Introduce Instance.kind.

runtime/vm/service/service.md

 # Dart VM Service Protocol 0.0
 
 This document describes _version 0.0_ of the Dart VM Service Protocol.
 This protocol is used to communicate with a running Dart Virtual
 Machine.
 
 To use the Service Protocol, start the VM with the *--observe* flag.
 The VM will start a webserver which services protocol requests via WebSocket.
 It is possible to make HTTP (non-WebSocket) requests,
 but this does not allow access to VM _events_ and is not documented
@@ skipping 24 matching lines @@
         - [getVersion](#getversion)
         - [getVM](#getvm)
         - [pause](#pause)
         - [removeBreakpoint](#removebreakpoint)
         - [resume](#resume)
         - [setName](#setname)
         - [setLibraryDebuggable](#setlibrarydebuggable)
         - [streamCancel](#streamcancel)
         - [streamListen](#streamlisten)
 - [Public Types](#public-types)
-        - [Bool](#bool)
         - [BoundField](#boundfield)
         - [BoundVariable](#boundvariable)
         - [Breakpoint](#breakpoint)
         - [Class](#class)
         - [ClassList](#classlist)
         - [Code](#code)
         - [CodeKind](#codekind)
-        - [Double](#double)
         - [Error](#error)
         - [Event](#event)
         - [EventKind](#eventkind)
         - [Field](#field)
         - [Flag](#flag)
         - [FlagList](#flaglist)
         - [Frame](#frame)
         - [Function](#function)
         - [Instance](#instance)
-        - [Int](#int)
         - [Isolate](#isolate)
         - [Library](#library)
         - [LibraryDependency](#librarydependency)
-        - [List](#list)
         - [ListElement](#listelement)
         - [Message](#message)
         - [Null](#null)
         - [Object](#object)
         - [Sentinel](#sentinel)
         - [SentinelKind](#sentinelkind)
         - [Script](#script)
         - [Stack](#stack)
         - [StepOption](#stepoption)
-        - [String](#string)
         - [Success](#success)
-        - [Type](#type)
         - [TypeArguments](#typearguments)
         - [Response](#response)
         - [Version](#version)
         - [VM](#vm)
 - [Revision History](#revision-history)
 
 ## RPCs, Requests, and Responses
 
 An RPC request is a JSON object sent to the server.  Here is an
 example [getVersion](#getversion) request:
@@ skipping 209 matching lines @@
 ```
 
 The major version number is incremented when the protocol is changed
 in a potentially _incompatible_ way.  An example of an incompatible
 change is removing a non-optional property from a result.
 
 The minor version number is incremented when the protocol is changed
 in a _backwards compatible_ way.  An example of a backwards compatible
 change is adding a property to a result.
 
+Certain changes that would normally not be backwards compatible are
+considered backwards compatible for the purposes of versioning.
+Specifically, additions can be made to the [EventKind](#eventkind) and
+[InstanceKind](#instancekind) enumerated types and the client must
+handle this gracefully.  See the notes on these enumerated types for more
+information.
+
 ## Private RPCs, Types, and Properties
 
 Any RPC, type, or property which begins with an underscore is said to
 be _private_.  These RPCs, types, and fields can be changed at any
 time without changing major or minor version numbers.
 
 The intention is that the Service Protocol will evolve by adding
 private RPCs which may, over time, migrate to the public api as they
 become stable.  Some private types and properties expose VM specific
 implementation state and will never be appropriate to add to
@@ skipping 366 matching lines @@
 ```
 enum PermittedValues {
   Value1,
   Value2
 }
 ```
 
 This means that _PermittedValues_ is a _string_ with two potential values,
 _Value1_ and _Value2_.
 
-### Bool
-
-```
-class @Bool extends @Instance {
-  // The value of this bool as a string, either 'true' or 'false'.
-  string valueAsString;
-}
-```
-
-_@Bool_ is a reference to a _Bool_.
-
-```
-class Bool extends Instance {
-  // The value of this bool as a string, either 'true' or 'false'.
-  string valueAsString;
-}
-```
-
-An _Bool_ represents an instance of the Dart language class _bool_.
-
 ### BoundField
 
 ```
 class BoundField {
   @Field decl;
   @Instance|Sentinel value;
 }
 ```
 
 A _BoundField_ represents a field bound to a particular value in an
@@ skipping 140 matching lines @@
 ```
 enum CodeKind {
   Dart,
   Native,
   Stub,
   Tag,
   Collected
 }
 ```
 
-### Double
+### Context
 
 ```
-class @Double extends @Instance {
-  // The value of this double as a string.
-  //
-  // Suitable for passing to double.parse().
-  string valueAsString;
+class @Context {
+  // The number of variables in this context.
+  int length;
 }
 ```
 
-_@Double_ is a reference to a _Double_.
+```
+class Context {
+  // The number of variables in this context.
+  int length;
 
-```
-class Double extends Instance {
-  // The value of this double as a string.
-  //
-  // Suitable for passing to double.parse().
-  string valueAsString;
+  // The variables in this context object.
+  ListElement[] variables;
 }
 ```
 
