Monoid implementation for Double. From ScalaZ 7.1 Double and Float are no longer supported as they don't totally fulfill the Monoid laws.

testing.sc

package com.example

import scalaz._
import Scalaz._


object testing {
  
  1 |+| 1                                         //> res0: Int = 2
  
  //ScalaZ no longer supports a Monoid of Double or Float as they don't
  //pass the monoid laws as per https://github.com/scalaz/scalaz/issues/334.
  // Here's my own:

  implicit val doubleInstance: Monoid[Double] = new Monoid[Double] {
  	def append(f1: Double, f2: => Double) = f1 + f2
  	def zero: Double = 0d
  }                                               //> doubleInstance  : scalaz.Monoid[Double] = com.example.testing$$anonfun$main$
                                                  //| 1$$anon$1@7aa34f04
  import Tags.{Multiplication}
  implicit val doubleMultiplicationNewType: Semigroup[Double @@ Multiplication] = new Semigroup[Double @@ Multiplication] {
		def append(f1: Double @@ Multiplication, f2: => Double @@ Multiplication) = Multiplication(Tag.unwrap(f1) * Tag.unwrap(f2))
		def zero: Double @@ Multiplication = Multiplication(1.0d)
	}                                         //> doubleMultiplicationNewType  : scalaz.Semigroup[scalaz.@@[Double,scalaz.Tags
                                                  //| .Multiplication]] = com.example.testing$$anonfun$main$1$$anon$2@75846058
  
  
  2.0 |+| 3.5                                     //> res1: Double = 5.5
  Multiplication(2.0) |+| Multiplication(3.5)     //> res2: scalaz.@@[Double,scalaz.Tags.Multiplication] = 7.0
  
  
  
  //List(true, true, false, true) foldMap { _. }
  
  
  
  case class Blah(security: String, price: Double)
  val myList = List(Blah("a", 2.0), Blah("b", 4.0))
                                                  //> myList  : List[com.example.testing.Blah] = List(Blah(a,2.0), Blah(b,4.0))

  //myList.foldMap(Tags.Monoidal(_.price))
  
}