Principle: Errors are values

Background

Most nontrivial programs contain functions that can fail, meaning that even if all their preconditions are met, they may not be able to perform their primary behavior. For example, a function that reads data from a remote server may fail if the server is unreachable, and a function that parses a string to return an integer may fail if the input string is not a properly-formatted integer.

In many cases, the function author wants these failures to be recoverable, meaning that a direct or transitive caller can respond to the failure in some way that enables the program to continue running.

Principle

A Carbon function that needs to report recoverable failures should return a sum type whose alternatives represent the success case and failure cases, such as Optional(T), Result(T, Error), or Bool. The function's successful return value, and any metadata about the failure, should be embedded in the alternatives of the sum type, rather than reported by way of output parameters or other side channels. Carbon's design will prioritize making this form of error handling efficient and ergonomic.

Applications of these principles

Carbon errors, unlike exceptions in C++ and similar languages, will not be propagated implicitly. Instead, Carbon will very likely need to provide some explicit but syntactically lightweight means of propagating errors, such as Rust's ? operator, so that error-propagation boilerplate doesn't make it hard for readers to follow the logic of the success path.

Carbon will not have a special syntax for specifying what kind of errors a function can emit, such as noexcept or dynamic exception specifications in C++, or throws in Java, because that information will be embedded in the function's return type. Similarly, Carbon errors will be statically typed, because Carbon return values are statically typed.