From 13c730f9d8dbeae6d9c571e1ce06bd49bf72c2c8 Mon Sep 17 00:00:00 2001
From: Yiyun Liu <liuyiyun@seas.upenn.edu>
Date: Wed, 4 Jun 2025 23:33:02 -0400
Subject: [PATCH] Update the implementation of step-sc so it errors when
 unsaturated

---
 semantics.rkt | 36 ++++++++++++++++--------------------
 1 file changed, 16 insertions(+), 20 deletions(-)

diff --git a/semantics.rkt b/semantics.rkt
index 968753a..ab4c45f 100644
--- a/semantics.rkt
+++ b/semantics.rkt
@@ -90,33 +90,29 @@
   (let ([stack (State-stack state)])
     (match stack
       [(cons addr rstack)
-       (step-node state (lookup-node (State-heap state) addr))]
+       (step-node rstack state (lookup-node (State-heap state) addr))]
       ['() (error "Invalid state")])))
 
-(: step-node (-> State Node State))
-(define (step-node st n)
+(: step-node (-> Stack State Node State))
+(define (step-node rstack st n)
   (match n
     [(? integer?) (error "A number cannot be stepped any further!")]
     [(list a _) (update-stack (λ (x) (cons a x)) st)]
     [(list 'define (list name args ...) body)
-     (step-sc st args body)]))
+     (step-sc rstack st args body)]))
 
-(: step-sc (-> State (Listof Name) CoreExpr State))
-(define (step-sc state arg-names body)
-  (let* ([stack (State-stack state)]
-         [heap (State-heap state)]
-         [new-env : Globals
-                      (for/fold ([globals : Globals (State-globals state)])
-                                ([arg-name : Name arg-names]
-                                 [addr : Addr (if (null? stack)
-                                                   (error "impossible")
-                                                   (cdr stack))])
-                        (update-globals globals arg-name addr))]
-         ;; [new-stack : Stack
-         ;;            (drop stack (add1 (length arg-names)))]
-         )
-    (let-values ([(new-heap addr) (instantiate-body body new-env heap)])
-      (let ([new-stack (cons addr (drop stack (add1 (length arg-names))))])
+(: step-sc (-> Stack State (Listof Name) CoreExpr State))
+(define (step-sc rstack state arg-names body)
+  (let ([heap (State-heap state)])
+    (let*-values ([(new-env new-stack)
+                  (for/fold ([globals : Globals (State-globals state)]
+                             [stack : Stack rstack])
+                            ([arg-name : Name arg-names])
+                    (if (null? stack)
+                        (error "Not enough arguments to apply the supercombinator")
+                        (values (update-globals globals arg-name (car stack)) (cdr stack))))]
+                  [(new-heap addr) (instantiate-body body new-env heap)])
+      (let ([new-stack (cons addr new-stack)])
         (struct-copy State state [stack new-stack] [heap new-heap])))))
 
 (: instantiate-body (-> CoreExpr Globals Heap (Values Heap Addr)))