new ZPure constructor 'modifyEither' (#445)

core/shared/src/main/scala/zio/prelude/fx/ZPure.scala

@@ -716,6 +716,15 @@ object ZPure {
   def modify[S1, S2, A](f: S1 => (S2, A)): ZPure[S1, S2, Any, Nothing, A] =
     Modify(f)
 
+  /**
+   * Constructs a computation that may fail from the specified modify function.
+   */
+  def modifyEither[S1, S2, E, A](f: S1 => Either[E, (S2, A)]): ZPure[S1, S2, Any, E, A] =
+    get.map(f).flatMap {
+      case Left(e)        => ZPure.fail(e)
+      case Right((s2, a)) => ZPure.succeed(a).asState(s2)
+    }
+
   /**
    * Constructs a computation that extracts the second element of a tuple.
    */

core/shared/src/test/scala/zio/prelude/fx/ZPureSpec.scala

@@ -603,7 +603,15 @@ object ZPureSpec extends DefaultRunnableSpec {
             assert(ZPure.fromEffect("a".toInt).runEither(()))(
               isLeft(equalTo(exception))
             )
-          }
+          },
+          suite("modifyEither")(
+            test("success") {
+              assert(ZPure.modifyEither((_: Int) => Right((1, "success"))).run(0))(equalTo((1, "success")))
+            },
+            test("failure") {
+              assert(ZPure.modifyEither((_: Int) => Left("error")).runEither(0))(isLeft(equalTo("error")))
+            }
+          )
         )
       )
     )