From 5f30a73936d675ddc15020591b7c7643bcd23cb4 Mon Sep 17 00:00:00 2001
From: Rohan Shah <rohan@blackfynn.com>
Date: Thu, 11 Jan 2018 22:51:41 -0500
Subject: [PATCH] Add leftFlatMap to Either (#2149)

* Add leftFlatMap to Either syntax

* Add tests for Either leftFlatMap
---
 core/src/main/scala/cats/syntax/either.scala      |  5 +++++
 tests/src/test/scala/cats/tests/EitherSuite.scala | 12 ++++++++++++
 2 files changed, 17 insertions(+)

diff --git a/core/src/main/scala/cats/syntax/either.scala b/core/src/main/scala/cats/syntax/either.scala
index bad47ec411..e9d440570d 100644
--- a/core/src/main/scala/cats/syntax/either.scala
+++ b/core/src/main/scala/cats/syntax/either.scala
@@ -148,6 +148,11 @@ final class EitherOps[A, B](val eab: Either[A, B]) extends AnyVal {
     case Right(b)    => f(b)
   }
 
+  def leftFlatMap[C, BB >: B](f: A => Either[C, BB]): Either[C, BB] = eab match {
+    case Left(a)      => f(a)
+    case r @ Right(_) => EitherUtil.leftCast(r)
+  }
+
   def compare[AA >: A, BB >: B](that: Either[AA, BB])(implicit AA: Order[AA], BB: Order[BB]): Int = eab match {
     case Left(a1)  =>
       that match {
diff --git a/tests/src/test/scala/cats/tests/EitherSuite.scala b/tests/src/test/scala/cats/tests/EitherSuite.scala
index 12f846965b..4d3a791ddd 100644
--- a/tests/src/test/scala/cats/tests/EitherSuite.scala
+++ b/tests/src/test/scala/cats/tests/EitherSuite.scala
@@ -282,4 +282,16 @@ class EitherSuite extends CatsSuite {
     }
   }
 
+  test("leftFlatMap consistent with leftMap") {
+    forAll { (either: Either[String, Int], f: String => String) =>
+      either.leftFlatMap(v => Left(f(v))) should === (either.leftMap(f))
+    }
+  }
+
+  test("leftFlatMap consistent with swap and then flatMap") {
+    forAll { (either: Either[String, Int], f: String => Either[String, Int]) =>
+      either.leftFlatMap(f) should === (either.swap.flatMap(a => f(a).swap).swap)
+    }
+  }
+
 }