如何为Scala中的泛型类型提供默认的类型类？ [英] How to provide a default typeclass for generic types in Scala?

问题描述

在Scala中，如果你创建了一个类型类，比如代数结构 Monoid [T] ，你可以为不同类型的monoids提供许多默认类型类实现。

假设monoid被定义为：

pre $ trait Monoid [T ] {
def op（x：T，y：T）：T
def id：T
}


由于连接操作下的字符串 s形成一个monoid，我们可以为 String s像这样：

 隐式对象StringMonoid extends Monoid [String] {
 override def op （a：String，b：String）：String = a + b 
覆盖def id：String =
} 
  

这很简单，因为 String 不是泛型类型。

我要求的是如何为 Seq [T] s提供一个默认monoid。 / strong>，其中type参数阻止我像上面那样创建隐式对象。

我可以这样做：

  class SeqMonoid [T]扩展Monoid [Seq [T]] {
覆盖def op（a：Seq [T]，b：Seq [T]） ：Seq [T] = a ++ b 
 override def id：Seq [T] = Nil 
} 
隐式对象intSeqMonoid extends SeqMonoid [Int] 
隐式对象doubleSeqMonoid extends SeqMonoid [Double] 
隐式对象stringSeqMonoid extends SeqMonoid [String] 
 ... 
  

但这种方法不利用泛型类型的美。

所以，一般来说，我的问题是：Scala中是否有任何方法可以为泛型类型提供类型类实现？

解决方案

您可以使用所需的类型提供隐式函数：

  implicit def SeqMonoid [T]：Monoid [Seq [T]] = new Monoid [Seq [T]] {
 override def op（a：Seq [T]，b：Seq [T ]）：Seq [T] = a ++ b 
 override def id：Seq [T] = Nil 
} 
  

In Scala, if you create a typeclass, say the algebraic structure Monoid[T], you can provide many default typeclass implementations for different types that are monoids.

Suppose a monoid is defined as:

trait Monoid[T] {
  def op(x: T, y: T): T
  def id: T
}

Since Strings under the concatenation operation form a monoid, we can provide a default monoid for Strings like this:

implicit object StringMonoid extends Monoid[String] {
  override def op(a: String, b: String): String = a + b
  override def id: String = ""
}

This is rather easy, since String is not a generic type.

What I am asking for is how to provide a default monoid for Seq[T]s, in which the type parameter prevents me from creating an implicit object like I did above.

I could do:

class SeqMonoid[T] extends Monoid[Seq[T]] {
  override def op(a: Seq[T], b: Seq[T]): Seq[T] = a ++ b
  override def id: Seq[T] = Nil
}
implicit object intSeqMonoid extends SeqMonoid[Int]
implicit object doubleSeqMonoid extends SeqMonoid[Double]
implicit object stringSeqMonoid extends SeqMonoid[String]
...

But this approach doesn't utilize the beauty of generic types.

So, in general, my question is: Is there any way in Scala I can provide typeclass implementations for generic types?

解决方案

You can provide an implicit function with the required type:

implicit def SeqMonoid[T]: Monoid[Seq[T]] = new Monoid[Seq[T]] {
  override def op(a: Seq[T], b: Seq[T]): Seq[T] = a ++ b
  override def id: Seq[T] = Nil
}

这篇关于如何为Scala中的泛型类型提供默认的类型类？的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！