问题描述
我想做这样的事情：
code>。 Ordering 是一个类型类，比 Ordered 更灵活（如果仅仅因为 Ordered 必须由要比较的类型实现，而 Ordering 可以在外部定义）。要为您的类型定义自然排序（默认 Ordering 实例），您只需在随播对象中定义一个隐式排序值。
$ b $因此，足够的序言。好的一点是，当使用 Ordering 时，你想要做的事很简单，因为对元组有一个隐式排序（假设元组本身有排序） ：object Foo {
implicit val FooOrdering = Ordering.by {foo：Foo =>
（foo.length，foo.x，foo.y，foo.z）
}
}
另外，还有一个隐式转换，它将任何具有 Ordering 类型实例的值转换为有序的值（见 Ordered.orderingToOrdered ），所以我们没有什么特别的事情可以自动地传递任何 Foo 添加到期望 Ordered [Foo] 的函数中）
更新：关于您的新问题：
一种方法是使用基于 Ordering .by 和转换为元组，但显式传递顺序来组合：
val byXOrdering = Ordering .by {foo：Foo => foo.x}
val byYOrdering = Ordering.by {foo：Foo => foo.y}
val byZOrdering = Ordering.by {foo：Foo => foo.z}
//由XOrdering和ByYrdering组成：
val byXThenYOrdering = Ordering.by {foo：Foo => （foo，foo）}（Ordering.Tuple2（byXOrdering，byYrrdering））
//由XOrdering和ByYrdering和byZOrdering组成：
val byXThenYThenZOrdering = Ordering.by {foo：Foo => （foo，foo，foo）}（Ordering.Tuple3（byXOrdering，byYrrdering，byZrrdering））
但是它相对嘈杂。
我只用标准库找不到任何东西，所以我实际上建议使用我们自己的帮助器：
final class CompositeOrdering [T]（val ord1：Ordering [T]，val ord2：Ordering [T]）extends Ordering [T] {
def compare（x：T，y：T）= {
val comp = ord1.compare（x，y）
if（comp！= 0）comp else ord2.compare（x，y）
}
}
object CompositeOrdering {
def apply [T]（orderings：Ordering [T] *）= orderings reduceLeft（_ orElse _）
}
implicit class OrderingOps [T]（val ord：Ordering [T] ）扩展AnyVal {
def orElse（ord2：Ordering [T]）= new CompositeOrdering [T]（ord，ord2）
}
可以这样使用：
val byXOrdering = Ordering.by {foo：Foo => foo.x}
val byYOrdering = Ordering.by {foo：Foo => foo.y}
val byZOrdering = Ordering.by {foo：Foo => foo.z}
//通过XOrdering和ByYrdering编写：
val byXThenYOrdering = byXOrdering orElse byYrrdering
//由XOrdering和ByYrdering和ByZrrdering组成：
val byXThenYThenZOrdering = byXOrdering orEsese byYrrdering orEsese byZrrdering
甚至更简单，如下所示：
//编写byXOrdering和byYrrdering：
val byXThenYOrdering = CompositeOrdering（byXOrdering，byYrrdering）
//通过XOrdering和byYrrdering and byZOrdering：
val byXThenYThenZOrdering = CompositeOrdering（byXOrdering，byYrrdering，byZrrdering）
CompositeOrdering.apply 基本上就是您在问题中称为 Ordering.multipleBy 的部分。
I want to do something like this:
class Foo extends Ordered[Foo] { val x val y val z . . . . def compare(that: Foo) = { val c0 = this.length compareTo that.length // primary comparison lazy val c1 = this.x compareTo that.x // secondary comparison lazy val c2 = this.y.size compareTo that.y.size // tertiary comparison lazy val c3 = this.z.head compareTo that.z.head // final tie breaker if (c0 != 0) c0 else if (c1 != 0) c1 else if (c2 != 0) c2 else if (c3 != 0) c3 else c4 } }I was wondering if there was any cleaner way to write this kind of thing. I am expecting some thing like Ordering.multipleBy(ordering: Ordered[A]*) signature which takes a varargs of comparables and selects first non zero.
解决方案It is often better to use Ordering instead of Ordered. Ordering is a type class and is much more flexible than Ordered (if only because Ordered must be implemented by the type to compare, while with Ordering you can define this outside). To define the natural ordering (default Ordering instance) for your type, you just define an implicit ordering value in the companion object.
So, enough with the preamble. The nice thing is that when using Ordering what you want to do is pretty simple, as there is an implicit ordering for tuples (provided that the tuple elements themselves have a orderings)`:
object Foo { implicit val FooOrdering = Ordering.by{ foo: Foo => (foo.length, foo.x, foo.y, foo.z) } }In addition, there is an implicit conversion that converts any value that has an Ordering type class instance into an Ordered value (see Ordered.orderingToOrdered) so we have nothing special to do to automagically being able to pass any instance of Foo to a function that expects an Ordered[Foo])
UPDATE: Concerning your new question:
One way to do it would be to use mostly the same technic based on Ordering.by and conversion to tuples, but explicitly passing the orderings to compose:
val byXOrdering = Ordering.by{ foo: Foo => foo.x } val byYOrdering = Ordering.by{ foo: Foo => foo.y } val byZOrdering = Ordering.by{ foo: Foo => foo.z } // Compose byXOrdering and byYOrdering: val byXThenYOrdering = Ordering.by{ foo: Foo => (foo, foo) }(Ordering.Tuple2(byXOrdering, byYOrdering)) // Compose byXOrdering and byYOrdering and byZOrdering: val byXThenYThenZOrdering = Ordering.by{ foo: Foo => (foo, foo, foo) }(Ordering.Tuple3(byXOrdering, byYOrdering, byZOrdering))But it is relatively "noisy".I could not find anything better using only the standard library, and so I would actually advise to use our own helper:
final class CompositeOrdering[T]( val ord1: Ordering[T], val ord2: Ordering[T] ) extends Ordering[T] { def compare( x: T, y: T ) = { val comp = ord1.compare( x, y ) if ( comp != 0 ) comp else ord2.compare( x, y ) } } object CompositeOrdering { def apply[T]( orderings: Ordering[T] * ) = orderings reduceLeft (_ orElse _) } implicit class OrderingOps[T]( val ord: Ordering[T] ) extends AnyVal { def orElse( ord2: Ordering[T] ) = new CompositeOrdering[T]( ord, ord2 ) }Which can be used like this:
val byXOrdering = Ordering.by{ foo: Foo => foo.x } val byYOrdering = Ordering.by{ foo: Foo => foo.y } val byZOrdering = Ordering.by{ foo: Foo => foo.z } // Compose byXOrdering and byYOrdering: val byXThenYOrdering = byXOrdering orElse byYOrdering // Compose byXOrdering and byYOrdering and byZOrdering: val byXThenYThenZOrdering = byXOrdering orElse byYOrdering orElse byZOrderingOr even simpler, like this:
// Compose byXOrdering and byYOrdering: val byXThenYOrdering = CompositeOrdering(byXOrdering, byYOrdering) // Compose byXOrdering and byYOrdering and byZOrdering: val byXThenYThenZOrdering = CompositeOrdering(byXOrdering, byYOrdering, byZOrdering)CompositeOrdering.apply is basically what you called Ordering.multipleBy in your question.
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