scala - Why there is no Semigroup instance for TrieMap in scalaz -

while can do

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

it seems there no support

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

why ?

scalaz doesn't provide every possible type+operation combination form semigroup, can implement it, instance 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 expected:

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

of course work every value semigroup has been defined in scalaz.

---

what monoid?

if want optimised version of implicit works monoid too:

  implicit def triemapmonoid[k: buildkeyconstraint, v: semigroup]: monoid[triemap[k, v]] = new monoid[triemap[k, v]] {     def 0 = 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 scalaz/std/map.scala)