Implementing a generic semigroup implementation in scala

Question

I was trying to implement SemiGroup in a generic manner for most of the scala types including collections. But when it comes to collection, I was stuck to implement the implicit SemiGroupImplicitTypes for collections.

For example: If I want to implement a SemiGroup[List[T]], It expects another param for the type of elements in List, I don't want to implement SemiGroup[List[Int]], SemiGroup[List[Double]] separately, want a single implementation for implicit, which will implement it for all types of List.

trait Semigroup[T] extends Any {
  def combine(a: T, b: T): T
}

object SemiGroup {
  def apply[T](a: T, b: T)(implicit ev: Semigroup[T]): T = ev.combine(a,b)
}

class SemiGroupList[T] extends Semigroup[List[T]] {
  override def combine(a: List[T], b: List[T]): List[T] = a ++ b
}

class SemiGroupSeq[T] extends Semigroup[Seq[T]] {
  override def combine(a: Seq[T], b: Seq[T]): Seq[T] = a ++ b
}

class SemiGroupMap[U, V] extends Semigroup[Map[U,V]] {
  override def combine(a: Map[U, V], b: Map[U, V]): Map[U, V] = a ++ b
}

class SemiGroupNumber[@specialized (Int, Double, Float, Long) T](implicit numeric: Numeric[T]) extends Semigroup[T] {
  override def combine(a: T, b: T): T = numeric.plus(a, b)
}

object SemiGroupImplicitTypes {
  implicit object IntSemiGroup extends SemiGroupNumber[Int]
  implicit object LongSemiGroup extends SemiGroupNumber[Long]
  implicit object DoubleSemiGroup extends SemiGroupNumber[Double]
  implicit object FloatSemiGroup extends SemiGroupNumber[Float]
}


import SemiGroupImplicitTypes._
SemiGroup[Long](1,2)

Answer 1

将其设为def：

 implicit def semigroupList[T] = new SemiGroupList[T]

Answer 2

You need to implement defs with type parameters:

object SemiGroupImplicitTypes {
    implicit object IntSemiGroup extends SemiGroupNumber[Int]
    implicit object LongSemiGroup extends SemiGroupNumber[Long]
    implicit object DoubleSemiGroup extends SemiGroupNumber[Double]
    implicit object FloatSemiGroup extends SemiGroupNumber[Float]

    implicit def listSemiGroup[T]: Semigroup[List[T]] = new SemiGroupList[T]
}

Answer 3

trait Semigroup[T] extends Any {
  def combine(a: T, b: T): T
}

object SemiGroup {
  def apply[T](a: T, b: T)(implicit ev: Semigroup[T]): T = ev.combine(a,b)
}

class SemiGroupList[T] extends Semigroup[List[T]] {
  override def combine(a: List[T], b: List[T]): List[T] = a ++ b
}

class SemiGroupSeq[T] extends Semigroup[Seq[T]] {
  override def combine(a: Seq[T], b: Seq[T]): Seq[T] = a ++ b
}

class SemiGroupMap[U, V] extends Semigroup[Map[U,V]] {
  override def combine(a: Map[U, V], b: Map[U, V]): Map[U, V] = a ++ b
}

class SemiGroupNumber[@specialized (Int, Double, Float, Long) T](implicit numeric: Numeric[T]) extends Semigroup[T] {
  override def combine(a: T, b: T): T = numeric.plus(a, b)
}

object SemiGroupImplicitTypes {
  implicit def numberSemiGroup[T](implicit numeric: Numeric[T]) = new SemiGroupNumber[T]()
  implicit def listSemiGroup[T] = new SemiGroupList[T]()
  implicit def mapSemiGroup[U,V] = new SemiGroupMap[U,V]()
  implicit def seqSemiGroup[U] = new SemiGroupSeq[U]()
}

import SemiGroupImplicitTypes._


SemiGroup[Int](1,2)
SemiGroup[List[Int]](List[Int](1,2,3), List(2,3,5))

The modified version of the implementation I implemented by using def's.