Add lang.runtime module

Functions to register and deregister a  listener.
Fixes #361.

lang/Makefile

@@ -15,7 +15,8 @@ ZIPFILE=$(DISTDIR).zip
 COPYFILES= \
   lib/array.bal lib/boolean.bal lib/decimal.bal lib/error.bal lib/float.bal \
   lib/future.bal lib/int.bal lib/map.bal lib/object.bal lib/stream.bal \
-  lib/string.bal lib/table.bal lib/typedesc.bal lib/value.bal lib/xml.bal \
+  lib/string.bal lib/table.bal lib/typedesc.bal lib/xml.bal \
+  lib/runtime.bal lib/value.bal \
   style/ballerina-language-specification.css \
   style/ballerina-language-specification.js
 DISTFILES=spec.html $(COPYFILES)

lang/lib/runtime.bal

@@ -0,0 +1,34 @@
+// Copyright (c) 2020 WSO2 Inc. (http://www.wso2.org) All Rights Reserved.
+//
+// WSO2 Inc. licenses this file to you under the Apache License,
+// Version 2.0 (the "License"); you may not use this file except
+// in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied.  See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+# A listener that is dynamically registered with a module.
+public type DynamicListener object {
+   public function 'start() returns error?;
+   public function gracefulStop() returns error?;
+   public function immedateStop() returns error?;
+};
+
+# Register a listener object with a module.
+# + listener - the listener object to be registered
+# The listener becomes a module listener of the module from which this
+# function is called.
+public isolated function registerListener(DynamicListener listener) = external;
+
+# Deregister a listener from a module.
+# + listener - the listener object to be unregistered
+# The `listener` ceases to be a module listener of the module from
+# which this function is called.
+public isolated function deregisterListener(DynamicListener listener) = external;

lang/spec.html

@@ -9134,10 +9134,10 @@ <h2 id="lang_library">10. Lang library</h2>
 </p>
 <p>
 The lang library comprises the following modules. With the exception of the
-<code>lang.value</code> module, each corresponds to a basic type.
+<code>lang.value</code> and <code>lang.runtime</code> modules, each corresponds
+to a basic type.
 </p>
 <ul>
-<li><code><a href="./lib/value.bal" type="text/plain">lang.value</a></code></li>
 <li><code><a href="./lib/array.bal" type="text/plain">lang.array</a></code> for
 basic type list</li>
 <li><code><a href="./lib/boolean.bal" type="text/plain">lang.boolean</a></code> for
@@ -9166,6 +9166,8 @@ <h2 id="lang_library">10. Lang library</h2>
 basic type typedesc</li>
 <li><code><a href="./lib/xml.bal" type="text/plain">lang.xml</a></code> for
 basic type xml</li>
+<li><code><a href="./lib/runtime.bal" type="text/plain">lang.runtime</a></code></li>
+<li><code><a href="./lib/value.bal" type="text/plain">lang.value</a></code></li>
 </ul>
 <p>
 Modules in the lang library can make use generic typing. Since generic typing