Merge pull request #49 from typelevel/bplommer/contravariant

contravariant

core/src/main/scala/org.typelevel/catapult/ContextEncoder.scala

@@ -17,16 +17,28 @@
 package org.typelevel.catapult
 
 import com.launchdarkly.sdk.{LDContext, LDUser}
+import cats.Contravariant
 
 /** A typeclass for converting values of type `Ctx` into [[https://javadoc.io/doc/com.launchdarkly/launchdarkly-java-server-sdk/latest/com/launchdarkly/sdk/LDContext.html LDContext]]. An instance must be in scope when
   * evaulating flags against a context represented by the `Ctx` type. Instances are provided for [[https://javadoc.io/doc/com.launchdarkly/launchdarkly-java-server-sdk/latest/com/launchdarkly/sdk/LDContext.html LDContext]]
   * and [[https://javadoc.io/doc/com.launchdarkly/launchdarkly-java-server-sdk/latest/com/launchdarkly/sdk/LDUser.html LDUser]]; custom instances can be created to allow other types to be used.
   */
 trait ContextEncoder[Ctx] {
   def encode(ctx: Ctx): LDContext
+
+  /** Create a new `ContextEncoder` by applying a function to the input before encoding. */
+  def contramap[A](f: A => Ctx): ContextEncoder[A] = a => encode(f(a))
 }
 
 object ContextEncoder {
+  def apply[Ctx](implicit ev: ContextEncoder[Ctx]): ContextEncoder[Ctx] = ev
+  def encode[Ctx](ctx: Ctx)(implicit ev: ContextEncoder[Ctx]): LDContext = ev.encode(ctx)
+
+  implicit val catapultContextEncoderContravariant: Contravariant[ContextEncoder] =
+    new Contravariant[ContextEncoder] {
+      def contramap[A, B](fa: ContextEncoder[A])(f: B => A): ContextEncoder[B] = fa.contramap(f)
+    }
+
   implicit val catapultContextEncoderForLdContext: ContextEncoder[LDContext] = identity(_)
 
   implicit val catapultContextEncoderForLdUser: ContextEncoder[LDUser] = LDContext.fromUser(_)

testkit/src/test/scala/org/typelevel/catapult/ContextEncoderTests.scala

@@ -0,0 +1,39 @@
+/*
+ * Copyright 2022 Typelevel
+ *
+ * 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 org.typelevel.catapult
+
+import com.launchdarkly.sdk.{ContextKind, LDContext}
+import weaver.SimpleIOSuite
+
+object ContextEncoderTests extends SimpleIOSuite {
+  pureTest("contramap allows changing the input type of ContextEncoder") {
+    // This is a trivial example - it would be just as simple to write a new ContextEncoder from scatch.
+    case class Country(name: String)
+
+    val enc = ContextEncoder[LDContext].contramap[Country](country =>
+      LDContext.builder(ContextKind.of("country"), country.name).build()
+    )
+
+    val franceCtx = LDContext.create(ContextKind.of("country"), "France")
+    val germanyCtx = LDContext.create(ContextKind.of("country"), "Germany")
+
+    expect.all(
+      enc.encode(Country("France")) == franceCtx,
+      enc.encode(Country("Germany")) == germanyCtx,
+    )
+  }
+}