Swan Lake Alpha5

Overview of Ballerina Swan Lake Alpha5

This is the fifth Alpha release in a series of planned Alpha and Beta releases leading up to the Ballerina Swan Lake GA release.

It introduces the new language features planned for the Swan Lake GA release and includes improvements and bug fixes done to the compiler, runtime, standard library, and developer tooling after the Swan Lake Alpha4 release.

Updating Ballerina

If you are already using Ballerina, you can use the update tool to directly update to Ballerina Swan Lake Alpha5 as follows.

To do this, first, execute the command below to get the update tool updated to its latest version.

bal update

If you are using an update tool version below 0.8.14, execute the ballerina update command to update it. Next, execute the command below to update to Swan Lake Alpha5.

bal dist pull slalpha5

Installing Ballerina

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

Language updates

New features

The lang.xml:data() Lang Library function

The lang.xml:data() lang library function has been introduced to get the character data from an xml value. This function returns the character data of an xml:Text value as a string. For an xml:Element value, this returns the concatenation of the result of invoking this function over the children of the element. Calling lang.xml:data() on comments, processing instructions, and empty sequences returns an empty string.

The lang.xml:getDescendants() Lang Library function

The lang.xml:getDescendants() lang library function has been introduced to retrieve a sequence of the descendant elements for a given element in document order.

Bitwise operations as constant expressions

Bitwise binary operations <<, >>, >>>, ^, &, | are now allowed as constant expressions.

const BASE = 0x10;
const int SHIFTED = (BASE << 2) | 0xa;


Improvements to the XML literal

XML template literals now support the inclusion of multiple XML items of the same xml subtype as well as different xml subtypes.

xml x1 = xml `<ele>item</ele><!--comment-->text<?foo?>`;
xml<xml:Element> x2 = xml `<ele>item1</ele><ele>item2</ele>`;
xml<xml:Comment> x3 = xml `<!--comment1--><!--comment2-->`;
xml<xml:ProcessingInstruction> x4 = xml `<?foo?><?bar?>`;
string interpolation = "Interpolation";
xml<xml:Text> x5 = xml `text ${interpolation}`;
Normalization of XML attribute values

In XML template literals, character references within XML attribute values are now normalized to the referring character. For example, the value of the e.attr attribute in the following example is now a&b.

xml:Element e = xml `<elem attr="a&amp;b" />`;

Breaking changes

  • The lang.array:lastIndexOf and lang.array:indexOf lang library functions can only be used with arrays of which the element types are subtypes of the anydata type.
  • A compilation error occurs if the field visibility qualifiers in the including type and the included type with object type inclusion are not the same.
  • Remote method call action cannot be used to call non-remote methods.
  • The xml and readonly type-descriptors can no longer be used as expressions. A reference to a type definition defined with such type-descriptors needs to be used instead.
  • The argument passed to the second parameter of the attach method of the listener for service declarations has been fixed as follows. If there is no attach-point in the service, () (nil) is passed as the argument. If the attach-point is the root resource path ("/"), an empty array is passed.
  • The lang.xml:getContent() function no longer accepts xml:Text as an argument. The new lang.xml:data() function can be used instead.
  • lang.value functions cannot be called on expressions of type object. Function call syntax (value:func(obj)) can be used instead.
  • The lang.stream:reduce() lang library function will only return the reduced value or an error (if the completion type includes a subtype of error). It no longer returns a nil () value. The return type of lang.stream:reduce() has been updated accordingly.
  • An issue causing the complement of numeric literals to return incorrect values has been fixed.
  • Negation has been disallowed with numeric literals.
  • Subtyping rules have been fixed to consider the never type, which denotes an empty set of values as a subtype of any type T.
  • Relational expressions with lists have been fixed to consider the relative order of lists with different sizes.
  • Any non-error value will match the wildcard match pattern at runtime even when the matched expression's static type includes a subtype of error.
  • An invalid attempt to transfer out a value via a return statement that uses self in an isolated object method results in a compilation error.
  • In a readonly and class intersection, the method qualifiers are now preserved from the class.

Bug fixes

To view bug fixes, see the GitHub milestone for Swan Lake Alpha5.

