Added liftT to WriterT (issue #1315) (#1325)

* Attempt to solve #1315 * Attempt to solve #1315
typelevel · Aug 23, 2016 · 7feff3e · 7feff3e
1 parent 06539fa
commit 7feff3e
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diff --git a/core/src/main/scala/cats/data/WriterT.scala b/core/src/main/scala/cats/data/WriterT.scala
@@ -57,7 +57,13 @@ final case class WriterT[F[_], L, V](run: F[(L, V)]) {
 
   def show(implicit F: Show[F[(L, V)]]): String = F.show(run)
 }
-object WriterT extends WriterTInstances with WriterTFunctions
+
+object WriterT extends WriterTInstances with WriterTFunctions {
+
+  def lift[F[_], L, V](fv: F[V])(implicit monoidL: Monoid[L], F: Applicative[F]): WriterT[F, L, V] =
+    WriterT(F.map(fv)(v => (monoidL.empty, v)))
+
+}
 
 private[data] sealed abstract class WriterTInstances extends WriterTInstances0 {
 

diff --git a/tests/src/test/scala/cats/tests/WriterTTests.scala b/tests/src/test/scala/cats/tests/WriterTTests.scala
@@ -65,7 +65,15 @@ class WriterTTests extends CatsSuite {
       WriterT.valueT[Id, Int, Int](i).value should === (i)
     }
   }
-
+
+  test("Writer.pure and WriterT.lift are consistent") {
+    forAll { (i: Int) =>
+      val writer: Writer[String, Int] = Writer.value(i)
+      val writerT: WriterT[Option, String, Int] = WriterT.lift(Some(i))
+      writer.run.some should === (writerT.run)
+    }
+  }
+
   test("show") {
     val writerT: WriterT[Id, List[String], String] = WriterT.put("foo")(List("Some log message"))
     writerT.show should === ("(List(Some log message),foo)")