Add WriterT MonadError instance

typelevel · Nov 21, 2015 · 266665f · 266665f
1 parent 482bc2e
commit 266665f
Showing 1 changed file with 18 additions and 1 deletion.
diff --git a/core/src/main/scala/cats/data/WriterT.scala b/core/src/main/scala/cats/data/WriterT.scala
@@ -104,12 +104,20 @@ private[data] sealed abstract class WriterTInstances3 extends WriterTInstances4
     }
 }
 
-private[data] sealed abstract class WriterTInstances4 {
+private[data] sealed abstract class WriterTInstances4 extends WriterTInstances5 {
   implicit def writerTFunctor[F[_], L](implicit F: Functor[F]): Functor[WriterT[F, L, ?]] = new WriterTFunctor[F, L] {
     implicit val F0: Functor[F] = F
   }
 }
 
+private[data] sealed abstract class WriterTInstances5 {
+  implicit def writerTMonadError[F[_], L, E](implicit F: MonadError[F, E], L: Monoid[L]): MonadError[WriterT[F, L, ?], E] =
+    new WriterTMonadError[F, L, E] {
+      implicit val F0: MonadError[F, E] = F
+      implicit val L0: Monoid[L] = L
+    }
+}
+
 private[data] sealed trait WriterTFunctor[F[_], L] extends Functor[WriterT[F, L, ?]] {
   implicit def F0: Functor[F]
 
@@ -149,6 +157,15 @@ private[data] sealed trait WriterTMonad[F[_], L] extends WriterTApplicative[F, L
     fa.flatMap(f)
 }
 
+private[data] sealed trait WriterTMonadError[F[_], L, E] extends MonadError[WriterT[F, L, ?], E] with WriterTMonad[F, L] {
+  override implicit def F0: MonadError[F, E]
+
+  def raiseError[A](e: E): WriterT[F, L, A] = WriterT(F0.raiseError[(L, A)](e))
+
+  def handleErrorWith[A](fa: WriterT[F, L, A])(f: E => WriterT[F, L, A]): WriterT[F, L, A] =
+    WriterT(F0.handleErrorWith(fa.run)(e => f(e).run))
+}
+
 trait WriterTFunctions {
   def putT[F[_], L, V](vf: F[V])(l: L)(implicit functorF: Functor[F]): WriterT[F, L, V] =
     WriterT(functorF.map(vf)(v => (l, v)))