为什么在这种情况下不考虑带有泛型参数的隐式转换? [英] Why is the implicit conversion not considered in this case with generic parameters?
问题描述
请考虑以下代码,这些代码来自 metascala项目:
Consider the following code, derived from the metascala project:
object Units {
case class Quantity[M <: MInt, T: Numeric](value: T) {
type This = Quantity[M, T]
def *[M2 <: MInt](m: Quantity[M2, T]) =
Quantity[M + M2, T](numeric[T].times(value, m.value))
def /[M2 <: MInt](m: Quantity[M2, T]) =
Quantity[M - M2, T](numeric[T].div(value, m.value))
def apply(v: T) = Quantity[M, T](numeric[T].times(v, value))
}
implicit def measure[T: Numeric](v: T) = Quantity[_0, T](v)
implicit def numericToQuantity[T: Numeric](v: T) =
new QuantityConstructor[T](v)
class QuantityConstructor[T: Numeric](v: T) {
def m = Quantity[_1, T](v)
}
}
(MInt
基本上是Peano编号的实现,其中_0
,_1
是来自
(the MInt
is basically the implementation of peano numbers where _0
, _1
are concrete "values" from metascala. Tell me if you need additional code, I just didn't want to paste everything in here.)
我想支持这样的代码,其中一些现有的数量可以乘以一个简单的数字,例如. g.
I want to support code where some existing Quantity can be multiplied by a simple number, e. g.
import Units._
val length1 = 5 * (5 m) //doesn't work <-----
val length2 = (5 m) * 5 // works
为什么我假设没有在第一行代码中考虑隐式方法measure
?
Why is the implicit method measure
not considered in the first line of code as I have assumed?
相反,我收到此错误消息:
Instead I get this error message:
overloaded method value * with alternatives:
(x: Double)Double <and>
(x: Float)Float <and>
(x: Long)Long <and>
(x: Int)Int <and>
(x: Char)Int <and>
(x: Short)Int <and>
(x: Byte)Int cannot be applied to
(scalax.units.Units3.Quantity[scalax.units.Integers._1,Int])
我正在使用Scala 2.10-trunk.
I'm using Scala 2.10-trunk.
这实际上是对.
推荐答案
要开始做这件事,这是一个出现相同问题的独立示例,
To get things started, here's a stand-alone example that gives the same problem,
object Units {
case class Quantity[T: Numeric](value: T) {
def *[M](m: Quantity[T]) = // type M can't be inferred below
Quantity[T](implicitly[Numeric[T]].times(value, m.value))
}
implicit def measure[T: Numeric](v: T) = Quantity[T](v)
val length0 = measure(5) * Quantity(5) // works
val length1 = 5 * Quantity(5) // doesn't work
}
由于某种原因,由于方法*
上的类型参数M
,找不到转换measure
.如果从*
中删除了type参数,则编译正常.也许其他人可以解释为什么?
For some reason, the conversion measure
isn't being found because of the type parameter M
on the method *
. If the type parameter is removed from *
, things compile fine. Maybe someone else can explain why?
编辑.这看起来像Scala编译器的局限性,因为将*
重命名为类似***
的名称可以解决此问题.可能Int.*
(没有类型参数)的存在排除了使用Quantity.*[M]
(带有类型参数)的隐式转换.这使我想起了这样的要求,即重写方法必须具有
Edit. This is looking like a limitation of the Scala compiler, since renaming *
to something like ***
resolves the problem. Perhaps the existence of Int.*
(without a type parameter) is precluding the implicit conversion for use of Quantity.*[M]
(with a type parameter). This reminds me of the requirement that overridden methods must have the same exact type parameters.
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