英文:
How to add custom Semigroup instance for Int
问题
以下是已经翻译好的内容：
import cats._
import cats.implicits._
object SemigroupInstances {
implicit val intSemigroup: Semigroup[Int] = new Semigroup[Int] {
override def combine(a: Int, b: Int) = a * b
}
}
import SemigroupInstances._
object Semigroup {
def apply[T](implicit instance: Semigroup[T]): Semigroup[T] = instance
}
val intSemigroup = Semigroup[Int]
intSemigroup.combine(4, 4) // 预期结果 16，实际结果 8
如何使自定义的 intSemigroup 正常工作？
英文:
import cats._
import cats.implicits._
object SemigroupInstances {
implicit val intSemigroup: Semigroup[Int] = new Semigroup[Int] {
override def combine(a: Int, b: Int) = a * b
}
}
import SemigroupInstances._
object Semigroup {
def apply[T](implicit instance: Semigroup[T]): Semigroup[T] = instance
}
val intSemigroup = Semigroup[Int]
intSemigroup.combine(4, 4) // expected 16, actual 8
So, how to make custom intSemigroup working?
答案1
得分: 0
你导入了两个Semigroup[Int]的实例：
- 通过
import cats.implicits._ - 通过
import SemigroupInstances._
编译器选择了来自cats.implicits._的实例。
如果你移除那个导入，它就可以工作：https://scastie.scala-lang.org/avsmEturTRGkNEOCDxuXOA
如果你想了解更多编译器如何选择隐式实例的信息，请参考这篇帖子：https://stackoverflow.com/questions/5598085/where-does-scala-look-for-implicits/5598107#5598107
在你的具体情况下，原因似乎是来自cats的实例的类型实际上是CommutativeGroup[Int]：
implicit val catsKernelStdGroupForInt: CommutativeGroup[Int] = new IntGroup
因此，作为一个派生类（CommutativeGroup扩展了Semigroup），它被认为更具体，因此更受欢迎。
如果你将你的实例类型更改为CommutativeGroup[Int]（尽管这在数学上是不正确的，但这不是关键），你会得到一个"模糊的给定实例"错误，因为编译器无法决定使用哪个实例：https://scastie.scala-lang.org/dzClDOYDSJ20P1SRpH72nA
英文:
You import two instances of Semigroup[Int]:
- via
import cats.implicits._ - via
import SemigroupInstances._
The compiler chooses the one from cats.implicits._ over your custom instance.
If you remove that import, it works: https://scastie.scala-lang.org/avsmEturTRGkNEOCDxuXOA
If you want to know more about how the compiler chooses which implicit instance to use, see this post: https://stackoverflow.com/questions/5598085/where-does-scala-look-for-implicits/5598107#5598107
In your specific case, the reason appears to be that the type of the instance from cats is actually CommutativeGroup[Int]:
implicit val catsKernelStdGroupForInt: CommutativeGroup[Int] = new IntGroup
Hence, being a derived class (CommutativeGroup extends Semigroup), it is considered more specific and hence preferred.
If you change the type of your instance to CommutativeGroup[Int] (which is not possible without being mathematically incorrect, but that's not the point here), you get an "Ambiguous given instances" error because the compiler cannot decide which instance to use: https://scastie.scala-lang.org/dzClDOYDSJ20P1SRpH72nA
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