EXn

Exn is a shorthand used in several programming language communities to refer to the concept of an exception value. An exception represents an error or unusual condition that disrupts normal program flow and can be raised, propagated, and eventually handled by a corresponding handler. Depending on the language, exn may be a simple tag or a structured object carrying a message and additional metadata such as a location, stack trace, or continuation information.

In OCaml, exn is the built-in type of all exceptions. Exceptions are values of type exn and are raised with the raise construct. The standard library provides several predefined exceptions, such as Failure of string, Invalid_argument of string, Division_by_zero, Not_found, and Stack_overflow. Users can also declare their own exceptions using the exception keyword, typically carrying additional payload data. Exceptions in OCaml are caught with a try...with construct, often using pattern matching to retrieve the payload.

In the Scheme family of languages, including Racket, exn (and related exn:… variants) denotes an exception object or record. These exceptions can include fields such as a message, a culprit, and source location. They are created and raised within the language's error-handling facilities and are caught by corresponding handlers, allowing structured and context-rich error management.

Overall, exn localization varies by language but shares the core idea: a first-class value used to signal, transport, and manage error conditions separate from regular return values. This approach supports modular error handling, composable libraries, and the ability to distinguish different error kinds through pattern matching or type-based dispatch.