-A _Double_ represents an instance of the Dart language class _double_.
-
 ### Error
 
 ```
 class @Error extends @Object {
   // A description of the error.
   string message;
 }
 ```
 
 _@Error_ is a reference to an _Error_.
@@ skipping 271 matching lines @@
   @Code code [optional];
 }
 ```
 
 A _Function_ represents a Dart language function.
 
 ### Instance
 
 ```
 class @Instance extends @Object {
+  // What kind of instance is this?
+  InstanceKind kind;
+
   // Instance references include their class.
   @Class class;
+
+  // The value of this instance as a string.
+  //
+  // Provided for the instance kinds:
+  //   Null (null)
+  //   Bool (true or false)
+  //   Double (suitable for passing to Double.parse())
+  //   Int (suitable for passing to int.parse())
+  //   String (value may be truncated)
+  string valueAsString [optional];
+
+  // The valueAsString for String references may be truncated.  If so,
+  // this property is added with the value 'true'.
+  bool valueAsStringIsTruncated [optional];
+
+  // The length of a List instance.
+  //
+  // Provided for instance kinds:
+  //   List
+  int length [optional];
+
+  // The name of a Type instance.
+  //
+  // Provided for instance kinds:
+  //   Type
+  string name [optional];
+
+  // The corresponding Class if this Type is canonical.
+  //
+  // Provided for instance kinds:
+  //   Type
+  @Class typeClass [optional];
+
+  // The parameterized class of a type parameter:
+  //
+  // Provided for instance kinds:
+  //   TypeParameter
+  @Class parameterizedClass [optional];
 }
 ```
 
 _@Instance_ is a reference to an _Instance_.
 
 ```
 class Instance extends Object {
+  // What kind of instance is this?
+  InstanceKind kind;
+
+  // Instance references include their class.
+  @Class class;
+
+  // The value of this instance as a string.
+  //
+  // Provided for the instance kinds:
+  //   Bool (true or false)
+  //   Double (suitable for passing to Double.parse())
+  //   Int (suitable for passing to int.parse())
+  //   String (value may be truncated)
+  string valueAsString [optional];
+
+  // The valueAsString for String references may be truncated.  If so,
+  // this property is added with the value 'true'.
+  bool valueAsStringIsTruncated [optional];
+
+  // The length of a List instance.
+  //
+  // Provided for instance kinds:
+  //   List
+  int length [optional];
+
+  // The name of a Type instance.
+  //
+  // Provided for instance kinds:
+  //   Type
+  string name [optional];
+
+  // The corresponding Class if this Type is canonical.
+  //
+  // Provided for instance kinds:
+  //   Type
+  @Class typeClass [optional];
+
+  // The parameterized class of a type parameter:
+  //
+  // Provided for instance kinds:
+  //   TypeParameter
+  @Class parameterizedClass [optional];
+
+  // The fields of this Instance.
   BoundField fields [optional];
+
+  // The elements of a List instance.
+  //
+  // Provided for instance kinds:
+  //   List
+  ListElement[] elements [optional];
+
+  // The function associated with a Closure instance.
+  //
+  // Provided for instance kinds:
+  //   Closure
+  @Function closureFunction [optional];
+
+  // The context associated with a Closure instance.
+  //
+  // Provided for instance kinds:
+  //   Closure
+  @Function closureContext [optional];
+
+  // The referent of a MirrorReference instance.
+  //
+  // Provided for instance kinds:
+  //   MirrorReference
+  @Instance mirrorReferent [optional];
+  
+  // The key for a WeakProperty instance.
+  //
+  // Provided for instance kinds:
+  //   WeakProperty
+  @Instance propertyKey [optional];
+  
+  // The key for a WeakProperty instance.
+  //
+  // Provided for instance kinds:
+  //   WeakProperty
+  @Instance propertyValue [optional];
+
+  // The type arguments for this type.
+  //
+  // Provided for instance kinds:
+  //   Type
+  @TypeArguments typeArguments [optional];
+
+  // The index of a TypeParameter instance.
+  //
+  // Provided for instance kinds:
+  //   TypeParameter
+  int parameterIndex [optional];
+
+  // The type bounded by a BoundedType instance
+  // or
+  // The referent of a TypeRef instance.
+  //
+  // The value will always be one of:
+  // Type, TypeRef, TypeParameter, BoundedType.
+  //
+  // Provided for instance kinds:
+  //   BoundedType
+  //   TypeRef
+  @Instance type [optional];
+
+  // The bound of a TypeParameter or BoundedType.
+  //
+  // The value will always be one of:
+  // Type, TypeRef, TypeParameter, BoundedType.
+  //
+  // Provided for instance kinds:
+  //   BoundedType
+  //   TypeParameter
+  @Instance bound [optional];
 }
 ```
 
 An _Instance_ represents an instance of the Dart language class _Object_.
 
-### Int
+### InstanceKind
 
