Method injection (enrich my library) 

If we were to write a function that sums two types using the Monoid, we need to call it like this.

scala> def plus[A: Monoid](a: A, b: A): A = implicitly[Monoid[A]].mappend(a, b)
plus: [A](a: A, b: A)(implicit evidence$1: Monoid[A])A

scala> plus(3, 4)
res25: Int = 7

We would like to provide an operator. But we don’t want to enrich just one type, but enrich all types that has an instance for Monoid. Let me do this in Scalaz 7 style.

scala> trait MonoidOp[A] {
         val F: Monoid[A]
         val value: A
         def |+|(a2: A) = F.mappend(value, a2)
defined trait MonoidOp

scala> implicit def toMonoidOp[A: Monoid](a: A): MonoidOp[A] = new MonoidOp[A] {
         val F = implicitly[Monoid[A]]
         val value = a
toMonoidOp: [A](a: A)(implicit evidence$1: Monoid[A])MonoidOp[A]

scala> 3 |+| 4
res26: Int = 7

scala> "a" |+| "b"
res28: String = ab

We were able to inject |+| to both Int and String with just one definition.

Standard type syntax 

Using the same technique, Scalaz also provides method injections for standard library types like Option and Boolean:

scala> 1.some | 2
res0: Int = 1

scala> Some(1).getOrElse(2)
res1: Int = 1

scala> (1 > 10)? 1 | 2
res3: Int = 2

scala> if (1 > 10) 1 else 2
res4: Int = 2

I hope you could get some feel on where Scalaz is coming from.