Fix circular imports.

Author: mbostock

README.md

@@ -96,7 +96,7 @@ Builtins must have constant values; unlike [variables](#variables), they cannot
 
 Returns a new [module](#modules) for this [runtime](#runtimes).
 
-If *define* is specified, it is a function which defines the new module’s [variables](#variables). If this runtime already has a module for the specified *define* function, the existing module is returned; otherwise, a new module is created, and the *define* function is called, being passed this runtime and the specified *observer* factory function. If *define* is not specified, a new module is created and returned.
+If *define* is specified, it is a function which defines the new module’s [variables](#variables) by calling *runtime*.module (with no arguments) and then calling [*module*.variable](#module_variable) on the returned module as desired. If this runtime already has a module for the specified *define* function, the existing module is returned; otherwise, a new module is created, and the *define* function is called, being passed this runtime and the specified *observer* factory function. If *define* is not specified, a new module is created and returned.
 
 If an *observer* factory function is specified, it is called for each named variable in the returned module, being passed the variable’s name. The [standard inspector](#inspector) is available as a ready-made observer: it displays DOM elements “as-is” and renders interactive displays for other arbitrary values such as numbers and objects.
 

src/runtime.js

@@ -17,6 +17,7 @@ export default function Runtime(builtins = new Library, global = window_global)
     _dirty: {value: new Set},
     _updates: {value: new Set},
     _computing: {value: null, writable: true},
+    _init: {value: null, writable: true},
     _modules: {value: new Map},
     _variables: {value: new Set},
     _disposed: {value: false, writable: true},
@@ -50,10 +51,23 @@ function runtime_dispose() {
 }
 
 function runtime_module(define, observer = noop) {
-  if (define === undefined) return new Module(this);
-  let module = this._modules.get(define);
+  let module;
+  if (define === undefined) {
+    if (module = this._init) {
+      this._init = null;
+      return module;
+    }
+    return new Module(this);
+  }
+  module = this._modules.get(define);
   if (module) return module;
-  this._modules.set(define, module = define(this, observer));
+  this._init = module = new Module(this);
+  this._modules.set(define, module);
+  try {
+    define(this, observer);
+  } finally {
+    this._init = null;
+  }
   return module;
 }