diff --git a/AppKit/Rakefile b/AppKit/Rakefile
index f4f3d30cc..a93beb55c 100644
--- a/AppKit/Rakefile
+++ b/AppKit/Rakefile
@@ -53,6 +53,7 @@ file_d $THEME_PRODUCT => ThemeFiles << $ENVIRONMENT_PRODUCT do
             puts str
         end
     end
+    rake abort if ($? != 0)
 end
 
 task :build => [:build_subprojects, :AppKit, $ENVIRONMENT_PRODUCT, $THEME_PRODUCT, $ENVIRONMENT_THEME_PRODUCT]
diff --git a/Objective-J/Rakefile b/Objective-J/Rakefile
index c57bb8edc..7debf3b85 100644
--- a/Objective-J/Rakefile
+++ b/Objective-J/Rakefile
@@ -48,9 +48,7 @@ def platform_flags(*platforms)
 end
 
 def build_product(path, flags)
-
     IO.popen("gcc #{flags} -E -x c -P -", "w+") do |preprocessor|
-
         Files.select { |name| name.match(/\.js$/) }.each do |fileName|
           preprocessor.puts IO.read(fileName)
         end
@@ -60,8 +58,8 @@ def build_product(path, flags)
         File.open(path, "w") do |file|
             file.write(IO.read("header.txt") + preprocessor.read)
         end
-
     end
+    rake abort if ($? != 0)
 end
 
 task :build => [:Products, $ENVIRONMENT_PRODUCT, :build_subprojects]
diff --git a/Objective-J/debug.js b/Objective-J/debug.js
index cbc5fd447..2c7624464 100644
--- a/Objective-J/debug.js
+++ b/Objective-J/debug.js
@@ -1,173 +1,184 @@
-function objj_backtrace_format(aReceiver, aSelector)
+// formatting helpers
+
+function objj_debug_object_format(aReceiver)
 {
-    return "[<" + GETMETA(aReceiver).name + " " + (typeof sprintf == "function" ? sprintf("%#08x", aReceiver.__address) : aReceiver.__address.toString(16)) + "> " + aSelector + "]";
+    return (aReceiver && aReceiver.isa) ? sprintf("<%s %#08x>", GETMETA(aReceiver).name, aReceiver.__address) : String(aReceiver);
 }
 
-function objj_msgSend_Backtrace(/*id*/ aReceiver, /*SEL*/ aSelector)
+function objj_debug_message_format(aReceiver, aSelector)
 {
-    if (aReceiver == nil)
-        return nil;
-
-    objj_debug_backtrace.push(objj_backtrace_format(aReceiver, aSelector));
-
-    try
-    {
-        var result = class_getMethodImplementation(aReceiver.isa, aSelector).apply(aReceiver, arguments);
-    }
-    catch (anException)
-    {
-        CPLog.error("Exception " + anException + " in " + objj_backtrace_format(aReceiver, aSelector));
-        objj_debug_print_backtrace();
-    }
-
-    objj_debug_backtrace.pop();
-
-    return result;
+    return sprintf("[%s %s]", objj_debug_object_format(aReceiver), aSelector);
 }
 
-function objj_msgSendSuper_Backtrace(/*id*/ aSuper, /*SEL*/ aSelector)
+
+// save the original msgSend implementations so we can restore them later
+var objj_msgSend_original = objj_msgSend,
+    objj_msgSendSuper_original = objj_msgSendSuper;
+
+
+// decorator management functions
+
+// reset to default objj_msgSend* implementations
+function objj_msgSend_reset()
 {
-    objj_debug_backtrace.push(objj_backtrace_format(aSuper.receiver, aSelector));
-    var super_class = aSuper.super_class;
-
-    arguments[0] = aSuper.receiver;
-    
-    try
-    {
-        var result = class_getMethodImplementation(super_class, aSelector).apply(aSuper.receiver, arguments);
-    }
-    catch (anException)
-    {
-        CPLog.error("Exception " + anException + " in " + objj_backtrace_format(aSuper.receiver, aSelector));
-        objj_debug_print_backtrace();
-    }
-
-    objj_debug_backtrace.pop();
-
-    return result;
+    objj_msgSend = objj_msgSend_original;
+    objj_msgSendSuper = objj_msgSendSuper_original;
 }
 
