I2E: Exceptions

Whenever there is a contract, the risk exists that someone will break it. This is where exceptions come in.

Exceptions -- contract violations -- may arise from several causes. One is an assertion violation, if you've selected run-time assertion monitoring. Another is a signal triggered by the hardware or operating system to indicate an abnormal condition such as arithmetic overflow, or an attempt to create a new object when there's not enough memory available.

Unless a routine has made specific provision to handle exceptions, it will fail if an exception arises during its execution. This in turn provides one more source of exceptions: a routine that fails triggers an exception in its caller.

A routine may, however, handle an exception through a rescue clause. This optional clause attempts to "patch things up" by bringing the current object to a stable state (one satisfying the class invariant). Then it can terminate in either of two ways:

  • The rescue clause may execute a retry instruction, which causes the routine to restart its execution from the beginning, attempting again to fulfill its contract, usually through another strategy. This assumes that the instructions of the rescue clause, before the retry, have attempted to correct the cause of the exception.
  • If the rescue clause does not end with retry, then the routine fails: it returns to its caller, immediately triggering an exception. (The caller's rescue clause will be executed according to the same rules.)

The principle is that a routine must either succeed or fail: it either fulfills its contract, or not; in the latter case it must notify its caller by triggering an exception.

Usually, only a few routines of a system will explicitly include a rescue clause. A routine that doesn't have an explicit rescue is considered to have an implicit one, which calls a routine default_rescue that by default does nothing, so that an exception will cause the routine to fail immediately, propagating the exception to the caller.

An example using the exception mechanism is a routine attempt_transmission that tries to transmit a message over a phone line. The actual transmission is performed by an external, low-level routine transmit; once started, however, transmit may abruptly fail, triggering an exception, if the line is disconnected. Routine attempt_transmission tries the transmission at most 50 times; before returning to its caller, it sets a boolean attribute successful to True or False depending on the outcome. Here is the text of the routine: attempt_transmission (message: STRING) -- Try to transmit message, at most 50 times. -- Set successful accordingly. local failures: INTEGER do if failures < 50 then transmit (message) successful := True else successful := False end rescue failures := failures + 1 retry end

Initialization rules ensure that failures, a local entity, is set to zero on entry.

This example illustrates the simplicity of the mechanism: the rescue clause never attempts to achieve the routine's original intent; this is the sole responsibility of the body (the do clause). The only role of the rescue clause is to clean up the objects involved, and then either to fail or to retry.

This disciplined exception mechanism is essential for software developers, who need protection against unexpected events, but cannot be expected to sacrifice safety and simplicity to pay for this protection.