为什么 Numeric 的行为与 Ordered 不同? [英] Why does Numeric behave differently than Ordered?

问题描述

Scala 有许多可以用作类型类的特征，例如 scala.math 包中的 Ordered 和 Numeric.

例如，我可以使用 Ordered 编写一个通用方法，如下所示:

def f[T <% Ordered[T]](a: T, b: T) = if (a 

我想用 Numeric 做类似的事情，但这不起作用:

def g[T <% Numeric[T]](a: T, b: T) = a * b

为什么Ordered 和Numeric 之间有明显的差异?

我知道还有其他方法可以做到这一点，以下方法可行(使用上下文绑定):

def g[T : Numeric](a: T, b: T) = 隐式[Numeric[T]].times(a, b)

但这看起来比仅仅使用 * 将两个数字相乘要复杂得多.为什么 Numeric trait 不包括像 * 这样的方法，而 Ordered 确实包括像 < 这样的方法?>

我知道还有 Ordering，您可以使用与 Numeric 相同的方式，另请参阅 这个答案:

def f[A : Ordering](a: A, b: A) = 隐式[Ordering[A]].compare(a, b)

解决方案

Ordered 只是一些简单的 pimped 方法，它们返回 Int 或 Boolean代码>，所以不需要类型技巧.

另一方面，

Numeric 具有根据所使用的确切子类返回不同类型的方法.因此，虽然 Ordered 只不过是一个标记特征，Numeric 是一个功能齐全的类型类.

要恢复您的运算符，您可以在 lhs 操作数上使用 mkNumericOps(在 Numeric 中定义).

更新

Miles 说的很对，mkNumericOps 是隐式的，所以只要导入 Numeric 的那个实例就会给你所有的魔法......

Scala has a number of traits that you can use as type classes, for example Ordered and Numeric in the package scala.math.

I can, for example, write a generic method using Ordered like this:

def f[T <% Ordered[T]](a: T, b: T) = if (a < b) a else b

I wanted to do a similar thing with Numeric, but this doesn't work:

def g[T <% Numeric[T]](a: T, b: T) = a * b

Why is there an apparent discrepancy between Ordered and Numeric?

I know there are other ways to do this, the following will work (uses a context bound):

def g[T : Numeric](a: T, b: T) = implicitly[Numeric[T]].times(a, b)

But that looks more complicated than just being able to use * to multiply two numbers. Why does the Numeric trait not include methods like *, while Ordered does include methods like <?

I know there's also Ordering which you can use in the same way as Numeric, see also this answer:

def f[A : Ordering](a: A, b: A) = implicitly[Ordering[A]].compare(a, b)

解决方案

Ordered is just a few simple pimped methods that return either Int or Boolean, so no type-trickery is needed.

Numeric, on the other hand, has methods that return different types depending on the exact subclass used. So while Ordered is little more than a marker trait, Numeric is a fully-featured type class.

To get your operators back, you can use mkNumericOps (defined in Numeric) on the lhs operand.

UPDATE

Miles is quite right, mkNumericOps is implicit, so just importing that instance of Numeric will give you back all the magic...

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