-function objj_msgSend_Profile(/*id*/ aReceiver, /*SEL*/ aSelector)
+// decorate both objj_msgSend and objj_msgSendSuper
+function objj_msgSend_decorate()
 {
-    if (aReceiver == nil)
-        return nil;
-
-    // profiling book keeping 
-    var profileRecord = {
-        parent      : objj_debug_profile,
-        receiver    : GETMETA(aReceiver).name,
-        selector    : aSelector,
-        calls       : []
-    }
-    objj_debug_profile.calls.push(profileRecord);
-    objj_debug_profile = profileRecord;
-    profileRecord.start = new Date();
-    
-    var result = class_getMethodImplementation(aReceiver.isa, aSelector).apply(aReceiver, arguments);
-    
-    profileRecord.end = new Date();
-    objj_debug_profile = profileRecord.parent;
-
-    return result;
-}
-
-function objj_msgSendSuper_Profile(/*id*/ aSuper, /*SEL*/ aSelector)
-{
-    // profiling book keeping 
-    var profileRecord = {
-        parent      : objj_debug_profile,
-        receiver    : GETMETA(aReceiver).name,
-        selector    : aSelector,
-        calls       : []
-    }
-    objj_debug_profile.calls.push(profileRecord);
-    objj_debug_profile = profileRecord;
-    profileRecord.start = new Date();
-
-    var super_class = aSuper.super_class;
-
-    arguments[0] = aSuper.receiver;
-    
-    var result = class_getMethodImplementation(super_class, aSelector).apply(aSuper.receiver, arguments);
-    
-    profileRecord.end = new Date();
-    objj_debug_profile = profileRecord.parent;
-
-    return result;
-}
-
-var objj_msgSend_Standard = objj_msgSend,
-    objj_msgSendSuper_Standard = objj_msgSendSuper;
-
-// FIXME: This could be much better.
-var objj_debug_backtrace;
-    
-function objj_backtrace_set_enabled(enabled)
-{
-    if (enabled)
+    for (var i = 0; i < arguments.length; i++)
     {
-        objj_debug_backtrace = [];
-        objj_msgSend = objj_msgSend_Backtrace;
-        objj_msgSendSuper = objj_msgSendSuper_Backtrace;
+        objj_msgSend = arguments[i](objj_msgSend);
+        objj_msgSendSuper = arguments[i](objj_msgSendSuper);
+    }
+}
+
+// reset then decorate both objj_msgSend and objj_msgSendSuper
+function objj_msgSend_set_decorators()
+{
+    objj_msgSend_reset();
+    objj_msgSend_decorate.apply(null, arguments);
+}
+
+
+// backtrace decorator
+
+var objj_backtrace = [];
+
+function objj_backtrace_print(stream) {
+    for (var i = 0; i < objj_backtrace.length; i++)
+        objj_fprintf(stream, objj_debug_message_format(objj_backtrace[i].receiver, objj_backtrace[i].selector));
+}
+
+function objj_backtrace_decorator(msgSend)
+{
+    return function(aReceiverOrSuper, aSelector)
+    {
+        var aReceiver = aReceiverOrSuper && (aReceiverOrSuper.receiver || aReceiverOrSuper);
+        
+        // push the receiver and selector onto the backtrace stack
+        objj_backtrace.push({ receiver: aReceiver, selector : aSelector });
+        try
+        {
+            return msgSend.apply(null, arguments);
+        }
+        catch (anException)
+        {
+            // print the exception and backtrace
+            objj_fprintf(warning_stream, "Exception " + anException + " in " + objj_debug_message_format(aReceiver, aSelector));
+            objj_backtrace_print(warning_stream);
+        }
+        finally
+        {
+            // make sure to always pop
+            objj_backtrace.pop();
+        }
+    }
+}
+
+// type checking decorator
+
+var objj_typechecks_reported = {},
+    objj_typecheck_prints_backtrace = false;
+
+function objj_typecheck_decorator(msgSend)
+{
+    return function(aReceiverOrSuper, aSelector)
