Why there is no Semigroup instance for TrieMap in scalaz

Question

While I can do

Map("a" -> 1) |+| Map("a" -> 2)

It seems there is no support for

TrieMap("a" -> 1) |+| TrieMap("a" -> 2)

Why ?

Answer 1

Scalaz doesn't provide every possible Type+Operation combination that form a Semigroup, but you can easily implement it, for instance with the following implicit:

import scala.collection.concurrent.TrieMap
import scalaz._, Scalaz._

implicit def trieMapSemigroup[K: BuildKeyConstraint, V: Semigroup]: Semigroup[TrieMap[K, V]] =
  new Semigroup[TrieMap[K, V]] {
    def append(m1: TrieMap[K, V], m2: => TrieMap[K, V]) =
      (m1 /: m2) { case (res, (k, v)) =>
        res += (k -> res.get(k).cata(Semigroup[V].append(_, v), v))
      }
  }

That gives you as expected:

scala> TrieMap("a" -> 1) |+| TrieMap("a" -> 2)
res: TrieMap(a -> 3)

Of course that will work for every value as semigroup that has been defined in scalaz.

---

What about Monoid?

If you want an optimised version of the implicit that works as a Monoid too:

  implicit def trieMapMonoid[K: BuildKeyConstraint, V: Semigroup]: Monoid[TrieMap[K, V]] = new Monoid[TrieMap[K, V]] {
    def zero = TrieMap.empty[K, V]

    def append(m1: TrieMap[K, V], m2: => TrieMap[K, V]) = {
      val m2Instance: TrieMap[K, V] = m2
      val (from, to, semigroup) = {
        if (m1.size > m2Instance.size) (m2Instance, m1, Semigroup[V].append(_: V, _: V))
        else (m1, m2Instance, (Semigroup[V].append(_: V, _: V)).flip)
      }
      from.foldLeft(to) {
        case (res, (k, v)) => res += (k -> res.get(k).map(semigroup(_, v)).getOrElse(v))
      }
    }
  }

(code shameless adapted from scalaz/std/Map.scala)