custom_error_4.bal

import ballerina/io;

const REASON = "ErrorReason";

type Detail record {|
    string message;
    error cause?;
    int code;
|};

type CustomError error<REASON, Detail>;

public function main() {
    // Use the direct error constructor to create an error
    // similar to what could be created as a `CustomError`.
    error err = error("ErrorReason", 
                      message = "error message",
                      code = 1122);

    // Since the error value is immutable it cannot be changed 
    // now. The type test evaluates to `true` since the reason is
    // the same as what is expected by `CustomError` and the expected 
    // detail fields (i.e., `message`, `code`) are also available 
    // and are of the expected types.
    io:println(err is CustomError);
}