Map implicits for scalaz

MapImplicits.scala

  implicit def MapMonoid[K, V](implicit valueSemigroup: Semigroup[V]): Monoid[Map[K, V]] = new Monoid[Map[K, V]] {
    override val zero = Map.empty[K, V]
    override def append(m1: Map[K, V], m2: => Map[K, V]) = { 
      val (from, to, semigroup) = { 
        if (m1.size > m2.size) (m2, m1, (v1: V, v2: V) => valueSemigroup.append(v1, v2))
        else (m1, m2, (v1: V, v2: V) => valueSemigroup.append(v2, v1))
      }   

      from.foldLeft(to) {
        case (to, (k, v)) => to + (k -> to.get(k).map(semigroup(_, v)).getOrElse(v))
      }   
    }   
  }

  implicit def mapPlus[V](implicit valueSemigroup: Semigroup[V]): Plus[({type λ[α] = Map[α, V]})#λ] = new Plus[({type λ[α] = Map[α, V]})#λ] {
    override def plus[K](m1: Map[K, V], m2: => Map[K, V]) = MapMonoid[K, V].append(m1, m2) 
  }

  implicit def mapMA[K, V](m: Map[K, V])(implicit valueSemigroup: Semigroup[V]): MA[({type λ[α] = Map[α, V]})#λ, K] = new MA[({type λ[α] = Map[α, V]})#λ, K] {
    override val value = m 
  }