Unapply

Represents a type MA that has been destructured into as a type constructor M[_] applied to type A, along with a corresponding type class instance TC[M].

The implicit conversions in the companion object provide a means to obtain type class instances for partially applied type constructors, in lieu of direct compiler support as described in SI-2712.

// Directly depending on Applicative[G]
def traverse[G[_], B](f: A => G[B])(implicit G: Applicative[G]): G[F[B]] =
 G.traverse(self)(f)

// Indirect lookup of the Applicative instance
def traverseI[GB](f: A => GB)(implicit G: Unapply[Applicative, GB]): G.M[F[G.A]] /*G[F[B]*/ = {
 G.TC.traverse(self)(a => G(f(a)))
}

// Deforested version of traverseI
def traverseI2[GB](f: A => GB)(implicit G: Unapply[Applicative, GB]): G.M[F[G.A]] /*G[F[B]*/ = {
 G.TC.traverse(self)(G.leibniz.onF(f))
}

// Old usage
def stateTraverse1 {
 import scalaz._, Scalaz._
 import State.{State, stateMonad}
 val ls = List(1, 2, 3)
 val traverseOpt: Option[List[Int]] = ls.traverse(a => Some(a))
 val traverseState: State[Int, List[Int]] = ls.traverse[State[Int, *], Int](a => State((x: Int) => (x + 1, a)))
}

// New usage
def stateTraverse2 {
 import scalaz._, Scalaz._
 val ls = List(1, 2, 3)
 val traverseOpt: Option[List[Int]] = ls.traverseI(a => some(a))
 val traverseState = ls.traverseI(a => State((x: Int) => (x + 1, a)))
}

Credits to Miles Sabin.