Issue 2304: Missing functions for RWST

core/src/main/scala/cats/data/IndexedReaderWriterStateT.scala

@@ -45,6 +45,21 @@ final class IndexedReaderWriterStateT[F[_], E, L, SA, SB, A](val runF: F[(E, SA)
       }
     }
 
+  /**
+   * Example:
+   * {{{
+   * scala> import cats.implicits._
+   * scala> val x: IndexedReaderWriterStateT[Option, String, String, Int, Int, Unit] = IndexedReaderWriterStateT.tell("something")
+   * scala> val y: IndexedReaderWriterStateT[Option, String, String, Int, Int, (Unit, String)] = x.listen
+   * scala> y.run("environment", 17)
+   * res0: Option[(String, Int, (Unit, String))] = Some((something,17,((),something)))
+   * }}}
+   */
+  def listen(implicit F: Functor[F]): IndexedReaderWriterStateT[F, E, L, SA, SB, (A, L)] =
+    transform { (l, s, a) =>
+      (l, s, (a, l))
+    }
+
   /**
    * Modify the result of the computation using `f`.
    */
@@ -437,6 +452,10 @@ abstract private[data] class RWSTFunctions extends CommonIRWSTConstructors {
   def modifyF[F[_], E, L, S](f: S => F[S])(implicit F: Applicative[F],
                                            L: Monoid[L]): ReaderWriterStateT[F, E, L, S, Unit] =
     ReaderWriterStateT((_, s) => F.map(f(s))((L.empty, _, ())))
+
+  def listen[F[_], E, L, S, A](rwst: ReaderWriterStateT[F, E, L, S, A])(
+    implicit F: Functor[F]
+  ): ReaderWriterStateT[F, E, L, S, (A, L)] = rwst.listen
 }
 
 /**
@@ -491,6 +510,9 @@ abstract private[data] class RWSFunctions {
    */
   def tell[E, L, S](l: L): ReaderWriterState[E, L, S, Unit] =
     ReaderWriterStateT.tell(l)
+
+  def listen[E, L, S, A](rws: ReaderWriterState[E, L, S, A]): ReaderWriterState[E, L, S, (A, L)] =
+    rws.listen
 }
 
 sealed abstract private[data] class IRWSTInstances extends IRWSTInstances1 {

tests/src/test/scala/cats/tests/FoldableSuite.scala

@@ -151,6 +151,13 @@ abstract class FoldableSuite[F[_]: Foldable](name: String)(implicit ArbFInt: Arb
       fa.collectFold(pf) should ===(fa.toList.collect(pf).fold(m.empty)(m.combine))
 
       def g(a: String): Option[String] = Some(a).filter(f)
+
+      // `collectSomeFold` (deprecated) is used here instead of `collectFoldSome` to
+      // keep testing the deprecated code until it's finally removed. This helps with
+      // the coverage and, most importantly, prevents from breaking deprecated code paths
+      // that might still be in use.
+      //
+      // https://github.com/typelevel/cats/pull/3278#discussion_r372841693
       fa.collectSomeFold(g) should ===(fa.toList.filter(f).fold(m.empty)(m.combine))
     }
   }

tests/src/test/scala/cats/tests/IndexedReaderWriterStateTSuite.scala

@@ -328,6 +328,26 @@ class ReaderWriterStateTSuite extends CatsSuite {
     }
   }
 
+  test("pure + listen + map(_._1) + runEmptyA is identity") {
+    forAll { i: Int =>
+      IndexedReaderWriterStateT
+        .pure[Id, String, String, Int, Int](i)
+        .listen
+        .map(_._1)
+        .runEmptyA("") should ===(i)
+    }
+  }
+
+  test("tell + listen + map(_._2) + runEmptyA is identity") {
+    forAll { s: String =>
+      IndexedReaderWriterStateT
+        .tell[Id, String, String, Int](s)
+        .listen
+        .map(_._2)
+        .runEmptyA("") should ===(s)
+    }
+  }
+
   implicit val iso: Isomorphisms[IndexedReaderWriterStateT[ListWrapper, String, String, Int, String, *]] =
     Isomorphisms.invariant[IndexedReaderWriterStateT[ListWrapper, String, String, Int, String, *]](
       IndexedReaderWriterStateT.catsDataFunctorForIRWST(ListWrapper.functor)