String]() 

    val im=ip 
    val sm=sp 
} 

任何想法，为什么第一个代码不能编译，而第二个呢？

Answer 1

隐式的vals和defs需要对它们进行类型注释。添加这些，并且会发现暗示。