+    {
+        var aReceiver = aReceiverOrSuper && (aReceiverOrSuper.receiver || aReceiverOrSuper);
+        
+        if (!aReceiver)
+            return msgSend.apply(null, arguments);
+
+        var types = aReceiver.isa.method_dtable[aSelector].types;
+        for (var i = 2; i < arguments.length; i++)
+        {
+            try
+            {
+                objj_debug_typecheck(types[i-1], arguments[i]);
+            }
+            catch (e)
+            {
+                var key = [GETMETA(aReceiver).name, aSelector, i, e].join(";");
+                if (!objj_typechecks_reported[key]) {
+                    objj_typechecks_reported[key] = true;
+                    objj_fprintf(warning_stream, "Type check failed on argument " + (i-2) + " of " + objj_debug_message_format(aReceiver, aSelector) + ": " + e);
+                    if (objj_typecheck_prints_backtrace)
+                        objj_backtrace_print(warning_stream);
+                }
+            }
+        }
+        
+        var result = msgSend.apply(null, arguments);
+
+        try
+        {
+            objj_debug_typecheck(types[0], result);
+        }
+        catch (e)
+        {
+            var key = [GETMETA(aReceiver).name, aSelector, "ret", e].join(";");
+            if (!objj_typechecks_reported[key]) {
+                objj_typechecks_reported[key] = true;
+                objj_fprintf(warning_stream, "Type check failed on return val of " + objj_debug_message_format(aReceiver, aSelector) + ": " + e);
+                if (objj_typecheck_prints_backtrace)
+                    objj_backtrace_print(warning_stream);
+            }
+        }
+
+        return result;
+    }
+}
+
+// type checking logic:
+function objj_debug_typecheck(expectedType, object)
+{
+    var objjClass;
+    
+    if (!expectedType)
+    {
+        return;
+    }
+    else if (expectedType === "id")
+    {
+        if (object !== undefined)
+            return;
+    }
+    else if (expectedType === "void")
+    {
+        if (object === undefined)
+            return;
+    }
+    else if (objjClass = objj_getClass(expectedType))
+    {
+        if (object === nil)
+        {
+            return;
+        }
+        else if (object && object.isa)
+        {
+            var theClass = object.isa;
+            for (; theClass; theClass = theClass.super_class)
+            if (theClass === objjClass)
+                return;
+        }
     }
     else
     {
-        objj_msgSend = objj_msgSend_Standard;
-        objj_msgSendSuper = objj_msgSendSuper_Standard;
-    }
-}
-
-function objj_debug_print_backtrace()
-{
-    alert(objj_debug_backtrace_string());
-}
-
-function objj_debug_backtrace_string()
-{
-    return objj_debug_backtrace ? objj_debug_backtrace.join("\n") : "";
-}
-
-var objj_debug_profile = null,
-    objj_currently_profiling = false,
-    objj_profile_cleanup;
-
-function objj_profile(title)
-{
-    if (objj_currently_profiling)
         return;
-    
-    var objj_msgSend_profile_saved = objj_msgSend,
-        objj_msgSendSuper_profile_saved = objj_msgSendSuper;
-    
-    objj_msgSend = objj_msgSend_Profile;
-    objj_msgSendSuper = objj_msgSendSuper_Profile;
-    
-    var root = { calls: [] };
-    objj_debug_profile = root;
-    
-    var context = {
-        start : new Date(),
-        title : title,
-        profile : root
-    };
-
-    objj_profile_cleanup = function() {
-        objj_msgSend = objj_msgSend_profile_saved;
-        objj_msgSendSuper = objj_msgSendSuper_profile_saved;
-        context.end = new Date();
-        return context;
     }
     
