即使是.head也行不通.
我需要做些什么改变来完成这项工作?
import shapeless._ import HList._ import Nat._ scala> case class Foo[A](a: A) defined class Foo scala> case class Bar[A](f: Foo[A]) defined class Bar scala> val xs = Foo(23) :: Foo("blah") :: HNil xs: shapeless.::[Foo[Int],shapeless.::[Foo[java.lang.String],shapeless.HNil]] = Foo(23) :: Foo(blah) :: HNil scala> object mapper extends (Foo ~> Bar) { | def apply[A](f: Foo[A]) = Bar(f) | } defined module mapper scala> xs map mapper res13: mapper.Out = Bar(Foo(23)) :: Bar(Foo(blah)) :: HNil scala> res13.apply[_1] <console>:38: error: Could not find implicit value for parameter at: shapeless.At[mapper.Out,shapeless.Nat._1] res13.apply[_1] ^ scala> res13.head <console>:38: error: Could not find implicit value for parameter c: shapeless.IsHCons[mapper.Out] res13.head ^
解决方法
完全按照我的书面编写,至少使用最新的2.10.0-SNAPSHOT,
import shapeless._ import HList._ import Nat._ scala> case class Foo[A](a: A) defined class Foo scala> case class Bar[A](f: Foo[A]) defined class Bar scala> val xs = Foo(23) :: Foo("blah") :: HNil xs: shapeless.::[Foo[Int],shapeless.::[Foo[String],shapeless.HNil]] = Foo(23) :: Foo(blah) :: HNil scala> object mapper extends (Foo ~> Bar) { | def apply[A](f: Foo[A]) = Bar(f) | } defined module mapper scala> xs map mapper res0: shapeless.::[Bar[Int],shapeless.::[Bar[String],shapeless.HNil]] = Bar(Foo(23)) :: Bar(Foo(blah)) :: HNil scala> res0[_1] res1: Bar[String] = Bar(Foo(blah)) scala> res0.head res2: Bar[Int] = Bar(Foo(23))
请注意,上面为res0推断的类型是Bar [Int] :: Bar [String] :: HNil而不是mapper.Out …我怀疑这是2.9.x和2.10.0-SNAPSHOT之间行为的差异.
如果你坚持使用2.9.x那么我认为你应该能够通过明确地将Bar [Int] :: Bar [String] :: HNil归因于你的res13来解决这个问题…显然这更详细,但是c ‘est la vie.