Introduce finch-generic module

build.sbt

@@ -161,6 +161,7 @@ lazy val finch = project.in(file("."))
       """
         |import io.finch._
         |import io.finch.circe._
+        |import io.finch.generic._
         |import io.finch.items._
         |import com.twitter.util.{Future, Await}
         |import com.twitter.concurrent.AsyncStream
@@ -178,15 +179,20 @@ lazy val finch = project.in(file("."))
     "io.spray" %%  "spray-json" % sprayVersion
   ))
   .aggregate(
-    core, argonaut, jackson, json4s, circe, playjson, sprayjson, benchmarks, test, jsonTest, oauth2,
-    examples, sse
+    core, generic, argonaut, jackson, json4s, circe, playjson, sprayjson, benchmarks, test, jsonTest,
+    oauth2, examples, sse
   )
-  .dependsOn(core, circe)
+  .dependsOn(core, generic, circe)
 
 lazy val core = project
   .settings(moduleName := "finch-core")
   .settings(allSettings)
 
+  lazy val generic = project
+    .settings(moduleName := "finch-generic")
+    .settings(allSettings)
+    .dependsOn(core % "compile->compile;test->test")
+
 lazy val test = project
   .settings(moduleName := "finch-test")
   .settings(allSettings)

core/src/main/scala/io/finch/Endpoint.scala

@@ -582,18 +582,6 @@ object Endpoint {
       }
   }
 
-  class GenericDerivation[A] {
-    def fromParams[Repr <: HList](implicit
-      gen: LabelledGeneric.Aux[A, Repr],
-      fp: FromParams[Repr]
-    ): Endpoint[A] = fp.endpoint.map(gen.from)
-  }
-
-  /**
-   * Generically derive a very basic instance of [[Endpoint]] for a given type `A`.
-   */
-  def derive[A]: GenericDerivation[A] = new GenericDerivation[A]
-
   implicit val endpointInstance: Alternative[Endpoint] = new Alternative[Endpoint] {
     final override def ap[A, B](ff: Endpoint[A => B])(fa: Endpoint[A]): Endpoint[B] =
       ff.productWith(fa)((f, a) => f(a))

core/src/main/scala/io/finch/internal/FromParams.scala → generic/src/main/scala/io/finch/generic/FromParams.scala

@@ -1,4 +1,4 @@
-package io.finch.internal
+package io.finch.generic
 
 import scala.reflect.ClassTag
 
@@ -32,7 +32,7 @@ object FromParams {
   }
 }
 
-private[internal] object Extractor extends Poly1 {
+private[generic] object Extractor extends Poly1 {
 
   implicit def optionalExtractor[V](implicit
     dh: DecodeEntity[V],

generic/src/main/scala/io/finch/generic/GenericDerivation.scala

@@ -0,0 +1,11 @@
+package io.finch.generic
+
+import io.finch._
+import shapeless._
+
+final class GenericDerivation[A] {
+  def fromParams[Repr <: HList](implicit
+    gen: LabelledGeneric.Aux[A, Repr],
+    fp: FromParams[Repr]
+  ): Endpoint[A] = fp.endpoint.map(gen.from)
+}

generic/src/main/scala/io/finch/generic/package.scala

@@ -0,0 +1,8 @@
+package io.finch
+
+package object generic {
+  /**
+   * Generically derive a very basic instance of [[Endpoint]] for a given type `A`.
+   */
+  def deriveEndpoint[A]: GenericDerivation[A] = new GenericDerivation[A]
+}

generic/src/test/scala/io/finch/generic/DerivedEndpointLaws.scala

@@ -0,0 +1,36 @@
+package io.finch
+
+import cats.Eq
+import cats.instances.AllInstances
+import cats.laws._
+import cats.laws.discipline._
+import org.scalacheck.{Arbitrary, Prop}
+import org.typelevel.discipline.Laws
+
+trait DerivedEndpointLaws[A] extends Laws with MissingInstances with AllInstances {
+
+  def endpoint: Endpoint[A]
+  def toParams: A => Seq[(String, String)]
+
+  def roundTrip(a: A): IsEq[A] = {
+    val i = Input.get("/", toParams(a): _*)
+    endpoint(i).awaitValueUnsafe().get <-> a
+  }
+
+  def evaluating(implicit A: Arbitrary[A], eq: Eq[A]): RuleSet =
+    new DefaultRuleSet(
+      name = "evaluating",
+      parent = None,
+      "roundTrip" -> Prop.forAll { (a: A) => roundTrip(a) }
+    )
+}
+
+object DerivedEndpointLaws {
+  def apply[A](
+    e: Endpoint[A],
+    tp: A => Seq[(String, String)]
+  ): DerivedEndpointLaws[A] = new DerivedEndpointLaws[A] {
+    val endpoint: Endpoint[A] = e
+    val toParams = tp
+  }
+}

generic/src/test/scala/io/finch/generic/GenericSpec.scala

@@ -0,0 +1,28 @@
+package io.finch.generic
+
+import cats.kernel.Eq
+import io.finch._
+import org.scalacheck.Arbitrary
+
+class GenericSpec extends FinchSpec {
+
+  behavior of "generic"
+
+  case class Foo(a: String, b: Int)
+
+  val e: Endpoint[Foo] = deriveEndpoint[Foo].fromParams
+
+  implicit val eq: Eq[Foo] = Eq.fromUniversalEquals
+
+  implicit val arbitraryFoo: Arbitrary[Foo] = Arbitrary(for {
+    s <- Arbitrary.arbitrary[String]
+    i <- Arbitrary.arbitrary[Int]
+  } yield Foo(s, i))
+
+  val f: Foo => Seq[(String, String)] = foo => Seq(
+    ("a" -> foo.a),
+    ("b" -> foo.b.toString)
+  )
+
+  checkAll("DerivedEndpoint[Foo]", DerivedEndpointLaws[Foo](e, f).evaluating)
+}