draft fix approach 2

src/derive.luau

@@ -11,6 +11,7 @@ local function derive<T>(source: () -> T): () -> T
 
     return function()
         push_child_to_scope(node)
+        evaluate_node(node)
         return node.cache
     end
 end

src/graph.luau

@@ -1,12 +1,16 @@
 local flags = require "./flags"
+local update_id = 0
 
 export type SourceNode<T> = {
     cache: T,
+    needs_queue_children: boolean,
     [number]: Node<T>
 }
 
 export type Node<T> =  {
     cache: T,
+    last_eval_update_id: number,
+    needs_queue_children: boolean,
     effect:  ((T) -> T) | false,
     cleanups: { () -> () } | false,
 
@@ -147,6 +151,7 @@ end
 local update_queue = { n = 0 } :: { n: number, [number]: Node<any> }
 
 local function evaluate_node<T>(node: Node<T>)
+    if update_id == node.last_eval_update_id then return node.needs_queue_children end
     if flags.strict then
         local initial_value = node.cache
 
@@ -169,7 +174,10 @@ local function evaluate_node<T>(node: Node<T>)
             node.cache = new_value :: T
         end
 
-        return initial_value ~= node.cache
+        local needs_queue_children = initial_value ~= node.cache
+        node.needs_queue_children = needs_queue_children
+        node.last_eval_update_id = update_id
+        return needs_queue_children
     else
         local cur_value = node.cache
 
@@ -185,18 +193,24 @@ local function evaluate_node<T>(node: Node<T>)
             update_queue.n = 0
             error(`effect error:\n{new_value}\n`, 0)
         end
-    
+        local needs_queue_children = cur_value ~= new_value
+        node.needs_queue_children = needs_queue_children
+        node.last_eval_update_id = update_id
         node.cache = new_value
-        return cur_value ~= new_value
+        return needs_queue_children
     end
 end
 
 local function queue_children_for_update<T>(node: SourceNode<T>)
+    node.needs_queue_children = false
     local i = update_queue.n
-    while node[1] do
+    local j = 1
+
+    while node[j] do
+        if node[j].last_eval_update_id == update_id then j += 1; continue end
         i += 1
-        update_queue[i] = node[1]
-        unparent(node[1])
+        update_queue[i] = node[j]
+        unparent(node[j])
     end
     update_queue.n = i
 end
@@ -222,8 +236,9 @@ local function flush_update_queue(from: number)
     update_queue.n = from
 end
 
-local function update_descendants<T>(root: SourceNode<T>)
+local function update_descendants<T>(root: SourceNode<T>, is_root_update: boolean?)
     local n0 = update_queue.n
+    if is_root_update then update_id += 1 end
     queue_children_for_update(root)
 
     if flags.batch then return end
@@ -258,6 +273,8 @@ local function create_node<T>(owner: false | Node<any>, effect: false | (T) -> T
         effect = effect,
         cleanups = false,
 
+        needs_queue_children = false,
+        last_eval_update_id = 0,
         context = false,
 
         owner = owner,
@@ -278,7 +295,7 @@ local function create_node<T>(owner: false | Node<any>, effect: false | (T) -> T
 end
 
 local function create_source_node<T>(value: T): SourceNode<T>
-    return { cache = value }
+    return { cache = value, needs_queue_children = false }
 end
 
 local function get_children<T>(node: Node<T>): { Node<unknown> }

src/source.luau

@@ -21,7 +21,7 @@ local function source<T>(initial_value: T): Source<T>
         end
 
         node.cache = v
-        update_descendants(node)
+        update_descendants(node, true)
         return v
     end
 

test/tests.luau

@@ -98,9 +98,9 @@ TEST("graph", function()
         pop_scope()
         
         CHECK(count == 1)
-        update_descendants(a)
+        update_descendants(a, true)
         CHECK(count == 2)
-        update_descendants(b)
+        update_descendants(b, true)
         CHECK(count == 3)
     end
 
@@ -119,7 +119,7 @@ TEST("graph", function()
         push_scope(c); push_child_to_scope(a); pop_scope()
         push_scope(d); push_child_to_scope(b); push_child_to_scope(c); pop_scope()
 
-        update_descendants(a)
+        update_descendants(a, true)
 
         CHECK(b_cnt == 1)
         CHECK(c_cnt == 1)
@@ -138,7 +138,7 @@ TEST("graph", function()
 
         push_scope(c); assert(type(c.effect) == "function" and c.effect)(NIL); pop_scope()
 
-        update_descendants(a)
+        update_descendants(a, true)
 
         CHECK(#get_children(a) == 1)
         CHECK(#get_children(b) == 1)
@@ -286,14 +286,14 @@ TEST("graph", function()
         local a, b, c, d, e, f = node(root), node(root), node(root), node(root), node(root), node(root)
 
         function b.effect(x)
-            update_descendants(d)
+            update_descendants(d, true)
             return not x
         end
 
         push_child(a, b); push_child(a, c)
         push_child(d, e); push_child(d, f)
 
-        update_descendants(a)
+        update_descendants(a, true)
 
         CHECK(true)
     end
@@ -2517,6 +2517,29 @@ TEST("graph edge cases", wrap_root(function()
     local effect = vide.effect
     local root = vide.root
 
+    do CASE "brother requires older brother"
+        --[[
+            A
+          ^   ^
+         B <-- C
+        ]]
+
+        local count = { b = 0, c = 0 }
+        local a = source(0)
+        local b
+
+        local function upd_c() count.c += 1; return a()* (b and b() or 0) end
+        local c = derive(upd_c)
+
+        local function upd_b() count.b += 1; return a()*2 end
+        b = derive(upd_b)
+
+        a(2)
+        print('CAVALO', count.b, count.c)
+        CHECK(count.b == 2)
+        CHECK(count.c == 2)
+    end
+
     do CASE "diamond A,B,C,D"
         --[[
 
@@ -2577,14 +2600,14 @@ TEST("graph edge cases", wrap_root(function()
         CHECK(count.b == 2)
         CHECK(count.c == 2)
         CHECK(count.d == 2)
-        CHECK(count.e == 3) -- todo: redundant re-eval
+        CHECK(count.e == 2)
         CHECK(e() == 4)
 
         a(3)
         CHECK(count.b == 2)
         CHECK(count.c == 3)
         CHECK(count.d == 3)
-        CHECK(count.e == 4)
+        CHECK(count.e == 3)
         CHECK(e() == 12)
     end