12.1: Introduction

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Modern programming languages, including Smalltalk offer a dedicated exception-handling mechanism that greatly simplifies the way in which exceptional situations are signaled and handled. Before the development of the ANSI Smalltalk standard in 1996, several exception handling mechanisms existed, mostly incompatible with each other. Pharo’s exception handling follows the ANSI standard, with some embellishments; we present it in this chapter from a user perspective.

The basic idea behind exception handling is that client code does not clutter the main logic flow with checks for error codes, but specifies instead an exception handler to “catch” exceptions. When something goes wrong, instead of returning an error code, the method that detects the exceptional situation interrupts the main flow of execution by signaling an exception. This does two things: it captures essential information about the context in which the exception occurred, and transfers control to the exception handler, written by the client, which decides what to do about it. The “essential information about the context” is saved in an Exception object; various classes of Exception are specified to cover the varied exceptional situations that may arise.

Pharo’s exception-handling mechanism is particularly expressive and flexible, covering a wide range of possibilities. Exception handlers can be used to ensure that certain actions take place even if something goes wrong, or to take action only if something goes wrong. Like everything in Smalltalk, exceptions are objects, and respond to a variety of messages. When an exception is caught by a handler, there are many possible responses: the handler can specify an alternative action to perform; it can ask the exception object to resume the interrupted operation; it can retry the operation; it can pass the exception to another handler; or it can reraise a completely different exception.