 ```
-class @Int extends @Instance {
-  // The value of this int as a string.
-  //
-  // Suitable for passing to int.parse().
-  string valueAsString;
+enum {
+  // A general instance of the Dart class Object.
+  PlainInstance,
+
+  // null instance.
+  Null,
+
+  // true or false.
+  Bool,
+
+  // An instance of the Dart class double.
+  Double,
+
+  // An instance of the Dart class int.
+  Int,
+
+  // An instance of the Dart class String.
+  String,
+
+  // An instance of the built-in VM List implementation.  User-defined
+  // Lists will be PlainInstance.
+  List,
+
+  // An instance of the built-in VM Map implementation.  User-defined
+  // Lists will be PlainInstance.
+  Map,
+
+  // An instance of the built-in VM Closure implementation.  User-defined
+  // Closures will be PlainInstance.
+  Closure,
+
+  // An instance of the Dart class MirrorReference.
+  MirrorReference,
+
+  // An instance of the Dart class WeakProperty.
+  WeakProperty,
+
+  // An instance of the Dart class Type
+  Type,
+
+  // An instance of the Dart class TypeParamer
+  TypeParameter,
+
+  // An instance of the Dart class TypeRef
+  TypeRef,
+
+  // An instance of the Dart class BoundedType
+  BoundedType,
 }
 ```
 
-_@Int_ is a reference to an _Int_.
-
-```
-class Int extends Instance {
-  // The value of this int as a string.
-  //
-  // Suitable for passing to int.parse().
-  string valueAsString;
-}
-```
-
-An _Int_ represents an instance of the Dart language class _int_.
+Adding new values to _InstanceKind_ is considered a backwards
+compatible change.  Clients should treat unrecognized instance kinds
+as _PlainInstance_.
 
 ### Isolate
 
 ```
 class @Isolate extends Response {
   // The id which is passed to the getIsolate RPC to load this isolate.
   string id;
 
   // A numeric id for this isolate, represented as a string.  Unique.
   string number;
@@ skipping 110 matching lines @@
   // The prefix of an 'as' import, or null.
   String prefix;
 
   // The library being imported or exported.
   @Library target;
 }
 ```
 
 A _LibraryDependency_ provides information about an import or export.
 
-### List
-
-```
-class @List extends @Instance {
-  // The length of this list.
-  int length;
-}
-```
-
-_@List_ is a reference to a _List_.
-
-```
-class List extends Instance {
-  // The length of this list.
-  int length;
-
-  // The elements of this list.
-  ListElement[] elements;
-}
-```
-
-A _List_ represents an built-in instance of the Dart class _List_.
-User-defined lists will be represented as _Instance_.
-
 ### ListElement
 
 ```
 class ListElement {
   int index;
   @Instance|Sentinel value;
 }
 ```
 
 ### Message
@@ skipping 169 matching lines @@
 ```
 enum StepOption {
   Into,
   Over,
   Out
 }
 ```
 
 A _StepOption_ indicates which form of stepping is requested in a [resume](#resume) RPC.
 
-### String
-
-```
-class @String extends @Instance {
-  // The value of this double as a string.
-  //
-  // Note that this may be truncated.
-  //
-  // Suitable for passing to double.parse().
-  string valueAsString;
-
-  // The valueAsString for String references may be truncated.  If so,
-  // this property is added with the value 'true'.
-  bool valueAsStringIsTruncated [optional];
-}
-```
-
-_@String_ is a reference to a _String_.
-
-```
-class String extends Instance {
-  // The value of this double as a string.
-  //
-  // Note that this will never be truncated.
-  //
-  // Suitable for passing to double.parse().
-  string valueAsString;
-}
-```
-
-An _String_ represents an instance of the Dart language class _String_.
-
-The _valueAsString_ property for an _@String_ may be truncated.  To get
-the untruncated _valueAsString_, call the [getObject](#getobject) RPC
-on the String's _id_.  A _String_ object never truncates the
-_valueAsString_.
-
 ### Success
 
 ```
 class Success extends Response {
 }
 ```
 
 The _Success_ type is used to indicate that an operation completed successfully.
 
-### Type
-
-```
-class @Type extends @Instance {
-  // The name of this type.
-  string name;
-
-  // The corresponding Class if this Type is canonical.
-  @Class typeClass [optional];
-}
-```
-
-_@Type_ is a reference to a _Type_.
-
-```
-class Type extends Instance {
-  // The name of this type.
-  string name;
-
-  // The corresponding Class if this Type is canonical.
-  @Class typeClass [optional];
-
-  // The type arguments for this type.
-  @TypeArguments typeArgs [optional];
-}
-```
-
-An _Type_ represents an instance of the Dart language class _Type_.
-
 ### TypeArguments
 
 ```
 class @TypeArguments extends @Object {
   // A name for this type argument list.
   string name;
 }
 ```
 
 _@TypeArguments_ is a reference to a _TypeArguments_ object.
@@ skipping 69 matching lines @@
   // A list of isolates running in the VM.
   @Isolate[] isolates
 }
 ```
 
 ## Revision History
 
 version | comments
 ------- | --------
 0.0 | draft