Merge pull request #3556 from TimWSpence/order-for-writert

Order for writert
typelevel · Aug 12, 2020 · 0640b67 · 0640b67
2 parents bb8baa7 + eacd388
commit 0640b67
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diff --git a/core/src/main/scala/cats/data/WriterT.scala b/core/src/main/scala/cats/data/WriterT.scala
@@ -320,6 +320,9 @@ final case class WriterT[F[_], L, V](run: F[(L, V)]) {
     G.map(
       F.traverse(run)(lv => G.tupleLeft(f(lv._2), lv._1))
     )(WriterT.apply)
+
+  def compare(that: WriterT[F, L, V])(implicit Ord: Order[F[(L, V)]]): Int =
+    Ord.compare(run, that.run)
 }
 
 object WriterT extends WriterTInstances with WriterTFunctions with WriterTFunctions0 {
@@ -377,6 +380,11 @@ sealed abstract private[data] class WriterTInstances1 extends WriterTInstances2
 
   implicit def catsDataFoldableForWriterTId[L](implicit F: Foldable[Id]): Foldable[WriterT[Id, L, *]] =
     catsDataFoldableForWriterT[Id, L](F)
+
+  implicit def catsDataOrderForWriterT[F[_], L, V](implicit Ord: Order[F[(L, V)]]): Order[WriterT[F, L, V]] =
+    new Order[WriterT[F, L, V]] {
+      def compare(x: WriterT[F, L, V], y: WriterT[F, L, V]): Int = x.compare(y)
+    }
 }
 
 sealed abstract private[data] class WriterTInstances2 extends WriterTInstances3 {

diff --git a/tests/src/test/scala/cats/tests/WriterTSuite.scala b/tests/src/test/scala/cats/tests/WriterTSuite.scala
@@ -11,6 +11,7 @@ import cats.syntax.option._
 import cats.syntax.eq._
 import org.scalacheck.Prop._
 import org.scalacheck.Test.Parameters
+import cats.kernel.laws.discipline.OrderTests
 
 class WriterTSuite extends CatsSuite {
   type Logged[A] = Writer[ListWrapper[Int], A]
@@ -386,6 +387,20 @@ class WriterTSuite extends CatsSuite {
     checkAll("Writer[Int, Int]", SemigroupTests[Writer[Int, Int]].semigroup)
   }
 
+  {
+    // F[(L, V)] has a semigroup
+    implicit val FLV: Order[ListWrapper[(Int, Int)]] = ListWrapper.order[(Int, Int)]
+
+    Order[WriterT[ListWrapper, Int, Int]]
+    checkAll("WriterT[ListWrapper, Int, Int]", OrderTests[WriterT[ListWrapper, Int, Int]].order)
+    checkAll("Order[WriterT[ListWrapper, Int, Int]]",
+             SerializableTests.serializable(Order[WriterT[ListWrapper, Int, Int]])
+    )
+
+    Order[Writer[Int, Int]]
+    checkAll("Writer[Int, Int]", OrderTests[Writer[Int, Int]].order)
+  }
+
   {
     // F has an Applicative and L has a Monoid
     implicit val L: Monoid[ListWrapper[Int]] = ListWrapper.monoid[Int]