爲什麼我不能索引到通過映射另一個HList獲得的HList？

Question

即使.head也不起作用。

我需要做些什麼改變才能完成這項工作？

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 
        ^

Answer 1

對我的作品完全一樣寫的，至少與最新的2.10.0-快照

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)) 

注意，類型推斷res0以上Bar[Int] :: Bar[String] :: HNil而非mapper.Out ...我懷疑這是2.9.x和2.10.0-SNAPSHOT之間的行爲差​​異。

如果你遇到2.9.x的問題，那麼我認爲你應該能夠通過明確地將Bar[Int] :: Bar[String] :: HNil歸因於res13來解決這個問題......顯然這是更詳細的，但c'est la vie。