-    objj_currently_profiling = true;
-}
-
-function objj_profileEnd()
-{
-    if (!objj_currently_profiling)
-        return;
-    
-    objj_debug_profile = null;
-    objj_currently_profiling = false;
-    
-    return objj_profile_cleanup();
+    var actualType;
+    if (object === null)
+        actualType = "null";
+    else if (object === undefined)
+        actualType = "void";
+    else if (object.isa)
+        actualType = GETMETA(object).name;
+    else
+        actualType = typeof object;
+        
+    throw ("expected=" + expectedType + ", actual=" + actualType);
 }
diff --git a/Objective-J/preprocess.js b/Objective-J/preprocess.js
index 47218a9ca..ad96215f9 100644
--- a/Objective-J/preprocess.js
+++ b/Objective-J/preprocess.js
@@ -20,7 +20,8 @@
  * Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA
  */
 
-var OBJJ_PREPROCESSOR_DEBUG_SYMBOLS = 1 << 0;
+var OBJJ_PREPROCESSOR_DEBUG_SYMBOLS = 1 << 0,
+    OBJJ_PREPROCESSOR_TYPE_SIGNATURES = 1 << 1;
 
 function objj_preprocess(/*String*/ aString, /*objj_bundle*/ aBundle, /*objj_file*/ aSourceFile, /*unsigned*/ flags) 
 {    
@@ -592,12 +593,15 @@ objj_preprocessor.prototype.method = function(tokens)
     var buffer = new objj_stringBuffer(),
         token,
         selector = "",
-        parameters = [];
+        parameters = [],
+        types = [null];
     
     while((token = tokens.skip_whitespace()) && token != TOKEN_OPEN_BRACE)
     {
         if (token == TOKEN_COLON)
         {
+            var type = "";
+
             // Colons are part of the selector name
             selector += token;
             
@@ -606,18 +610,30 @@ objj_preprocessor.prototype.method = function(tokens)
             if (token == TOKEN_OPEN_PARENTHESIS)
             {
                 // Swallow parameter/return type.  Perhaps later we can use this for debugging?
-                while((token = tokens.skip_whitespace()) && token != TOKEN_CLOSE_PARENTHESIS) ;
+                while((token = tokens.skip_whitespace()) && token != TOKEN_CLOSE_PARENTHESIS)
+                    type += token;
     
                 token = tokens.skip_whitespace();
             }
             
+            // Add the type. If it's empty, add null instead.
+            types[parameters.length+1] = type || null;
+
             // Since this follows a colon, this must be the parameter name.
             parameters[parameters.length] = token;
         }
         
         else if (token == TOKEN_OPEN_PARENTHESIS)
+        {
+            var type = "";
+
             // Since :( is handled above, this must be the return type, just swallow it.
-            while((token = tokens.skip_whitespace()) && token != TOKEN_CLOSE_PARENTHESIS) ;
+            while((token = tokens.skip_whitespace()) && token != TOKEN_CLOSE_PARENTHESIS)
+                type += token;
+
+            // types[0] is the return argument
+            types[0] = type || null;
+        }
         
         // Argument list ", ..."
         else if (token == TOKEN_COMMA)
@@ -654,7 +670,11 @@ objj_preprocessor.prototype.method = function(tokens)
 
     CONCAT(buffer, ")\n{ with(self)\n{");
     CONCAT(buffer, this.preprocess(tokens, NULL, TOKEN_CLOSE_BRACE, TOKEN_OPEN_BRACE));
-    CONCAT(buffer, "}\n})");
+    CONCAT(buffer, "}\n}");
+    // TODO: actually use OBJJ_PREPROCESSOR_TYPE_SIGNATURES flag instead of tying to OBJJ_PREPROCESSOR_DEBUG_SYMBOLS
+    if (this._flags & OBJJ_PREPROCESSOR_DEBUG_SYMBOLS) //OBJJ_PREPROCESSOR_TYPE_SIGNATURES)
+        CONCAT(buffer, ","+JSON.stringify(types));
+    CONCAT(buffer, ")");
 
     return buffer;
 }
diff --git a/Rakefile b/Rakefile
index 4f0a3da62..e06fdf743 100644
--- a/Rakefile
+++ b/Rakefile
@@ -86,7 +86,9 @@ file_d $STARTER_DOWNLOAD_APPLICATION => [$TOOLS_DOWNLOAD_ENV] do
 
     rm_rf($STARTER_DOWNLOAD_APPLICATION)
     mkdir_p($STARTER_DOWNLOAD)
+    
     system %{capp gen #{$STARTER_DOWNLOAD_APPLICATION} -t Application --noconfig }
+    rake abort if ($? != 0)
 
