发散隐式扩展

Question

我有一个关于隐含解析的问题。 说，我有以下类型的类：

trait Foo[In <: Base, Out <: Base] { 
    def factor : Double 
} 

而

sealed trait Base 
object Base { 
    implicit def symmetricFoo[In <: Base, Out <: Base](implicit foo : Foo[In, Out]) : Foo[Out, In] = 
    new Foo[Out, In] { 
     def factor : Double = 1.0/foo.factor 
    } 

    implicit def transitiveFoo[In <: Base, Mid <: Base, Out <: Base](implicit foo1 : Foo[In, Mid], foo2 : Foo[Mid, Out]) : Foo[In, Out] = 
    new Foo[In, Out] { 
     def factor : Double = foo1.factor * foo2.factor 
    } 
} 

case object A extends Base 

case object B extends Base { 
    implicit def bFoo : Foo[B.type, A.type] = 
    new Foo[B.type, A.type] { 
     def factor : Double = 2.0 
    } 
} 

case object C extends Base { 
    implicit def cFoo : Foo[C.type, A.type] = 
    new Foo[C.type, A.type] { 
     def factor : Double = 3.0 
    } 
} 

case object D extends Base { 
    implicit def dFoo : Foo[D.type, C.type] = 
    new Foo[D.type, C.type] { 
     def factor : Double = 5.0 
    } 
} 

我希望能够得到，如果我有沿路径中介富的富[X，Y]的情况下， X到Y.有时它确实有效，例如， G。

println(implicitly[Foo[D.type, A.type]].factor) // 15.0 (D->C, C->A = 5 * 3) 
println(implicitly[Foo[D.type, B.type]].factor) // 7.5 (D->C, C->A, A->B = 5 * 3 * 1/2) 

，但如果我改变行的顺序，它不会：

println(implicitly[Foo[D.type, B.type]].factor) // 7.5 
println(implicitly[Foo[D.type, A.type]].factor) // does not compile 

有很多或多或少相同的错误消息像

test.this.Base.transitiveFoo is not a valid implicit value for test.Foo[test.C.type,test.A.type] because: hasMatchingSymbol reported error: diverging implicit expansion for type test.Foo[test.C.type,Mid] starting with method transitiveFoo in object Base

diverging implicit expansion for type test.Foo[test.A.type,Mid] starting with method transitiveFoo in object Base

not enough arguments for method implicitly: (implicit e: test.Foo[test.D.type,test.A.type])test.Foo[test.D.type,test.A.type]. Unspecified value parameter e.

完整的日志有点长。

给出Foo [X，Y]得到Foo [Y，X]，给出Foo [X，Y]和Foo [Y，Z]得到Foo [X，Z]的正确方法是什么]为定义的Foo的任何组合？ 会像没形状的'懒惰的帮助在这里？提前致谢！

Answer 1

这种变异似乎工作：

import shapeless.Lazy 

    trait Foo[In <: Base, Out <: Base] { 
    def factor : Double 
    } 

    sealed trait Base 
    case object A extends Base 
    case object B extends Base 
    case object C extends Base 
    case object D extends Base 

    trait LowPriorityImplicits { 
    implicit def symmetricFoo[In <: Base, Out <: Base](implicit foo : Lazy[Foo[In, Out]]) : Foo[Out, In] = 
     new Foo[Out, In] { 
     def factor : Double = 1.0/foo.value.factor 
     } 
    } 

    object Base extends LowPriorityImplicits { 
    implicit def bFoo : Foo[B.type, A.type] = 
     new Foo[B.type, A.type] { 
     def factor : Double = 2.0 
     } 

    implicit def cFoo : Foo[C.type, A.type] = 
     new Foo[C.type, A.type] { 
     def factor : Double = 3.0 
     } 

    implicit def dFoo : Foo[D.type, C.type] = 
     new Foo[D.type, C.type] { 
     def factor : Double = 5.0 
     } 

    implicit def transitiveFoo[In <: Base, Out <: Base, Mid <: Base](implicit foo1 : Foo[In, Mid], foo2 : Foo[Mid, Out]) : Foo[In, Out] = 
     new Foo[In, Out] { 
     def factor : Double = foo1.factor * foo2.factor 
     } 
    } 

    def main(args: Array[String]): Unit = { 
    println(implicitly[Foo[D.type, B.type]].factor) 
    println(implicitly[Foo[D.type, A.type]].factor) 
    } 

    7.5 
    15.0 

    def main(args: Array[String]): Unit = { 
    println(implicitly[Foo[D.type, A.type]].factor) 
    println(implicitly[Foo[D.type, B.type]].factor) 
    } 

    15.0 
    7.5 

以防万一我build.sbt：

scalaOrganization := "org.typelevel" 
scalaVersion := "2.12.3-bin-typelevel-4" 
libraryDependencies += "com.chuusai" %% "shapeless" % "2.3.2"