From d827f9f68355b5d2cac7436c6a9f1acf96dfc82c Mon Sep 17 00:00:00 2001
From: Yiyun Liu <liuyiyun@seas.upenn.edu>
Date: Sat, 7 Jun 2025 19:05:26 -0400
Subject: [PATCH] Add in-place update

---
 semantics.rkt | 23 ++++++++++++-----------
 1 file changed, 12 insertions(+), 11 deletions(-)

diff --git a/semantics.rkt b/semantics.rkt
index 650bfd6..4b02bae 100644
--- a/semantics.rkt
+++ b/semantics.rkt
@@ -150,18 +150,18 @@
   (let ([stack (State-stack state)])
     (match stack
       [(cons addr rstack)
-       (step-node rstack state (lookup-node (State-heap state) addr))]
+       (step-node addr rstack state (lookup-node (State-heap state) addr))]
       ['() (error "Invalid state")])))
 
-(: step-node (-> Stack State Node State))
-(define (step-node rstack st n)
+(: step-node (-> Addr Stack State Node State))
+(define (step-node addr rstack st n)
   (cond
     [(integer-node? n) (error "A number cannot be stepped any further!")]
     [(ap-node? n) (update-stack (λ (stack) (push-addr stack (ap-fun n))) st)]
     [(undef-node? n) (error "An undefined node is being stepped!")]
     [(sc-node? n)
      (let-values ([(args body) (match-sc n)])
-       (step-sc rstack st args body))]
+       (step-sc addr rstack st args body))]
     [(indirect-node? n)
      (update-stack (λ (_) (push-addr rstack (indirect-addr n))) st)]))
 
@@ -171,18 +171,19 @@
     [(list _ a) a]
     [_ (error "Not an application node")]))
 
-(: step-sc (-> Stack State (Listof Name) CoreExpr State))
-(define (step-sc rstack state arg-names body)
+(: step-sc (-> Addr Stack State (Listof Name) CoreExpr State))
+(define (step-sc root rstack state arg-names body)
   (let ([heap (State-heap state)])
-    (let*-values ([(new-env new-stack)
+    (let*-values ([(new-env new-stack root)
                   (for/fold ([globals : Globals (State-globals state)]
-                             [stack : Stack rstack])
+                             [stack : Stack rstack]
+                             [root : Addr root])
                             ([arg-name : Name arg-names])
                     (if (null? stack)
                         (error "Not enough arguments to apply the supercombinator")
-                        (values (update-globals globals arg-name (get-arg heap (car stack))) (cdr stack))))]
-                  [(heap addr) (instantiate-and-allocate body new-env heap)])
-      (let ([new-stack (cons addr new-stack)])
+                        (values (update-globals globals arg-name (get-arg heap (car stack))) (cdr stack) (car stack))))])
+      (let* ([heap (instantiate-and-assign body new-env heap root)]
+             [new-stack (cons root new-stack)])
         (struct-copy State state [stack new-stack] [heap heap])))))
 
 (: instantiate-and-allocate (-> CoreExpr Globals Heap (Values Heap Addr)))