Swan Lake Preview 7

Overview of Ballerina Swan Lake Preview 7

This release is the seventh preview version of Ballerina Swan Lake. It is the successor of the fifth preview version (SLP5) and SLP6 is an internal-only release.

This release includes a new set of language features along with improvements and bug fixes to the compiler, runtime, standard library, and developer tooling.

Updating Ballerina

You can use the update tool to update to Ballerina Swan Lake Preview 7 as follows.

For existing users

If you are already using Ballerina, you can directly update your distribution to the Swan Lake channel using the Ballerina update tool. To do this, first, execute the command below to get the update tool updated to its latest version.

ballerina update

Next, execute the command below to update to Swan Lake Preview 7.

ballerina dist pull slp7

However, if you are using a Ballerina version below 1.1.0, install via the installers.

For new users

If you have not installed Ballerina, then download the installers to install.


  • Introducing Ballerina Packages
  • Support for configuring module-level, basic variables at the program execution
  • Ability to mark module-variable declarations with an initial value expression as isolated
  • Ability to use the isolated qualifier with an object-constructor expression to construct an isolated object
  • Support to mark an object-type definition as a distinct object type
  • Allow using the object constructor to create distinct objects
  • Support to register and deregister a dynamic listener at the runtime
  • Introduction of the new runtime APIs
  • Revamp of the File/Filepath modules
  • Introduction of new high-level APIs to provide I/O operations
  • Introduction of a new code-action Add Type Cast
  • Revamp of the code-action extension APIs
  • Introduction of code to cloud (c2c) to simplify deploying Ballerina code in the cloud

What is new in Ballerina Swan Lake Preview 7

Ballerina packages

With this release we will be introducing Ballerina Packages. Ballerina Package is a bundle that consist of one or more Ballerina modules. Going forward, the Ballerina Package will be used to share Ballerina libraries.

With the introduction of Ballerina packages Ballerina project structure and build tools has also been changed. Please refer to the Organize Ballerina Code for more details.

For Swan Lake releases Ballerina Central will support users to push and pull Ballerina Packages. Going forward It will not be possible to push individual modules with swan lake releases. With the preview releases you have the ability to push and pull Ballerina Packages to/from Central. However the Packages pushed using preview releases will be cleared when we release the swan lake GA release.You can use the existing Ballerina Central key to push Packages.


Support for configurability

Now, Ballerina supports configuring module-level, basic variables at the program execution. The value provided in the program will be overridden by the value specified in the configuration file. These variables can be initialized using the configurable keyword in the following ways.

A configurable variable that is defined with ? as the initializer expression requires a value to be specified for such a variable in the configuration file.

configurable string hostName = ?;

A configurable variable that is defined with any other initializer expression can be overridden by a value specified in the configuration file. If a value is not provided in the configuration file, the initializer expression is used.

configurable string hostName =”;

Currently, configuration is supported via a TOML (v0.4.0) file named configuration.toml. This file should be located in the current directory. The format of the configuration.toml file is as follows.

# The above toml table specification is not needed for single file execution
intVar = 24
floatVar = 4.8
stringVar = "hello world"
booleanVar = false

Currently, Ballerina supports configurable variables of the types int, float, boolean, and string. Future versions will add support for configurable variables of any type that is a subtype of readonly&anydata.

Isolated variables

Module variable declarations with an initial value expression can now be marked as isolated. An isolated variable can be accessed only within a lock statement.

Similar to isolated objects, isolated variables are also considered as isolated roots, which guarantee that their mutable state is only reachable via the isolated root. As with isolated objects, when an isolated variable is accessed within a lock statement, additional rules apply to how values are transferred in and out of the lock statement.

An isolated variable can also be accessed in an isolated function.

type Coordinates record {|
    decimal latitude;
    decimal longitude;

isolated map<Coordinates> cities = {};

isolated function getLocation(string city) returns Coordinates? {
    lock {
        return cities[city].clone();

isolated function resetCities() {
    lock {
        cities = {};
Isolated object constructor expressions

Several changes have been introduced to how an object constructor expression can construct an isolated object. Now, the isolated qualifier can be used with an object constructor expression to construct an isolated object.

class Identifier {
    int id = 1;

public function main() {
    Identifier nonIsolatedIdentifier = new;

    Identifier isolatedIdentifier = isolated object {
        final int id = 1;
        private string name = "default";

        function updateName(string name) {
            lock {
                self.name = name;

    boolean b1 = <any> nonIsolatedIdentifier is isolated object {}; // false
    boolean b2 = <any> isolatedIdentifier is isolated object {}; // true

An object-constructor expression also constructs an isolated object if all of the fields of the object constructor expression are final and the types of the fields are subtypes of readonly or isolated object {}.

isolated class Identifier {
    final int i;
    private int j;

    isolated function init(int i, int j) {
        self.i = i;
        self.j = j;

public function main() {
    var ob = object { // Constructs an `isolated` object.
        final int a = 1000;
        final Identifier b = new (1, 2);

    isolated object {} isolatedOb = ob; // Now valid.
Support for distinct objects
Distinct object type

An object type definition can now define a distinct object type.

type Circle distinct object {
    int x;
    int y;
    int r;

type Color [int, int, int];

class ColoredCircle {
    Color color;

    function init(int x, int y, int r, Color color) {
        self.x = x;
        self.y = y;
        self.r = r;
        self.color = color;

class ColoredCircleLookAlike {
    int x;
    int y;
    int r;
    Color color;

    function init(int x, int y, int r, Color color) {
        self.x = x;
        self.y = y;
        self.r = r;
        self.color = color;

Circle c = new ColoredCircle(0, 0, 0, [123, 100, 100]);
Circle d = new ColoredCircleLookAlike(0, 0, 0, [123, 100, 100]); // Not allowed.

