From 70d2c44baf7798a9d5627a312d344e7095784676 Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Jo=C3=A3o=20Ferreira?= <jtjeferreira@gmail.com>
Date: Mon, 24 Apr 2017 10:10:25 +0100
Subject: [PATCH] improve EitherT inference for use in for-comprehension

---
 core/src/main/scala/cats/data/EitherT.scala        | 6 +++---
 tests/src/test/scala/cats/tests/EitherTTests.scala | 7 +++++++
 2 files changed, 10 insertions(+), 3 deletions(-)

diff --git a/core/src/main/scala/cats/data/EitherT.scala b/core/src/main/scala/cats/data/EitherT.scala
index 1b51488624..8f7b6149c7 100644
--- a/core/src/main/scala/cats/data/EitherT.scala
+++ b/core/src/main/scala/cats/data/EitherT.scala
@@ -71,19 +71,19 @@ final case class EitherT[F[_], A, B](value: F[Either[A, B]]) {
   def applyAlt[D](ff: EitherT[F, A, B => D])(implicit F: Apply[F]): EitherT[F, A, D] =
     EitherT[F, A, D](F.map2(this.value, ff.value)((xb, xbd) => Apply[Either[A, ?]].ap(xbd)(xb)))
 
-  def flatMap[AA >: A, D](f: B => EitherT[F, AA, D])(implicit F: Monad[F]): EitherT[F, AA, D] =
+  def flatMap[D](f: B => EitherT[F, A, D])(implicit F: Monad[F]): EitherT[F, A, D] =
     EitherT(F.flatMap(value) {
       case l @ Left(_) => F.pure(l.rightCast)
       case Right(b) => f(b).value
     })
 
-  def flatMapF[AA >: A, D](f: B => F[Either[AA, D]])(implicit F: Monad[F]): EitherT[F, AA, D] =
+  def flatMapF[D](f: B => F[Either[A, D]])(implicit F: Monad[F]): EitherT[F, A, D] =
     flatMap(f andThen EitherT.apply)
 
   def transform[C, D](f: Either[A, B] => Either[C, D])(implicit F: Functor[F]): EitherT[F, C, D] =
     EitherT(F.map(value)(f))
 
-  def subflatMap[AA >: A, D](f: B => Either[AA, D])(implicit F: Functor[F]): EitherT[F, AA, D] =
+  def subflatMap[D](f: B => Either[A, D])(implicit F: Functor[F]): EitherT[F, A, D] =
     transform(_.flatMap(f))
 
   def map[D](f: B => D)(implicit F: Functor[F]): EitherT[F, A, D] = bimap(identity, f)
diff --git a/tests/src/test/scala/cats/tests/EitherTTests.scala b/tests/src/test/scala/cats/tests/EitherTTests.scala
index dbe79220ce..a611d12d26 100644
--- a/tests/src/test/scala/cats/tests/EitherTTests.scala
+++ b/tests/src/test/scala/cats/tests/EitherTTests.scala
@@ -378,4 +378,11 @@ class EitherTTests extends CatsSuite {
       }
     }
   }
+  test("inference works in for-comprehension") {
+    val either: Id[Either[String, Int]] = 0.asRight.pure[Id]
+    for {
+      a <- EitherT(either)
+      b <- EitherT.right(1.pure[Id])
+    } yield (a + b)
+  }
 }