fix(repl): detect cycles when trying to stringify the value

Closes #123

lib/ag-solo/vats/repl.js

@@ -2,7 +2,7 @@ import { makeEvaluators } from '@agoric/evaluate';
 import harden from '@agoric/harden';
 
 // A REPL-specific JSON stringify.
-export function stringify(value, spaces) {
+export function stringify(value, spaces, already = new WeakSet()) {
   if (Object(value) !== value) {
     return JSON.stringify(value, spaces);
   }
@@ -17,11 +17,17 @@ export function stringify(value, spaces) {
     return '[Promise]';
   }
 
+  // Detect cycles.
+  if (already.has(value)) {
+    return '[Circular]';
+  }
+  already.add(value);
+
   if (Array.isArray(value)) {
     let ret = '[';
     let sep = '';
     for (let i = 0; i < value.length; i++) {
-      ret += sep + stringify(value[i]);
+      ret += sep + stringify(value[i], spaces, already);
       sep = ',';
     }
     ret += ']';
@@ -37,6 +43,7 @@ export function stringify(value, spaces) {
     ret += `${sep}${JSON.stringify(key, undefined, spaces)}:${stringify(
       value[key],
       spaces,
+      already,
     )}`;
     sep = ',';
   }