Runtime updates

New features

Support for configurable variables with records having fields of record types
public type Person readonly & record {
     string name;
     int id;
     Address address;

public type Address  record {
    string city;
    County country;

public type County  record {
    string name;

configurable Person person = ?;

The Config.toml would be as follows.

name = "waruna"
id = 10
address.country.name="Sri Lanka"
Support for configurable variables with arrays having fields of record types
configurable Person[] & readonly personArray = ?;

The Config.toml would be as follows.

name = "manu"
id = 11
address.city="New York"

name = "hinduja"
id = 12
Support for configurable variables with multidimensional arrays
configurable int[][] & readonly int2DArr = ?;

The Config.toml would be as follows.

int2DArr = [[1,2],[3,4]]
Support for optional module name in TOML syntax of configurable variables

When providing values for configurable variables, the module information should be provided in the Config.toml file according to the following specifications.

  • The org-name and module-name are optional for configurable variables defined in the root module of the program.
  • The org-name is optional only for configurable variables defined in the root package of the program.

For example, consider a package with the organization name as myOrg and root module name as main.

The main.bal would be as follows.

import main.foo;
import importedOrg/mod;
configurable string mainVar = ?;
public function main() {
// use imported modules

The foo.bal file of the main.foo module will be as follows.

configurable string fooVar = ?;

In the mod.bal, which is from another package with the organization name importedOrg and module name mod,

configurable string modVar = ?;

the values can be provided in Config.toml as follows.

mainVar = "variable from root module"
fooVar = "variable from non-root module of the root package"
modVar = "variable from non-root package"


Improved command-line argument parsing

The command-line arguments are now parsed into the following.

  • options and option arguments
  • operands
Options and option arguments

The record parameter is included as the last of the parameter specify options.

In the example below, name and score are options. Alice and 99.9 are arguments of the option. Both the operand and option parameters can be of types int, float, decimal, string, array of any of these types, and union of any of these types with nil.

public type Person record {
	string name;
	float? score = 0;

public function main(*Person person) {
	// Process data here
bal run file.bal -- --name Alice --score=99.9

An array value is specified by repeatedly specifying the option parameter. In the example below, suppose scores is an int array.

bal run file.bal -- --scores=10 --scores=20 --scores=30

This produces the following int array.

[10, 20, 30]

Note: Additionally, option parameters can be of types boolean, boolean[], or boolean?. When there’s an option of one of these types, it does not take an option argument. The presence of the option is considered to be true and the absence of it is considered to be false.

In the example below, suppose results is a boolean array.

bal run file.bal -- --results --results --results

This produces the following boolean array.

[true, true, true]

Other parameters that are not included records specify operands. For these parameters, the position is significant and the name is not.

In the example below, it includes the two operands 100 and Good, which get mapped to efficiency and character respectively.

public type Person record {
	string name;
	float? score = 0;
public function main(int efficiency, string character, *Person person) {
	// Process data here
bal run file.bal -- --name alice  100 --score=99.9 Good

Note: If there is an operand parameter of type O[], then it cannot be followed by parameters of type O[], O?, and O x = d. Here, O stands for a type that is a subtype of one of string, float, or decimal.

Bug fixes

To view bug fixes, see the GitHub milestone for Swan Lake Alpha5.

Standard library updates

New features

Added compiler plugin validation for services in the following packages: email, file, graphql, grpc, http, kafka, nats, rabbitmq, stan, tcp, udp, websocket, websub, and websubhub`.

graphql package
  • Added named fragment support in GraphQL queries.
  • Added enum support for GraphQL services.
  • Added map support for GraphQL fields.


jballerina.java package

The java:cast function is now a dependently-typed function. If the typedesc argument is not provided, it is inferred from the contextually-expected type.

FileInputStream|error obj1 = java:cast(inputStream, FileInputStream);
FileInputStream|error obj2 = java:cast(inputStream); // The second argument is inferred to be `FileInputStream`.
file package
  • Changed the path field in the ListenerConfig from optional to mandatory.
  • Improved the compiler plugin validation for the service.
log package
  • Changed the time format in the log output to RFC3339.
  • Added validation to check log levels in the Config.toml file.
sql package

If return types of the SQL time-related data types are expected as records, then the type of those records will be validated now and should be as follows.

  • DATE -> time:Date
  • TIME -> time:TimeOfDay
  • TIME WITH TIME ZONE -> time:TimeOfDay
  • TIMESTAMP -> time:Civil
websubhub package

Added the websubhub:ServiceConfig annotation.

Bug fixes

To view bug fixes, see the GitHub milestone for Swan Lake Alpha5.

Developer tools updates

Bug fixes

To view bug fixes, see the GitHub milestone for Swan Lake Alpha5 of the repositories below.