Welcome, Alleycats (#1984)

* moved alleycats in

* address feedback / removed duplicated class

alleycats-core/src/main/scala/alleycats/ConsK.scala

@@ -0,0 +1,20 @@
+package alleycats
+
+import cats.SemigroupK
+import export.imports
+import simulacrum.typeclass
+
+@typeclass trait ConsK[F[_]] {
+  def cons[A](hd: A, tl: F[A]): F[A]
+}
+
+object ConsK extends ConsK0 {
+  implicit def pureSemigroupKIsConsK[F[_]](implicit p: Pure[F], s: SemigroupK[F]): ConsK[F] =
+    new ConsK[F] {
+      def cons[A](hd: A, tl: F[A]): F[A] = s.combineK(p.pure(hd), tl)
+    }
+}
+
+@imports[ConsK]
+trait ConsK0
+

alleycats-core/src/main/scala/alleycats/Empty.scala

@@ -0,0 +1,36 @@
+package alleycats
+
+import cats.{Eq, Monoid}
+import cats.syntax.eq._
+import export.imports
+import simulacrum.typeclass
+import scala.collection.generic.CanBuildFrom
+
+@typeclass trait Empty[A] {
+  def empty: A
+
+  def isEmpty(a: A)(implicit ev: Eq[A]): Boolean =
+    empty === a
+
+  def nonEmpty(a: A)(implicit ev: Eq[A]): Boolean =
+    empty =!= a
+}
+
+object Empty extends EmptyInstances0 {
+  def apply[A](a: => A): Empty[A] =
+    new Empty[A] { lazy val empty: A = a }
+}
+
+trait EmptyInstances0 extends EmptyInstances1 {
+  implicit def iterableIsEmpty[CC[X] <: Iterable[X], A](implicit cbf: CanBuildFrom[CC[A], A, CC[A]]): Empty[CC[A]] =
+    Empty(cbf().result)
+}
+
+trait EmptyInstances1 extends EmptyInstances2 {
+  // If Monoid extended Empty then this could be an exported subclass instance provided by Monoid
+  implicit def monoidIsEmpty[A: Monoid]: Empty[A] =
+    Empty(Monoid[A].empty)
+}
+
+@imports[Empty]
+trait EmptyInstances2

alleycats-core/src/main/scala/alleycats/EmptyK.scala

@@ -0,0 +1,22 @@
+package alleycats
+
+import export._
+import simulacrum.typeclass
+
+@typeclass trait EmptyK[F[_]] { self =>
+  def empty[A]: F[A]
+
+  def synthesize[A]: Empty[F[A]] =
+    new Empty[F[A]] {
+      def empty: F[A] = self.empty[A]
+    }
+}
+
+@imports[EmptyK]
+object EmptyK
+
+@exports
+object EmptyKInstances {
+  @export(Instantiated)
+  implicit def instantiate[F[_], T](implicit ekf: EmptyK[F]): Empty[F[T]] = ekf.synthesize[T]
+}

alleycats-core/src/main/scala/alleycats/Extract.scala

@@ -0,0 +1,28 @@
+package alleycats
+
+import cats.{CoflatMap, Comonad}
+import export.imports
+import simulacrum.typeclass
+
+@typeclass trait Extract[F[_]] {
+  def extract[A](fa: F[A]): A
+}
+
+object Extract extends Extract0 {
+  // Ideally this would be an exported subclass instance provided by Comonad
+  implicit def comonadIsExtract[F[_]](implicit ev: Comonad[F]): Extract[F] =
+    new Extract[F] {
+      def extract[A](fa: F[A]): A = ev.extract(fa)
+    }
+
+  // Ideally this would be an instance exported to Comonad
+  implicit def extractCoflatMapIsComonad[F[_]](implicit e: Extract[F], cf: CoflatMap[F]): Comonad[F] =
+    new Comonad[F] {
+      def extract[A](fa: F[A]): A = e.extract(fa)
+      override def map[A, B](fa: F[A])(f: A => B): F[B] = cf.map(fa)(f)
+      def coflatMap[A, B](fa: F[A])(f: F[A] => B): F[B] = cf.coflatMap(fa)(f)
+    }
+}
+
+@imports[Extract]
+trait Extract0

alleycats-core/src/main/scala/alleycats/One.scala

@@ -0,0 +1,24 @@
+package alleycats
+
+import cats.{Eq, Monoid}
+import cats.syntax.eq._
+import export.imports
+import simulacrum.typeclass
+
+@typeclass trait One[A] {
+  def one: A
+
+  def isOne(a: A)(implicit ev: Eq[A]): Boolean =
+    one === a
+
+  def nonOne(a: A)(implicit ev: Eq[A]): Boolean =
+    one =!= a
+}
+
+object One extends One0 {
+  def apply[A](a: => A): One[A] =
+    new One[A] { lazy val one: A = a }
+}
+
+@imports[One]
+trait One0

alleycats-core/src/main/scala/alleycats/Pure.scala

@@ -0,0 +1,29 @@
+package alleycats
+
+import cats.{Applicative, FlatMap, Monad}
+import export.imports
+import simulacrum.typeclass
+
+@typeclass trait Pure[F[_]] {
+  def pure[A](a: A): F[A]
+}
+
+object Pure extends Pure0 {
+  // Ideally this would be an exported subclass instance provided by Applicative
+  implicit def applicativeIsPure[F[_]](implicit ev: Applicative[F]): Pure[F] =
+    new Pure[F] {
+      def pure[A](a: A): F[A] = ev.pure(a)
+    }
+
+  // Ideally this would be an instance exported to Monad
+  implicit def pureFlatMapIsMonad[F[_]](implicit p: Pure[F], fm: FlatMap[F]): Monad[F] =
+    new Monad[F] {
+      def pure[A](a: A): F[A] = p.pure(a)
+      override def map[A, B](fa: F[A])(f: A => B): F[B] = fm.map(fa)(f)
+      def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B] = fm.flatMap(fa)(f)
+      def tailRecM[A, B](a: A)(f: (A) => F[Either[A, B]]): F[B] = fm.tailRecM(a)(f)
+    }
+}
+
+@imports[Pure]
+trait Pure0

alleycats-core/src/main/scala/alleycats/Zero.scala

@@ -0,0 +1,24 @@
+package alleycats
+
+import cats.Eq
+import cats.syntax.eq._
+import export.imports
+import simulacrum.typeclass
+
+@typeclass trait Zero[A] {
+  def zero: A
+
+  def isZero(a: A)(implicit ev: Eq[A]): Boolean =
+    zero === a
+
+  def nonZero(a: A)(implicit ev: Eq[A]): Boolean =
+    zero =!= a
+}
+
+object Zero extends Zero0 {
+  def apply[A](a: => A): Zero[A] =
+    new Zero[A] { lazy val zero: A = a }
+}
+
+@imports[Zero]
+trait Zero0

alleycats-core/src/main/scala/alleycats/std/all.scala

@@ -0,0 +1,13 @@
+package alleycats
+package std
+
+import export._
+
+@reexports(
+  EmptyKInstances,
+  ListInstances,
+  OptionInstances,
+  SetInstances,
+  TryInstances,
+  IterableInstances
+) object all extends LegacySetInstances with LegacyTryInstances with LegacyIterableInstances

alleycats-core/src/main/scala/alleycats/std/iterable.scala

@@ -0,0 +1,27 @@
+package alleycats
+package std
+
+import cats.{Eval, Foldable}
+import export._
+
+@reexports(IterableInstances)
+object iterable extends LegacyIterableInstances
+
+@exports
+object IterableInstances {
+  @export(Orphan)
+  implicit val exportIterableFoldable: Foldable[Iterable] =
+    new Foldable[Iterable] {
+      override def foldLeft[A, B](fa: Iterable[A], b: B)(f: (B, A) => B): B = fa.foldLeft(b)(f)
+
+      override def foldRight[A, B](fa: Iterable[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
+        Foldable.iterateRight(fa, lb)(f)
+    }
+
+}
+
+// TODO: remove when cats.Foldable support export-hook
+trait LegacyIterableInstances {
+  implicit def legacyIterableFoldable(implicit e: ExportOrphan[Foldable[Iterable]]): Foldable[Iterable] = e.instance
+
+}

