From 2dc2814d76139fb8fe927547b69f90cc066f8098 Mon Sep 17 00:00:00 2001
From: aaron <83140718+centau@users.noreply.github.com>
Date: Fri, 19 Jul 2024 11:21:49 +0100
Subject: [PATCH] Fix regression with derived sources

---
 CHANGELOG.md    |  8 ++++++++
 src/graph.luau  |  9 +++++----
 test/tests.luau | 21 +++++++++++++++++++++
 3 files changed, 34 insertions(+), 4 deletions(-)

diff --git a/CHANGELOG.md b/CHANGELOG.md
index 83d16e5..89cf8e7 100644
--- a/CHANGELOG.md
+++ b/CHANGELOG.md
@@ -6,6 +6,14 @@ The format is based on [Keep a Changelog](https://keepachangelog.com/en/1.0.0/).
 
 --------------------------------------------------------------------------------
 
+## Unreleased
+
+### Fixed
+
+- Error stack traces being lost.
+
+--------------------------------------------------------------------------------
+
 ## [0.2.0] - 2023-11-22
 
 ### Added
diff --git a/src/graph.luau b/src/graph.luau
index a90dd6c..f35b9b6 100644
--- a/src/graph.luau
+++ b/src/graph.luau
@@ -131,15 +131,16 @@ local update_queue = { n = 0 } :: { n: number, [number]: Node<any> }
 
 local function evaluate_node<T>(node: Node<T>)
     if flags.strict then
-        local ok, cur_value, new_value
+        local initial_value = node.cache
+
         for i = 1, 2 do
-            cur_value = node.cache
+            local cur_value = node.cache
 
             flush_cleanups(node)
             destroy_owned(node)
     
             push_scope(node)
-            ok, new_value = ycall(node.effect :: (T) -> T, cur_value)
+            local ok, new_value = ycall(node.effect :: (T) -> T, cur_value)
             pop_scope()
             
             if not ok then
@@ -151,7 +152,7 @@ local function evaluate_node<T>(node: Node<T>)
             node.cache = new_value :: T
         end
 
-        return cur_value ~= new_value
+        return initial_value ~= node.cache
     else
         local cur_value = node.cache
 
diff --git a/test/tests.luau b/test/tests.luau
index 02e2e6c..c0f69d1 100644
--- a/test/tests.luau
+++ b/test/tests.luau
@@ -2261,6 +2261,27 @@ TEST("strict", wrap_root(function()
         src(not src())
         CHECK(count == 4)
     end
+
+    do CASE "effect using derived source"
+        local input = source(true)
+
+        local output = derive(function()
+            return input()
+        end)
+
+        local count = 0
+
+        effect(function()
+            output()
+            count += 1
+        end)
+
+        CHECK(count == 2)
+
+        input(false)
+
+        CHECK(count == 4)
+    end
 end))
 
 local ok = FINISH()