在scalaz中使用Monoid运算符：| + | 不是Some [Double]的成员

Question

What's missing to get this working?

import scalaz._
import Scalaz._

val r = Some(1.0) |+| None
val r1 = None[Double] |+| Some(1.0)

I am getting the following:

Error:(4, 25) value |+| is not a member of Some[Double] lazy val r = Some(1.0) |+| None

                   ^

Answer 1

The problem is that a Monoid over Double is not lawful as the associativity rule can be invalidated in some cases by floating point arithmetic approximation. For this reason scalaz left that instance out of the main project and included it instead in the scalaz-outlaws one.

Include that library if you need to have an instance for Double but remember that there is a reason for this and consider your use case (eg if you are processing money transactions using floating point arithmetic you are probably doing something wrong).