How to find out what happens when I use Monoid for Map in scalaz

Question

• How to find all instances Monoid. For example, how do you know if there is an instance Monoidfor Mapin scalaz? And if so, where is it in the source code. I tried the following without success

  @ implicitly[Monoid[Map[_, _]]]
  Main.scala:1146: could not find implicit value for parameter e: scalaz.Monoid[Map[_, _]]
  implicitly[Monoid[Map[_, _]]]
            ^
  Compilation Failed
  

• How can I see what happens (implicit conversions, ...) when I execute code from REPL, for example

  Map("a", 1) |+| Map("a", 1)
  

Answer 1

• Unable to find all instances of type class.

  In particular, Mapthis depends on the type of values, because a monoid Map[K, V]instance needs an instance Semigroup[V].

  Code Map Monoidcan be found at scalaz.std.map.

• :

  import scalaz.std.map._
  import scalaz.std.anyVal._
  import scalaz.syntax.semigroup._
  
  import scala.reflect.runtime.universe._
  
  showCode(reify { Map("a" -> 1) |+| Map("a" -> 1)  }.tree)
  // `package`.monoid.ToSemigroupOps(
  //   Predef.Map.apply(Predef.ArrowAssoc("a").->(1)))
  //   (map.mapMonoid(Predef.this.DummyImplicit.dummyImplicit, anyVal.intInstance))
  //   .|+|(Predef.Map.apply(Predef.ArrowAssoc("a").->(1)))
  

  Scala :

  • ToSemigroupOps, |+| Map.
  • Monoid Map[String, Int], Semigroup[Int].