The ColoredCircle type is a distinct type that is a subtype of the Circle distinct object type even though ColoredCircle is not marked as a distinct class.

Although ColoredCircleLookAlike is structurally a subtype of the Circle type, since Circle is a distinct type and ColoredCircleLookAlike does not include it via object type inclusion,ColoredCircleLookAlike is not a subtype of Circle.

Distinct object constructor

If no type reference is provided in the object constructor, the contextually-expected type must be definite and the type-ids of the constructed object will be the type-ids of the contextually-expected type. This allows using the object constructor to create distinct objects.


Support for DynamicListener

A dynamic listener can be registered and deregistered at runtime as follows.

ModListener listenerVar = new(); // Where ModListener is a DynamicListener 

These methods and the definition of DynamicListener have been added to the new lang.runtime module.

public type DynamicListener object {
    public function __start() returns error?;
    public function __gracefulStop() returns error?;
    public function __immediateStop() returns error?;
New runtime APIs

The io.ballerina.runtime.api. package will only be exposed to the outside. The runtime API package and its sub packages will contain the required APIs to handle the runtime constructs as follows.

io.ballerina.runtime.api.creatorsProvides APIs to create runtime types, values, and errors.
io.ballerina.runtime.api.utilsProvides APIs to manage XML, JSON, string, and type-related operations.
io.ballerina.runtime.api.types and io.ballerina.runtime.api.valuesHolds the Ballerina runtime API types and values.
io.ballerina.runtime.api.EnvironmentInterop functions can have an Environment parameter as the first parameter. This provides APIs to get the current module, strand ID, strand name, and strand metadata. Also, with the Environment instance, you can set and get the strand local properties.

| io.ballerina.runtime.api.Runtime | The Environment also provides an instance of the current runtime class, which will contain APIs to invoke Ballerina object functions. | | io.ballerina.runtime.api.* | Contains Future, Module, PredefinedTypes, and TypeTags classes to handle the other runtime constructs.

None of the Ballerina runtime internal exceptions will be exposed. The BError class should be used to handle Ballerina runtime errors. The io.ballerina.runtime.api.creators.ErrorCreator class provides the required APIs to create runtime errors.

Standard library

Revamp file and filepath modules

The file and filepath APIs have been revamped in this release. A summarized list of the changes done is as follows.

  1. The file and filepath modules have been merged together. There will be no filepath module anymore.

  2. Some APIs have been removed.

    • From filepath - extension, getPathSeparator, getPathListSeparator, isReserved, and matches
    • From file - exists, tempDir
  3. The implementation of some APIs has been modified.

    • From filepath - normalize
    • From file - createDir, remove, copy, readDir, getMetaData (earlier getFileInfo)
  4. Some new APIs have been introduced.

    • createTemp - to create a temporary file in either the default temp directory of the OS or in a specific directory.
    • createTempDir - to create a temporary directory as above.
    • test - checks if a file/directory exists, readable, writable, etc.
Improved io module

The new high-level APIs below were introduced to provide I/O operations.

  1. Added a new set of APIs to read from files supporting different data types.

    E.g., io:fileReadBytes, io:fileReadJson, io:fileReadCsv, and io:fileReadXml

  2. Added a new set of APIs to read files as streams.

    E.g., io:fileReadBlocksAsStream, io:fileReadLinesAsStream, and io:fileReadCsvAsStream

Language server

  1. Introduced a new code-action Add Type Cast to add a typecast when variable assignment fails due to incompatible types.

    Before execution:

    int myInt = 1.1;
    int myInt = getFloat();

    After execution:

    int myInt = <int>1.1;
    int myInt = <int>getFloat();
  2. The code-action extension APIs have been revamped in this release. A summarized list of changes is as follows.

    • The MatchedNode, matchedSymbol, and matchedExprType for the cursor position are available now through the PositionDetails construct in the CodeActionContext.
    • Listing the priorities can be set for the code-actions now (lower the number, greater the priority).

Code to Cloud

  1. Introduced code to cloud (c2c) to simplify the experience of developing and deploying Ballerina code in the cloud. Code to cloud builds the containers and required artifacts by deriving the required values from the code without using any annotations.

    import ballerina/http;
    import ballerina/c2c as _;
    service hello on new http:Listener(9090) {
        resource function sayHello(http:Caller caller, http:Request req) returns error? {
            check caller->respond("Hello, World!");
  2. Removed the Kubernetes module and replaced it by the c2c module.

Breaking changes

  1. It is no longer possible to specify the version in an import declaration. A specific version can be imported by specifying the package dependency along with the version in the Ballerina.toml file.

    org = "ballerina"
    name = "stringutils"
    version = "0.5.2"
  2. As the initial step of upcoming changes for the error detail, the detail type descriptor of an error type descriptor can no longer be a closed record.

  3. The Kubernetes module is removed and replaced by the c2c module.