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:
rescueclause may execute a
retryinstruction, 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
rescueclause, before the
retry, have attempted to correct the cause of the exception.
- If the
rescueclause does not end with
retry, then the routine fails: it returns to its caller, immediately triggering an exception. (The caller's
rescueclause 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
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.
if failures < 50 then
successful := True
successful := False
failures := failures + 1
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.