import ballerina/io;function main (string[] args) {
    function (string, string) returns (string) lambda =
            (string x, string y) => (string) {
                string r = x + y;
                return r;
            };
    io:println("Output: " + lambda("Hello ", "world.!!!"));
}

Lambda

Lambdas are simply a syntactic shortcut for defining a function inline. In comparison to a normal function definition, the only missing part here is the name. Semantically, these inline anonymous functions will not be allowed to have any references to in-scope variables other than to global or package level variables. Thus, it behaves exactly like a regular function except that it cannot be called in another place as it has no name.

import ballerina/io;
function main (string[] args) {
    function (string, string) returns (string) lambda =
            (string x, string y) => (string) {
                string r = x + y;
                return r;
            };
    io:println("Output: " + lambda("Hello ", "world.!!!"));
}

Define lambda function in-line to function pointer var.

$ ballerina run lambda.bal
Output: Hello world.!!!