fix: make the NaturalTransformation lazy in order to not duplicate ef…

…fects

for Id ~> Action

build.sbt

@@ -120,7 +120,13 @@ lazy val mimaSettings =
       // Because, in that case, it is very possible to confuse the ownEnv with the env and shutdown the env
       // which breaks the execution of the whole specification
       ProblemFilters.exclude[DirectMissingMethodProblem]("org.specs2.specification.core.OwnEnv.ownEnv"),
-      ProblemFilters.exclude[DirectMissingMethodProblem]("org.specs2.specification.core.OwnExecutionEnv.env")
+      ProblemFilters.exclude[DirectMissingMethodProblem]("org.specs2.specification.core.OwnExecutionEnv.env"),
+
+      // issue #1277 NaturalTransformation needs to be made lazier
+      ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.specs2.fp.NaturalTransformation.apply"),
+      ProblemFilters.exclude[ReversedMissingMethodProblem]("org.specs2.fp.NaturalTransformation.apply"),
+      ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.specs2.fp.NaturalTransformation#naturalId.apply"),
+      ProblemFilters.exclude[IncompatibleMethTypeProblem]("org.specs2.control.Operation#operationToAction.apply")
     )
   )
 

common/shared/src/main/scala/org/specs2/control/Operation.scala

@@ -163,7 +163,7 @@ object Operation:
       "Applicative[Operation]"
 
   given operationToAction: NaturalTransformation[Operation, Action] with
-    def apply[A](operation: Operation[A]): Action[A] =
+    def apply[A](operation: =>Operation[A]): Action[A] =
       operation.toAction
 
   given SafeOperation: Safe[Operation] with

core/jvm/src/test/scala/org/specs2/fp/FoldSpec.scala

@@ -0,0 +1,27 @@
+package user
+
+import org.specs2.*
+import org.specs2.concurrent.*
+import org.specs2.control.*
+import org.specs2.control.origami.*
+import org.specs2.control.producer.*
+import org.specs2.fp.*
+
+class FoldSpec(ee: ExecutionEnv) extends Specification {
+  def is = s2"""
+
+  A correct implementation of NaturalTransformation from Id to Action must not duplicate effects (see issue #1277) $dontDuplicateEffects
+
+"""
+
+  def dontDuplicateEffects = {
+    val p = Producer.emitAll[Action, Int](1, 2, 3)
+
+    // The NaturalTransformation from Id (which is Id[X] X) to Action must
+    // make sure to not run side effects of X twice.
+    // This tests guarantees that during the evaluation of `zip` and the various folds
+    // we don't run the action for collection elements into a mutable map more than necessary.
+    val vs = p.fold(Folds.list[Int].into[Action] `zip` Folds.list[Int].into[Action])
+    vs.run(ee) === (List(1, 2, 3), List(1, 2, 3))
+  }
+}

fp/src/main/scala/org/specs2/fp/NaturalTransformation.scala

@@ -1,10 +1,10 @@
 package org.specs2.fp
 
 trait NaturalTransformation[-F[_], +G[_]]:
-  def apply[A](fa: F[A]): G[A]
+  def apply[A](fa: =>F[A]): G[A]
 
 object NaturalTransformation:
 
   given naturalId[M[_]: Monad]: NaturalTransformation[Id, M] with
-    def apply[A](fa: Id[A]): M[A] =
+    def apply[A](fa: =>Id[A]): M[A] =
       summon[Monad[M]].point(fa)

html/src/main/scala/org/specs2/reporter/HtmlPrinter.scala

@@ -40,15 +40,16 @@ case class HtmlPrinter(env: Env, searchPage: SearchPage, logger: Logger = Consol
 
   /** @return a SinkTask for the Html output */
   def sink(spec: SpecStructure): AsyncSink[Fragment] =
-    ((Statistics.fold `zip` list[Fragment].into[Action] `zip` SimpleTimer.timerFold.into[Action]) <*
-      fromStart((getHtmlOptions(env.arguments) >>= (options => copyResources(env, options))).void.toAction))
+    val copyHtmlResources = (getHtmlOptions(env.arguments) >>= (options => copyResources(env, options))).void.toAction
+    val htmlSink = (Statistics.fold `zip` list[Fragment].into[Action] `zip` SimpleTimer.timerFold.into[Action])
       .mapFlatten { case ((stats, fragments), timer) =>
         val executedSpec = spec.copy(lazyFragments = () => Fragments(fragments*))
         getPandoc(env).flatMap {
           case Some(pandoc) => printHtmlWithPandoc(env, executedSpec, stats, timer, pandoc).toAction
           case _            => printHtml(env, executedSpec, stats, timer).toAction
         }
       }
+    htmlSink.startWith(copyHtmlResources)
 
   /** WITHOUT PANDOC
     */