Show and Eq instances for cats

build.sbt

@@ -9,6 +9,7 @@ val gitHubOwner = "fthomas"
 val gitPubUrl = s"https://github.com/$gitHubOwner/$projectName.git"
 val gitDevUrl = s"git@github.com:$gitHubOwner/$projectName.git"
 
+val catsVersion = "0.9.0"
 val macroCompatVersion = "1.1.1"
 val macroParadiseVersion = "2.1.0"
 val shapelessVersion = "2.3.2"
@@ -23,7 +24,7 @@ val macroParadise = compilerPlugin(
   "org.scalamacros" % "paradise" % macroParadiseVersion % Test cross CrossVersion.patch)
 
 val allSubprojects =
-  Seq("core", "eval", "scalacheck", "scalaz", "scodec", "pureconfig")
+  Seq("cats", "core", "eval", "scalacheck", "scalaz", "scodec", "pureconfig")
 val allSubprojectsJVM = allSubprojects.map(_ + "JVM")
 val allSubprojectsJS = {
   val jvmOnlySubprojects = Seq("pureconfig")
@@ -34,7 +35,9 @@ val allSubprojectsJS = {
 
 lazy val root = project
   .in(file("."))
-  .aggregate(coreJVM,
+  .aggregate(catsJVM,
+             catsJS,
+             coreJVM,
              coreJS,
              docs,
              evalJVM,
@@ -55,6 +58,19 @@ lazy val root = project
     parallelExecution in Test in ThisBuild := false
   )
 
+lazy val cats = crossProject
+  .configureCross(moduleCrossConfig("cats"))
+  .dependsOn(core % "compile->compile;test->test")
+  .settings(
+    libraryDependencies += "org.typelevel" %%% "cats-core" % catsVersion,
+    initialCommands += s"""
+      import $rootPkg.cats._
+    """
+  )
+
+lazy val catsJVM = cats.jvm
+lazy val catsJS = cats.js
+
 lazy val core = crossProject
   .configureCross(moduleCrossConfig("core"))
   .enablePlugins(BuildInfoPlugin)

latestVersion.sbt

@@ -5,4 +5,5 @@ latestVersionInSeries in ThisBuild := Some("0.8.1")
 unreleasedModules in ThisBuild := Set(
   // Example:
   // "refined-eval"
+  "refined-cats"
 )

modules/cats/shared/src/main/scala/eu/timepit/refined/cats/package.scala

@@ -0,0 +1,23 @@
+package eu.timepit.refined
+
+import _root_.cats.kernel.Eq
+import _root_.cats.Show
+import _root_.cats.syntax.contravariant._
+import eu.timepit.refined.api.RefType
+
+package object cats {
+
+  /**
+   * `Eq` instance for refined types that delegates to the `Eq`
+   * instance of the base type.
+   */
+  implicit def refTypeEq[F[_, _], T: Eq, P](implicit rt: RefType[F]): Eq[F[T, P]] =
+    Eq[T].contramap(rt.unwrap)
+
+  /**
+   * `Show` instance for refined types that delegates to the `Show`
+   * instance of the base type.
+   */
+  implicit def refTypeShow[F[_, _], T: Show, P](implicit rt: RefType[F]): Show[F[T, P]] =
+    Show[T].contramap(rt.unwrap)
+}

modules/cats/shared/src/test/scala/eu/timepit/refined/cats/CatsSpec.scala

@@ -0,0 +1,23 @@
+package eu.timepit.refined.cats
+
+import cats.instances.int._
+import cats.syntax.eq._
+import cats.syntax.show._
+import eu.timepit.refined.auto._
+import eu.timepit.refined.types.numeric.PosInt
+import org.scalacheck.Prop._
+import org.scalacheck.Properties
+
+class CatsSpec extends Properties("cats") {
+
+  property("Equal") = secure {
+    val x: PosInt = 5
+    val y: PosInt = 5
+    x === y
+  }
+
+  property("Show") = secure {
+    val x: PosInt = 5
+    x.show ?= "5"
+  }
+}