Question

scala> implicit def transitive[A, B, C](implicit f: A => B, g: B => C): A => C = f andThen g
transitive: [A, B, C](implicit f: A => B, implicit g: B => C)A => C

scala> class Foo; class Bar; class Baz { def lol = println("lol") }
defined class Foo
defined class Bar
defined class Baz

scala> implicit def foo2Bar(f: Foo) = new Bar
foo2Bar: (f: Foo)Bar

scala> implicit def bar2Baz(f: Bar) = new Baz
bar2Baz: (f: Bar)Baz

scala> implicitly[Foo => Baz]
<console>:14: error: diverging implicit expansion for type Foo => Baz
starting with method transitive in object $iw
              implicitly[Foo => Baz]

从上面的代码中可以明显看出，我正在尝试编写一个方法，该方法在范围内导入时会使隐式转换具有传递性。我期待这段代码可以工作，但令人惊讶的是它没有。上述错误消息的含义是什么，以及如何使此代码生效？

Answer 1

更新：正如评论中所述，此方法无法在2.8上编译，而implicitly[Foo => Baz]正常工作，(new Foo).lol则无效。

如果您将transitive重命名为conforms以隐藏Predef中的方法，则此功能正常：

implicit def conforms[A, B, C](implicit f: A => B, g: B => C): A => C = f andThen g

有关详细信息，请参阅this answer。

作为旁注：使用-Xlog-implicits启动REPL是在这种情况下获取更多信息性错误消息的便捷方法。在这种情况下，起初并没有太多帮助：

scala> implicitly[Foo => Baz]
scala.this.Predef.conforms is not a valid implicit value for Foo => Baz because:
type mismatch;
 found   : <:<[Foo,Foo]
 required: Foo => Baz
<console>:14: error: diverging implicit expansion for type Foo => Baz
starting with method transitive in object $iw
              implicitly[Foo => Baz]
                        ^
scala.this.Predef.conforms is not a valid implicit value for Foo => Baz because:
type mismatch;
 found   : <:<[Foo,Foo]
 required: Foo => Baz
transitive is not a valid implicit value for Unit => Foo => Baz because:
not enough arguments for method transitive: (implicit f: A => B, implicit g: B => C)A => C.
Unspecified value parameter g.
transitive is not a valid implicit value for => Unit => Foo => Baz because:
not enough arguments for method transitive: (implicit f: A => B, implicit g: B => C)A => C.
Unspecified value parameter g.

但如果我们暂时将foo2Bar和bar2Baz重写为函数，我们会收到一条错误消息，突出显示歧义：

implicit val foo2Bar = (_: Foo) => new Bar
implicit val bar2Baz = (_: Bar) => new Baz

scala> implicitly[Foo => Baz]
transitive is not a valid implicit value for Foo => Baz because:
ambiguous implicit values:
 both method conforms in object Predef of type [A]=> <:<[A,A]
 and value foo2Bar in object $iw of type => Foo => Bar
 match expected type Foo => B

现在很清楚，我们只需要隐藏conforms。

关于implicits的奇怪错误消息

1 个答案: