Add instances for functions of all arities

`Monad` and `Distributive` instances.
`Defer` was not stack safe.

core/src/main/scala/cats/derived/function.scala

@@ -0,0 +1,42 @@
+package cats.derived
+
+import cats.derived.util.FnGeneric
+import cats.instances.function._
+import cats.{Distributive, Functor, Monad}
+import shapeless._
+
+object function extends FunctionInstances {
+
+  implicit def kittensMkMonadForFunctionN[F[_], L <: _ :: _ :: _](
+    implicit gen: FnGeneric.Aux[F, L]
+  ): Monad[F] = new Monad[F] {
+    private[this] val F = Monad[L => *]
+
+    def pure[A](x: A): F[A] =
+      gen.from(F.pure(x))
+
+    override def map[A, B](fa: F[A])(f: A => B): F[B] =
+      gen.from(F.map(gen.to(fa))(f))
+
+    def flatMap[A, B](fa: F[A])(f: A => F[B]): F[B] =
+      gen.from(F.flatMap(gen.to(fa))(a => gen.to(f(a))))
+
+    def tailRecM[A, B](a: A)(f: A => F[Either[A, B]]): F[B] =
+      gen.from(F.tailRecM(a)(a => gen.to(f(a))))
+  }
+}
+
+private[derived] abstract class FunctionInstances {
+
+  implicit def kittensMkDistributiveForFunctionN[F[_], L <: _ :: _ :: _](
+    implicit gen: FnGeneric.Aux[F, L]
+  ): Distributive[F] = new Distributive[F] {
+    private[this] val F = Distributive[L => *]
+
+    def map[A, B](fa: F[A])(f: A => B): F[B] =
+      gen.from(F.map(gen.to(fa))(f))
+
+    def distribute[G[_]: Functor, A, B](ga: G[A])(f: A => F[B]): F[G[B]] =
+      gen.from(F.distribute(ga)(a => gen.to(f(a))))
+  }
+}

core/src/main/scala/cats/derived/util/fnGeneric.scala

@@ -0,0 +1,23 @@
+package cats.derived.util
+
+import shapeless.HList
+import shapeless.ops.function.{FnFromProduct, FnToProduct}
+
+trait FnGeneric[F[_]] {
+  type A <: HList
+  def to[B](f: F[B]): A => B
+  def from[B](f: A => B): F[B]
+}
+
+object FnGeneric {
+  type x
+  type Aux[F[_], L <: HList] = FnGeneric[F] { type A = L }
+
+  implicit def instance[F[_], L <: HList](
+    implicit toP: FnToProduct.Aux[F[x], L => x], fromP: FnFromProduct.Aux[L => x, F[x]]
+  ): Aux[F, L] = new FnGeneric[F] {
+    type A = L
+    def to[B](f: F[B]): A => B = toP(f.asInstanceOf[F[x]]).asInstanceOf[A => B]
+    def from[B](f: A => B): F[B] = fromP(f.asInstanceOf[A => x]).asInstanceOf[F[B]]
+  }
+}

core/src/test/scala/cats/derived/function.scala

@@ -0,0 +1,60 @@
+/*
+ * Copyright (c) 2016 Miles Sabin
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package cats.derived
+
+import cats.Eq
+import cats.laws.discipline.{DistributiveTests, ExhaustiveCheck, MonadTests}
+import cats.derived.function._
+import cats.instances.all._
+import cats.laws.discipline.arbitrary._
+import cats.laws.discipline.eq._
+
+class FunctionSuite extends KittensSuite {
+  type B = Boolean
+  type F0[R] = () => R
+  type F1[R] = B => R
+  type F2[R] = (B, B) => R
+  type F4[R] = (B, B, B, B) => R
+  type F8[R] = (B, B, B, B, B, B, B, B) => R
+
+  implicit val exhaustiveCheck4: ExhaustiveCheck[(B, B, B, B)] =
+    ExhaustiveCheck.instance(for {
+      (a, b) <- ExhaustiveCheck[(B, B)].allValues
+      (c, d) <- ExhaustiveCheck[(B, B)].allValues
+    } yield (a, b, c, d))
+
+  implicit val exhaustiveCheck8: ExhaustiveCheck[(B, B, B, B, B, B, B, B)] =
+    ExhaustiveCheck.instance(for {
+      (a, b, c, d) <- ExhaustiveCheck[(B, B, B, B)].allValues
+      (e, f, g, h) <- ExhaustiveCheck[(B, B, B, B)].allValues
+    } yield (a, b, c, d, e, f, g, h))
+
+  implicit def eqFn4[R: Eq]: Eq[F4[R]] = Eq.by(_.tupled)
+  implicit def eqFn8[R: Eq]: Eq[F8[R]] = Eq.by(_.tupled)
+
+  checkAll("Monad[Function0]", MonadTests[F0].monad[B, B, B])
+  checkAll("Monad[Function1]", MonadTests[F1].monad[B, B, B])
+  checkAll("Monad[Function2]", MonadTests[F2].monad[B, B, B])
+  checkAll("Monad[Function4]", MonadTests[F4].monad[B, B, B])
+  checkAll("Monad[Function8]", MonadTests[F8].monad[B, B, B])
+
+  checkAll("Distributive[Function0]", DistributiveTests[F0].distributive[B, B, B, Option, Function0])
+  checkAll("Distributive[Function1]", DistributiveTests[F1].distributive[B, B, B, Option, Function0])
+  checkAll("Distributive[Function2]", DistributiveTests[F2].distributive[B, B, B, Option, Function0])
+  checkAll("Distributive[Function4]", DistributiveTests[F4].distributive[B, B, B, Option, Function0])
+  checkAll("Distributive[Function8]", DistributiveTests[F8].distributive[B, B, B, Option, Function0])
+}