Cats 0.9.0

The biggest user-facing change in this release is to the behavior of the flatMap (and related methods) provided by EitherOps for the standard library's Either for Scala 2.10 and 2.11. These methods now match the behavior of the flatMap on Either in Scala 2.12 in that they don't require the left-hand side types to match.

For example, the following would previously compile on 2.12, but not 2.10 or 2.11:

import cats.syntax.either._

sealed abstract class AppError
case object Error1 extends AppError
case object Error2 extends AppError

val either1: Either[Error1.type, String] = Right("hi")
val either2: Either[Error2.type, String] = Right("bye")

val result: Either[AppError, String] = for {
  v1 <- either1
  v2 <- either2
} yield v1 + v2

This code now works equivalently on all supported Scala versions.

Changes:

#1424: NonEmptyList and NonEmptyVector are now covariant
#1506: flatMap provided by Either syntax matches 2.12's Either#flatMap
#1466: Improved stack safety for StateT
#1510: catchNonFatal for Future is now asynchronous

Bug fixes:

#1465 and #1507: Stack safety law for Monad#tailRecM is less eager and doesn't throw exceptions

New type class instances:

#1475: Reducible instances for Eval and Id
#1484: Show instance for Symbol

Other additions:

#1446: Cofree comonad
#1520 and #1522: intercalate for Foldable (and intercalate1 for Reducible)
#1454: asLeft and asRight syntax methods for creating Either values
#1468: tupleLeft and tupleRight for Functor
#1500: putLeft, putRight, mergeLeft, and mergeRight methods for Ior
#1495: show string interpolator
#1448: Validated#findValid (like orElse but error accumulating)
#1455: reverse for NonEmptyList
#1517: zipWithIndex for NonEmptyList
#1512 and #1514: filterNot for NonEmptyList and NonEmptyVector
#1480: FunctionK#and
#1481: EitherT.cond

Miscellaneous improvements (syntax, documentation, tests):

#1513: Improved documentation for Functor, Applicative, and Traverse
#1440: Improved type class documentation
#1442: Improved documentation for Semigroup and Monoid
#1479: Some instance traits are now package-private
#1445: Workaround for Tut issue
#1477: Use new kind-projector syntax for polymorphic lambdas
#1483: Binary compatibility checking is now part of the build for cats-kernel
#1469: More consistent instance names
#1496: Simpler creation of some SemigroupK and MonoidK instances
#1490: Avoid some duplication in build via sbt-travisci
#1497: Site list clean-up

And version updates:

#1499: 2.12 version is now 2.12.1
#1509: Scala.js version is 0.6.14

As always thanks to everyone who filed issues, participated in the Cats Gitter channel, submitted code, or helped review pull requests.

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Cats 0.9.0

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