     # No tools means no objective-j gem
     rm(File.join($STARTER_DOWNLOAD_APPLICATION, 'Rakefile'))
@@ -103,7 +105,9 @@ task :install => [:tools_download] do
     else
         prefix = ''
     end
+    
     system %{cd #{$TOOLS_DOWNLOAD} && sudo sh ./install-tools #{prefix} }
+    rake abort if ($? != 0)
 end
 
 task :test => [:build] do
@@ -122,6 +126,8 @@ end
 task :docs do
     if executable_exists? "doxygen"
       system %{doxygen #{$DOXYGEN_CONFIG} }
+      rake abort if ($? != 0)
+      
       rm_rf $DOCUMENTATION_BUILD
       mv "debug.txt", "Documentation"
       mv "Documentation", $DOCUMENTATION_BUILD
@@ -133,6 +139,7 @@ end
 task :submodules do
     if executable_exists? "git"
         system %{git submodule init && git submodule update}
+        rake abort if ($? != 0)
     else
         puts "Git not installed"
         rake abort
diff --git a/Tools/Rake/lib/objective-j/bundletask.rb b/Tools/Rake/lib/objective-j/bundletask.rb
index eeee33003..95b77e138 100644
--- a/Tools/Rake/lib/objective-j/bundletask.rb
+++ b/Tools/Rake/lib/objective-j/bundletask.rb
@@ -390,6 +390,7 @@ module ObjectiveJ
                                 puts str
                             end
                         end
+                        #rake abort if ($? != 0)
                     end
 
                     { 'copy' => include_nibs, 'copied_resources' => [copied_resource] }
@@ -424,6 +425,7 @@ module ObjectiveJ
                             puts str
                         end
                     end
+                    rake abort if ($? != 0)
                 end
 
                 enhance([frameworks_path])
@@ -510,6 +512,7 @@ module ObjectiveJ
                             puts str
                         end
                     end
+                    rake abort if ($? != 0)
                 end
 
                 BundleTask.compact(build_path) if needs_compact
diff --git a/Tools/capp/Templates/Application/index-debug.html b/Tools/capp/Templates/Application/index-debug.html
index 5ebc00c9a..54b8f7fbe 100644
--- a/Tools/capp/Templates/Application/index-debug.html
+++ b/Tools/capp/Templates/Application/index-debug.html
@@ -21,7 +21,22 @@
         </script>
         
         <script src = "Frameworks/Debug/Objective-J/Objective-J.js" type = "text/javascript"></script>
-
+        
+        <script type = "text/javascript">
+            objj_msgSend_reset();
+            
+            // DEBUG OPTIONS:
+            
+            // Uncomment to enable printing of backtraces on exceptions:
+            //objj_msgSend_decorate(objj_backtrace_decorator);
+            
+            // Uncomment to enable runtime type checking:
+            //objj_msgSend_decorate(objj_typecheck_decorator);
+            
+            // Uncomment (along with both above) to print backtraces on type check errors:
+            //objj_typecheck_prints_backtrace = true;
+        </script>
+        
         <style type = "text/css">
             body{margin:0; padding:0;}
             #container {position: absolute; top:50%; left:50%;}
diff --git a/Tools/capp/Templates/ThemeDescriptor/Rakefile b/Tools/capp/Templates/ThemeDescriptor/Rakefile
index c88a1aeae..9421af8e1 100644
--- a/Tools/capp/Templates/ThemeDescriptor/Rakefile
+++ b/Tools/capp/Templates/ThemeDescriptor/Rakefile
@@ -16,6 +16,7 @@ file_d $BLEND_PRODUCT => ['ThemeDescriptors.j'] do
             puts str
         end
     end
+    rake abort if ($? != 0)
 end
 
 task :default => [$BLEND_PRODUCT]
diff --git a/common.rb b/common.rb
index 1ae320f40..eeb3ac534 100644
--- a/common.rb
+++ b/common.rb
@@ -77,8 +77,8 @@ end
 def subrake(directories, task_name)
     directories.each do |directory|
       if (File.directory?(directory) && File.file?(File.join(directory, "Rakefile")))
-        ok = system(%{cd #{directory} && #{$serialized_env} #{$0} #{task_name}})
-        rake abort unless ok
+        system(%{cd #{directory} && #{$serialized_env} #{$0} #{task_name}})
+        rake abort if ($? != 0)
       else
         puts "warning: subrake missing: " + directory +" (this is not necessarily an error, "+directory+" may be optional)"
       end
@@ -147,4 +147,5 @@ task :clobberall => ['clobber-all']
 
 def spawn_rake(task_name)
     system %{#{$serialized_env} #{$0} #{task_name}}
+    rake abort if ($? != 0)
 end