From 0c2c630626d3490f83927492fadf885d6d115c83 Mon Sep 17 00:00:00 2001
From: etorreborre <etorreborre@yahoo.com>
Date: Sat, 12 Oct 2024 16:38:56 +0200
Subject: [PATCH] fix: make the NaturalTransformation lazy in order to not
 duplicate effects

for Id ~> Action
---
 .../scala/org/specs2/control/Operation.scala  |  2 +-
 .../test/scala/org/specs2/fp/FoldSpec.scala   | 27 +++++++++++++++++++
 .../org/specs2/fp/NaturalTransformation.scala |  4 +--
 3 files changed, 30 insertions(+), 3 deletions(-)
 create mode 100644 core/jvm/src/test/scala/org/specs2/fp/FoldSpec.scala

diff --git a/common/shared/src/main/scala/org/specs2/control/Operation.scala b/common/shared/src/main/scala/org/specs2/control/Operation.scala
index 4b0d46b6bf..312f41dd6c 100644
--- a/common/shared/src/main/scala/org/specs2/control/Operation.scala
+++ b/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
diff --git a/core/jvm/src/test/scala/org/specs2/fp/FoldSpec.scala b/core/jvm/src/test/scala/org/specs2/fp/FoldSpec.scala
new file mode 100644
index 0000000000..99ebd1d711
--- /dev/null
+++ b/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))
+  }
+}
diff --git a/fp/src/main/scala/org/specs2/fp/NaturalTransformation.scala b/fp/src/main/scala/org/specs2/fp/NaturalTransformation.scala
index 8fa5d7f046..0a0f0713f2 100644
--- a/fp/src/main/scala/org/specs2/fp/NaturalTransformation.scala
+++ b/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)