Add Show[Set]

core/src/main/scala/cats/std/set.scala

@@ -1,6 +1,8 @@
 package cats
 package std
 
+import cats.syntax.show._
+
 trait SetInstances extends algebra.std.SetInstances {
   implicit val setInstance: Foldable[Set] with MonoidK[Set] =
     new Foldable[Set] with MonoidK[Set] {
@@ -25,4 +27,9 @@ trait SetInstances extends algebra.std.SetInstances {
     }
 
     implicit def setMonoid[A]: Monoid[Set[A]] = MonoidK[Set].algebra[A]
+
+  implicit def setShow[A:Show]: Show[Set[A]] = new Show[Set[A]] {
+    def show(fa: Set[A]): String =
+      fa.toIterator.map(_.show).mkString("Set(", ", ", ")")
+  }
 }

tests/src/test/scala/cats/tests/SetTests.scala

@@ -11,4 +11,21 @@ class SetTests extends CatsSuite {
 
   checkAll("Set[Int]", FoldableTests[Set].foldable[Int, Int])
   checkAll("Foldable[Set]", SerializableTests.serializable(Foldable[Set]))
+
+  test("show"){
+    Set(1, 1, 2, 3).show should === ("Set(1, 2, 3)")
+    Set.empty[String].show should === ("Set()")
+
+    forAll { fs: Set[String] =>
+      fs.show should === (fs.toString)
+    }
+  }
+
+  test("show keeps separate entries for items that map to identical strings"){
+    implicit val intShow: Show[Int] = Show.show(_ => "1")
+    // an implementation implemented as set.map(_.show).mkString(", ") would
+    // only show one entry in the result instead of 3, because Set.map combines
+    // duplicate items in the codomain.
+    Set(1, 2, 3).show should === ("Set(1, 1, 1)")
+  }
 }