Using Scala type tags to ensure that a String is lower case. At compile time we can require a String @@ LowerCase (that is, a String that we are promising is lower case). However at runtime there is no extra boxing, so we don't have the overhead we would have if we wrapped strings in a LowerCase class.

gistfile1.txt

scala> import scalaz.{ @@, Tag }
import scalaz.{$at$at, Tag}

scala> sealed trait LowerCase
defined trait LowerCase

// as an optimization you could also define a TrustMeLowerCase method that doesn't call toLowerCase if you know your input is already lower case
scala> def LowerCase(s: String): String @@ LowerCase = Tag[String, LowerCase](s.toLowerCase)
LowerCase: (s: String)scalaz.@@[String,LowerCase]

scala> val canonicalConcepts: Map[String @@ LowerCase, String] = Map(LowerCase("United States of America") -> "Murica")
canonicalConcepts: Map[scalaz.@@[String,LowerCase],String] = Map(united states of america -> Murica)

scala> canonicalConcepts.get("United States of America")
<console>:12: error: type mismatch;
 found   : String("United States of America")
 required: scalaz.@@[String,LowerCase]
    (which expands to)  String with AnyRef{type Tag = LowerCase}
              canonicalConcepts.get("United States of America")
                                    ^

scala> canonicalConcepts.get(LowerCase("United States of America"))
res1: Option[String] = Some(Murica)