feat: contravariance for R, A in Effect, EagerEffect and in for *EffectScope

arrow-libs/core/arrow-core/build.gradle.kts

@@ -40,3 +40,8 @@ kotlin {
     }
   }
 }
+
+// enables context receivers for Jvm Tests
+tasks.named<org.jetbrains.kotlin.gradle.tasks.KotlinCompile>("compileTestKotlinJvm") {
+  kotlinOptions.freeCompilerArgs += "-Xcontext-receivers"
+}

arrow-libs/core/arrow-core/src/jvmTest/kotlin/arrow/core/continuations/EffectUsage.kt

@@ -0,0 +1,56 @@
+package arrow.core.continuations
+
+import kotlinx.coroutines.runBlocking
+import kotlinx.coroutines.Dispatchers
+
+sealed interface MyError
+sealed interface SubError : MyError
+sealed interface OtherError : MyError {
+  object Actual : OtherError
+}
+
+context(EffectScope<SubError>)
+  suspend fun subprogram(): Unit =
+  println("Hello SubProgram!")
+
+context(EffectScope<OtherError>)
+  suspend fun otherprogram(): Unit =
+  println("Hello OtherProgram!")
+
+context(EffectScope<OtherError>)
+  suspend fun fail(): MyResponse =
+  shift(OtherError.Actual)
+
+fun main() =
+  runBlocking(Dispatchers.Default) {
+    effect<MyError, Unit> {
+      subprogram()
+      otherprogram()
+      fail()
+    }.fold(::println) { }
+    // Hello SubProgram!
+    // Hello OtherProgram!
+    // OtherError$Actual@7cd62f43
+  }
+
+sealed interface MyResponse
+object EmptyResponse : MyResponse
+data class ErrorResponse(val error: Throwable) : MyResponse
+data class BodyResponse(val body: String) : MyResponse
+
+context(EffectScope<SubError>)
+  suspend fun respondWithBody(): BodyResponse =
+  BodyResponse("Hello Program!")
+
+context(EffectScope<OtherError>)
+  suspend fun attemptOrError(): MyResponse =
+  ErrorResponse(RuntimeException("Oh no!"))
+
+fun respond(): Effect<MyError, MyResponse> =
+  effect {
+    when (attemptOrError()) {
+      is BodyResponse -> respondWithBody()
+      EmptyResponse -> EmptyResponse
+      is ErrorResponse -> fail()
+    }
+  }