[英]How to use implicit conversion instead of structural typing when only one collection method is needed
I have a method in which I only need to access one method ( appended
, or :+
) of a type.我有一个方法,我只需要访问一个类型的一个方法( appended
或:+
)。 I want to be able to use both a custom case class and a normal List
.我希望能够同时使用自定义案例 class 和普通List
。 Coming from a TypeScript background, I know how to do this using structural typing:来自 TypeScript 背景,我知道如何使用结构类型来做到这一点:
case class CustomContainer[T](data: List[T]) {
def :+(other: T): CustomContainer[T] = copy(data = data :+ other)
}
def append[T, A <: { def :+(other: T): A }](appendable: A, other: T): A = appendable :+ other
append(List(1,2,3), 4) // List(1,2,3,4)
append(CustomContainer(List(1,2,3)), 4) // CustomContainer(List(1,2,3,4))
However, I also know there are performance and maintainability disadvantages to using structural types.但是,我也知道使用结构类型存在性能和可维护性方面的缺点。 I'd like to find a better way, so I'm trying to do this using traits instead, but the sticking point is that I can't make List
extend some trait that I defined.我想找到一个更好的方法,所以我尝试使用特征来做到这一点,但关键是我不能让List
扩展我定义的一些特征。 I have a feeling the answer is implicit conversion, but I can't figure out how to implicitly convert something to a trait, which is an abstract type.我感觉答案是隐式转换,但我不知道如何将某些东西隐式转换为特征,这是一种抽象类型。
Here's what I've tried so far:这是我到目前为止所尝试的:
trait Appendable[T, C] {
def :+(other: T): C
}
case class CustomContainer[T](data: List[T]) extends Appendable[T, CustomContainer[T]] {
override def :+(other: T): CustomContainer[T] = copy(data = data :+ other)
}
implicit def listToAppendable[T](list: List[T]): Appendable[T, List[T]] = ??? // What goes here?
def append[T, A <: Appendable[T, A]](appendable: A, other: T): A = appendable :+ other
append(List(1,2,3), 4) // List(1,2,3,4)
append(CustomContainer(List(1,2,3)), 4) // CustomContainer(List(1,2,3,4))
What goes in the ???
里面有什么???
spot?点? Alternatively is there a better way to do this?或者有没有更好的方法来做到这一点?
I am on Scala 2.10.4.我在 Scala 2.10.4 上。
You can use a type class for this:您可以为此使用 class 类型:
final case class CustomContainer[T](data: List[T])
trait Appendable[T, C[_]] {
def specialAppend(other: T, acc: C[T]): C[T]
}
object Appendable {
implicit def listAppendable[A]: Appendable[A, List] = new Appendable[A, List] {
override def specialAppend(other: A, acc: List[A]): List[A] = other :: acc
}
implicit def customContainerAppendable[A]: Appendable[A, CustomContainer] = new Appendable[A, CustomContainer] {
override def specialAppend(other: A, acc: CustomContainer[A]): CustomContainer[A] = acc.copy(data = other :: acc.data)
}
}
object Foo {
implicit class AppendableOps[A, C[_]](val c: C[A]) {
def :+!(elem: A)(implicit appendable: Appendable[A, C]): C[A] = appendable.specialAppend(elem, c)
}
def main(args: Array[String]): Unit = {
println(List(1,2,3,4) :+! 6)
println(CustomContainer(List(1)) :+! 2)
()
}
}
Yields:产量:
List(6, 1, 2, 3, 4)
CustomContainer(List(2, 1))
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