Copy/paste of some of tpolecat's doobie catchables.

At his suggestion, see issue rubicon-project#12 comments.

src/main/scala/rubiz/syntax/catchable.scala

@@ -1,8 +1,9 @@
 package rubiz
 package syntax
 
-import scalaz.{ Catchable, Monad }
+import scalaz.{Catchable, Monad, \/}
 import scalaz.syntax.monad._
+
 trait CatchableSyntax {
   implicit final def catchableOps[M[_]: Catchable, A](ma: M[A]): CatchableOps[M, A] = new CatchableOps[M, A](ma)
 }
@@ -14,4 +15,34 @@ final class CatchableOps[M[_], A](val ma: M[A])(implicit catchableM: Catchable[M
   def ensure(error: => Throwable)(f: A => Boolean)(implicit monadM: Monad[M]): M[A] = ma.flatMap { a =>
     if (f(a)) ma else catchableM.fail(error)
   }
+
+  /**
+    * Like `attempt` but catches (and maps) only where defined.
+    */
+  def attemptSome[M[_]: Monad, A, B](ma: M[A])(p: PartialFunction[Throwable, B])(implicit c: Catchable[M]): M[B \/ A] =
+    c.attempt(ma).map(_.leftMap(e => p.lift(e).getOrElse(throw e)))
+
+  /**
+    * Executes the handler, for exceptions propagating from `ma`.
+    */
+  def except[M[_]: Monad, A](ma: M[A])(handler: Throwable => M[A])(implicit c: Catchable[M]): M[A] =
+    c.attempt(ma).flatMap(_.bimap(handler, _.pure[M]).merge)
+
+  /**
+    * Executes the handler where defined, for exceptions propagating from `ma`.
+    */
+  def exceptSome[M[_]: Monad: Catchable, A](ma: M[A])(pf: PartialFunction[Throwable, M[A]]): M[A] =
+    except(ma)(e => pf.lift(e).getOrElse((throw e): M[A]))
+
+  /**
+    * Like "finally", but only performs the final action if there was an exception.
+    */
+  def onException[M[_]: Monad, A, B](ma: M[A])(action: M[B])(implicit c: Catchable[M]): M[A] =
+    except(ma)(e => action *> c.fail(e))
+
+  /**
+    * Always execute `sequel` following `ma`; generalizes `finally`.
+    */
+  def ensuring[M[_]: Monad: Catchable, A, B](ma: M[A])(sequel: M[B]): M[A] =
+    onException(ma)(sequel) <* sequel
 }