Better error message for assignment to final local variables

runtime/lib/errors_patch.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 patch class Error {
   /* patch */ static String _objectToString(Object object) {
     return Object._toString(object);
   }
 
   /* patch */ StackTrace get stackTrace => _stackTrace;
@@ skipping 194 matching lines @@
 
 
   String _developerMessage(args_mismatch) {
     if (_invocation_type < 0) {
       return "";
     }
     var type = _invocation_type & _InvocationMirror._TYPE_MASK;
     var level = (_invocation_type >> _InvocationMirror._CALL_SHIFT) &
          _InvocationMirror._CALL_MASK;
     var type_str =
-        (const ["method", "getter", "setter", "getter or setter"])[type];
+        (const ["method", "getter", "setter", "getter or setter", "variable"])[type];
     var args_message = args_mismatch ? " with matching arguments" : "";
     var msg;
     var memberName =
         (_memberName == null) ? "" : internal.Symbol.getName(_memberName);
+
+    if (type == _InvocationMirror._LOCAL_VAR) {
+      return "cannot assign to final variable '$memberName'.\n\n";

[Ivan Posva, 2014/02/25 07:10:07]

How about just assigning to msg here and let the rest of the function deal with
massaging the resulting string?

[hausner, 2014/02/25 18:47:05]

I'm not sure what massaging you mean. Adding the two newlines?

The problem is that I'd have to special-case the local variable case again in
the switch below, or put the entire switch statement into an else branch. Is
that really more readable?

+    }
     switch (level) {
       case _InvocationMirror._DYNAMIC: {
         if (_receiver == null) {
           msg = "The null object does not have a $type_str '$memberName'"
               "$args_message.";
         } else {
           if (_receiver is Function) {
             msg = "Closure call with mismatched arguments: "
                 "function '$memberName'";
           } else {
@@ skipping 57 matching lines @@
     String receiver_str;
     var level = (_invocation_type >> _InvocationMirror._CALL_SHIFT) &
         _InvocationMirror._CALL_MASK;
     if ( level == _InvocationMirror._TOP_LEVEL) {
       receiver_str = "top-level";
     } else {
       receiver_str = Error.safeToString(_receiver);
     }
     var memberName =
         (_memberName == null) ? "" : internal.Symbol.getName(_memberName);
-    if (!args_mismatch) {
+    var type = _invocation_type & _InvocationMirror._TYPE_MASK;
+    if (type == _InvocationMirror._LOCAL_VAR) {
       msg_buf.write(
-          "NoSuchMethodError : method not found: '$memberName'\n"
+          "NoSuchMethodError: cannot assign to final variable '$memberName'");
+    } else if (!args_mismatch) {
+      msg_buf.write(
+          "NoSuchMethodError: method not found: '$memberName'\n"
           "Receiver: $receiver_str\n"
           "Arguments: [$actual_buf]");
     } else {
       String actualParameters = actual_buf.toString();
       StringBuffer formal_buf = new StringBuffer();
       for (int i = 0; i < _existingArgumentNames.length; i++) {
         if (i > 0) {
           formal_buf.write(", ");
         }
         formal_buf.write(_existingArgumentNames[i]);
       }
       String formalParameters = formal_buf.toString();
       msg_buf.write(
           "NoSuchMethodError: incorrect number of arguments passed to "
           "method named '$memberName'\n"
           "Receiver: $receiver_str\n"
           "Tried calling: $memberName($actualParameters)\n"
           "Found: $memberName($formalParameters)");
     }
     return msg_buf.toString();
   }
 }
 
 class _JavascriptIntegerOverflowError extends Error {
   final Object _value;
 
   _JavascriptIntegerOverflowError(this._value);
   String toString() => "Javascript Integer Overflow: $_value";
 }