The sections below explain how the explicit error handling, static types, and concurrency safety combined with a familiar, readable syntax make programs reliable and maintainable.
Explicit error handling
Error handling refers to the act of reacting to and recovering from errors. Error handling plays a critical role in producing reliable, maintainable applications. Ballerina is designed with a conscious decision to have explicit errors instead of exceptions. As a result, Ballerina has explicit error reporting and explicit error testing and handling. It is explained further using examples.
Errors are expected in network handling. In fact, “The network is reliable” is one of the fallacies of distributed computing. Therefore, explicit error handling is a must when dealing with network services because it forces you to check for errors and handle or pass them to the caller.
In Ballerina, errors are reported by functions returning values of the
error type. The error values are part of their basic type.
This example defines a TCP client, which communicates with a remote TCP server. Both
write methods interact with the network. Hence, errors can occur at any time.
read method is completed successfully, it returns a
byte value. In the case of a failure, it returns an error value. You can describe this behavior using union types in Ballerina. The union type
byte | error means that the value could be either a
byte or an
error. The fact that this method can fail is explicit in the method signature.
write method does not return anything if it completes successfully. However, it returns an error otherwise. Optional types in Ballerina can be used to describe this behavior. Options types are syntactic sugar for union types containing the nil type, which is written as
error? is sugar for
error | (). The examples below show how you can check whether a function returns the expected value or an error.
Ballerina has language constructs for explicit error checking. Both explicit error reporting and checking improve code readability and maintainability simply because they are explicit. When you read Ballerina code, you can quickly notice the code, which reports errors, and you can check for errors.
Usually, a function handles errors by passing them up to the caller. Even the main function, which is the program entry point can return an error resulting in an error printed in your terminal.
The function below returns information about the provided domain name. It internally does a WHOIS database lookup via the provided TCP client. The code is correct and readable. However, it is verbose. Most of the time, you don’t need to handle every error. Instead, you pass errors to the caller.
is operator-based error checking pattern is very common and you would end up having too many of them in your code.
Ballerina provides a much more lightweight, shorthand for this pattern. The behavior of the function below is the same as the previous version. However, it is much more elegant. The
check expr check expression performs an explicit error check, and the control flow also remains explicit.
This function shows another pattern that handles errors in a single place. You can attach an
on fail clause to some Ballerina statements such as
foreach, etc. In this example, check does not simply return on error. The enclosing block decides how to handle the error. If the enclosing block has an
on fail clause, it catches the error. If the enclosing block does not have an
on fail block, it passes the error up to its enclosing block. Finally, the function handles the error by returning the error. This behavior is different from exceptions in that control flow is explicit.
Ignore return values and error
Ballerina does not allow ignoring return values of expressions.
However, you can explicitly ignore a return value of an expression by assigning the result of the expressions to
_;. This is like an implicitly declared variable of the
any type that cannot be referenced.
However, Ballerina does not allow ignoring the value of an expression if the type includes an error. You are forced to handle the error explicitly.
As explained earlier,
_ is like an implicitly declared variable of the
any type; this is a union type that includes all the types in Ballerina except for the error type. Therefore, the type, which includes all values supported by Ballerina is
any|error. As per the typing rules in Ballerina, the above statement causes a compilation error because
string|error is not a subtype of the
Deal with abnormal errors
Ballerina has made a conscious decision to distinguish normal errors from abnormal errors. The sections above explained how to deal with normal errors. Out of memory, division by zero, programming bugs are examples of abnormal errors in Ballerina. Such errors typically result in immediate program termination.
Abnormal errors can be reported using the
panic statement. Some language constructs such as type casts generate panics.
A panic always has an associated error value as illustrated in the example below.
Panics can be trapped with a
trap expression. Ballerina raises a panic on an integer overflow. You can convert this panic to an error with a trap expression.