knocked out the painful bug with call-with-continuation-prompt: I was incorrectly computing environment size when constructing the prompt frame.

whalesong/js-assembler/assemble.rkt

@@ -583,7 +583,7 @@ EOF
                     (assemble-label (make-Label (LinkedLabel-label label)))])))]
       
       [(PushControlFrame/Prompt? stmt)
-       (format "M.c.push(new RT.CallFrame(~a,M.p)); M.addPrompt(~a,false);" 
+       (format "M.c.push(new RT.CallFrame(~a,M.p)); M.addPrompt(~a,false,M.e.length);" 
                (let: ([label : (U Symbol LinkedLabel) (PushControlFrame/Prompt-label stmt)])
                  (cond
                    [(symbol? label) 

whalesong/js-assembler/runtime-src/baselib-primitives.js

@@ -3177,7 +3177,7 @@
                 M.e.pop();
                 M.p = proc;
                 M.a = 0;
-                M.addPrompt(promptTag, false);
+                M.addPrompt(promptTag, false, M.e.length);
                 baselib.functions.rawApply(M);
             },
             []);
@@ -3272,13 +3272,17 @@
                     handler = makeDefaultPromptHandler(promptTag);
                 }
             }
-
             M.p = proc;
             if (M.a >= 1) { M.e.pop(); } // the test is redundant, but I want the parallelism.
             if (M.a >= 2) { M.e.pop(); }
             if (M.a >= 3) { M.e.pop(); }
             M.a = Math.max(M.a - 3, 0);
-            M.addPrompt(promptTag, handler);
+
+            // subtle: the prompt's environment is the one _after_ the current call!
+            // That's why we need to do M.e.length - M.a: the environment currently
+            // has extra values due to us calling the prompt handler here.
+            M.addPrompt(promptTag, handler, M.e.length - M.a); 
+
             baselib.functions.rawApply(M);
         });
 

whalesong/js-assembler/runtime-src/runtime.js

@@ -499,10 +499,11 @@
         M.c.pop();
         return(M.p.mvr)(M);
     };
-    Machine.prototype.addPrompt = function(promptTag, abortHandlerClosure) {
+
+    Machine.prototype.addPrompt = function(promptTag, abortHandlerClosure, envLength) {
         this.c.push(new PromptFrame(justReturn,
                                     promptTag,
-                                    this.e.length,
+                                    envLength,
                                     abortHandlerClosure));
     };