Using EquaSets passed to methods

gistfile1.scala

//
// This gist shows how the compiler will deal with EquaSets as method parameters.
// In short, the compiler will let you do anything that is known to be OK. In
// particular, it won't let you union, intersect, or diff two EquaSets unless
// the full path dependent type is known and matches.
//
scala> import org.scalactic._

scala> val trimmed = EquaSets[String](StringNormalizations.trimmed.toHashingEquality)
trimmed: org.scalactic.EquaSets[String] = org.scalactic.EquaSets@2c011b16

scala> val lowered = SortedEquaSets[String](StringNormalizations.lowerCased.toOrderingEquality)
lowered: org.scalactic.SortedEquaSets[String] = org.scalactic.SortedEquaSets@3b2d73bb

// If I take explicit paths, it won't let me union in the body, which is what we'd want
scala> def takesSets(tset: trimmed.EquaSet, wset: lowered.EquaSet) = tset union wset
<console>:12: error: type mismatch;
 found   : lowered.EquaSet
 required: trimmed.EquaSet
       def takesSets(tset: trimmed.EquaSet, wset: lowered.EquaSet) = tset union wset
                                                                                ^

// But it will let me do something like call size, which is also what we want
scala> def takesSets(tset: trimmed.EquaSet, wset: lowered.EquaSet) = tset.size + wset.size
takesSets: (tset: trimmed.EquaSet, wset: lowered.EquaSet)Int

scala> takesSets(trimmed.EquaSet("hi"), lowered.SortedEquaSet("ho"))
res0: Int = 2

// It won't let me pass the wrong path in, which is what we'd want
scala> takesSets(trimmed.EquaSet("hi"), trimmed.EquaSet("ho"))
<console>:14: error: type mismatch;
 found   : trimmed.EquaSet
 required: lowered.EquaSet
              takesSets(trimmed.EquaSet("hi"), trimmed.EquaSet("ho"))
                                                              ^

// It will let me any kind of EquaSet, and let me do things like call size on it, which
// is again what we'd ant
scala> def takesSets(tset: EquaSets[String]#EquaSet, wset: EquaSets[String]#EquaSet) = tset.size + wset.size
takesSets: (tset: org.scalactic.EquaSets[String]#EquaSet, wset: org.scalactic.EquaSets[String]#EquaSet)Int

// But it won't let me union, which is also what we want, because union requires more information than
// we have in these types. The error message is a bit non-obvious though.
scala> def takesSets(tset: EquaSets[String]#EquaSet, wset: EquaSets[String]#EquaSet) = tset union wset
<console>:10: error: type mismatch;
 found   : org.scalactic.EquaSets[String]#EquaSet
 required: _10.EquaSet where val _10: org.scalactic.EquaSets[String]
       def takesSets(tset: EquaSets[String]#EquaSet, wset: EquaSets[String]#EquaSet) = tset union wset
                                                                                                  ^

// This is the build.sbt I used to generate the above REPL session. Just
// place this in a directory and say:
// $ sbt
// ...
// > console
scalaVersion := "2.11.2"

libraryDependencies += "org.scalactic" % "scalactic_2.11" % "3.0.0-SNAP1"

libraryDependencies += "org.scalatest" % "scalatest_2.11" % "3.0.0-SNAP1" % "test"

initialCommands in console := "import org.scalactic._"

initialCommands in Test in console := """|import org.scalatest._
                                         |import org.scalactic._
                                         |import Matchers._""".stripMargin
                                         
resolvers += "Sonatype OSS Releases" at "https://oss.sonatype.org/content/repositories/releases"