alleycats-core/src/main/scala/alleycats/std/list.scala

@@ -0,0 +1,22 @@
+package alleycats
+package std
+
+import export._
+
+@reexports(ListInstances)
+object list
+
+@exports
+object ListInstances {
+  @export(Orphan)
+  implicit val exportListEmptyK: EmptyK[List] =
+    new EmptyK[List] {
+      def empty[A]: List[A] = Nil
+    }
+
+  @export(Orphan)
+  implicit val exportListConsK: ConsK[List] =
+    new ConsK[List] {
+      def cons[A](hd: A, tl: List[A]): List[A] = hd :: tl
+    }
+}

alleycats-core/src/main/scala/alleycats/std/option.scala

@@ -0,0 +1,16 @@
+package alleycats
+package std
+
+import export._
+
+@reexports(OptionInstances)
+object option
+
+@exports
+object OptionInstances {
+  @export(Orphan)
+  implicit val exportOptionEmptyK: EmptyK[Option] =
+    new EmptyK[Option] {
+      def empty[A]: Option[A] = None
+    }
+}

alleycats-core/src/main/scala/alleycats/std/set.scala

@@ -0,0 +1,87 @@
+package alleycats.std
+
+import cats.{Applicative, Eval, Foldable, Monad, Traverse}
+import export._
+
+import scala.annotation.tailrec
+
+@exports
+object SetInstances {
+  // This method advertises parametricity, but relies on using
+  // universal hash codes and equality, which hurts our ability to
+  // rely on free theorems.
+  //
+  // Another problem some people have pointed out is the
+  // non-associativity of map when using impure functions. For
+  // example, consider the following expressions:
+  //
+  //   import scala.util.Random
+  //
+  //   val f = (_: Int) => 1
+  //   val g = (_: Int) => Random.nextInt
+  //
+  //   Set(1, 2, 3).map(f).map(g)
+  //   Set(1, 2, 3).map(f andThen g)
+  //
+  // The first Set will contain one random number, and the second will
+  // contain three. Since `g` is not a function (speaking strictly)
+  // this would not be considered a law violation, but it still makes
+  // people uncomfortable.
+  @export(Orphan)
+  implicit val setMonad: Monad[Set] =
+    new Monad[Set] {
+      def pure[A](a: A): Set[A] = Set(a)
+      override def map[A, B](fa: Set[A])(f: A => B): Set[B] = fa.map(f)
+      def flatMap[A, B](fa: Set[A])(f: A => Set[B]): Set[B] = fa.flatMap(f)
+
+      def tailRecM[A, B](a: A)(f: (A) => Set[Either[A, B]]): Set[B] = {
+        val bldr = Set.newBuilder[B]
+
+        @tailrec def go(set: Set[Either[A, B]]): Unit = {
+          val lefts = set.foldLeft(Set[A]()) { (memo, either) =>
+            either.fold(
+              memo + _,
+              b => {
+                bldr += b
+                memo
+              }
+            )
+          }
+          if (lefts.isEmpty)
+            ()
+          else
+            go(lefts.flatMap(f))
+        }
+        go(f(a))
+        bldr.result()
+      }
+    }
+
+  // Since iteration order is not guaranteed for sets, folds and other
+  // traversals may produce different results for input sets which
+  // appear to be the same.
+  @export(Orphan)
+  implicit val setTraverse: Traverse[Set] =
+    new Traverse[Set] {
+      def foldLeft[A, B](fa: Set[A], b: B)(f: (B, A) => B): B =
+        fa.foldLeft(b)(f)
+      def foldRight[A, B](fa: Set[A], lb: Eval[B])(f: (A, Eval[B]) => Eval[B]): Eval[B] =
+        Foldable.iterateRight(fa, lb)(f)
+      def traverse[G[_]: Applicative, A, B](sa: Set[A])(f: A => G[B]): G[Set[B]] = {
+        val G = Applicative[G]
+        sa.foldLeft(G.pure(Set.empty[B])) { (buf, a) =>
+          G.map2(buf, f(a))(_ + _)
+        }
+      }
+    }
+}
+
+@reexports(SetInstances)
+object set extends LegacySetInstances
+
+// TODO: remove when cats.{ Set, Traverse } support export-hook
+trait LegacySetInstances {
+  implicit def legacySetMonad(implicit e: ExportOrphan[Monad[Set]]): Monad[Set] = e.instance
+
+  implicit def legacySetTraverse(implicit e: ExportOrphan[Traverse[Set]]): Traverse[Set] = e.instance
+}