union

• A union type describes a value that can be one of several types.
• Why union types?
  • Union is useful when the program expects a value to be either many different types. e.g., A parameter to the function or return value can be either int or float etc.
  • In traditional object-oriented code, we have to abstract over the two types by creating a hierarchy of types to achieve this.
• T1|T2|T3 represents a union of the sets T1, T2 and T3.
  • e.g., The type int|float|string can hold either int, float or string.
• The is operator is used to check whether the value belongs to a specific type. The type will be narrowed with the use of the is operator. Narrowing occurs when Ballerina can deduce a more specific type for a value based on the structure of the code.
• T1|() can be written as T? as well.

Sample

import ballerina/io;

type Address record {
    string city;
    string country;
    int code;
};

//Instead of writing longer union, it is  easy to  use a  type defintion.
type Location Address|string|int;

public function main() {
    int|string|float u1 = "this is a string";
    if u1 is int {
        io:println("union contains a int  value");
    } else if u1 is float {
        io:println("union contains a float  value");
    } else { //Type narrowed  to  string  here.
        io:println("union contains a string  value");
    }

    //Union types with nil.
    int|() i1 = 4;
    io:println(i1);
    float? f1 = ();
    io:println(f1);

    //Union type with type descriptors
    Location loc = "UK";
    io:println(loc);
}

Output

union contains a string  value
4

UK