考虑以下代码:
trait Foo {
type T
def value: T
}
object Foo {
def apply[A](v: A): Foo = new Foo {
override type T = A
override def value = v
}
}
trait Decode[A] {
def apply(x: A): String
}
object Decode {
def apply[A](f: A => String): Decode[A] = new Decode[A] {
override def apply(x: A) = f(x)
}
implicit val decodeStr: Decode[String] = Decode(identity)
}
class Sandbox {
def decodeFoo(foo: Foo)(implicit decoder: Decode[foo.T]): String =
decoder(foo.value)
val foo = Foo("hello")
println(decodeFoo(foo))
}
上面的代码应该可以正常工作并打印hello
,但它无法编译:
could not find implicit value for parameter decoder: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo))
即使我明确传入隐式参数:
println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
我现在仍然收到此错误:
type mismatch;
[error] found : Decode[String]
[error] required: Decode[Sandbox.this.foo.T]
[error] println(decodeFoo(foo = foo)(decoder = Decode.decodeStr))
[error] ^
当然,我可以为它制作Foo
和Foo[T]
定义解码器,但这不是这个问题的重点——我想了解为什么上面的代码无法编译。