Add tail call optimization to interpretation

1 files changed, 10 insertions, 3 deletions

diff --git a/lib/js/urweb.js b/lib/js/urweb.js
index 2ebb13c5..653f8d2f 100644
--- a/lib/js/urweb.js
+++ b/lib/js/urweb.js
@@ -856,6 +856,13 @@ function execP(env, p, v) {
 function exec0(env, e) {
   var stack = null;
 
+  var saveEnv = function() {
+    if (stack.next != null && stack.next.data.c != "<")
+      stack = cons({c: "<", env: env}, stack.next);
+    else
+      stack = stack.next;
+  };
+
   while (true) {
     switch (e.c) {
     case "c":
@@ -891,7 +898,7 @@ function exec0(env, e) {
         if (fr.f == null)
           whine("Ur: applying null function");
         else if (fr.f.body) {
-          stack = cons({c: "<", env: env}, stack.next);
+          saveEnv();
           env = cons(v, fr.f.env);
           e = fr.f.body;
         } else {
@@ -923,7 +930,7 @@ function exec0(env, e) {
         stack = stack.next;
         break;
       case "=":
-        stack = cons({c: "<", env: env}, stack.next);
+        saveEnv();
         env = cons(v, env);
         e = fr.e2;
         break;
@@ -932,7 +939,7 @@ function exec0(env, e) {
         for (ps = fr.p; ps != null; ps = ps.next) {
           var r = execP(env, ps.data.p, v);
           if (r != false) {
-            stack = cons({c: "<", env: env}, stack.next);
+            saveEnv();
             env = r;
             e = ps.data.b;
             break;