Restarts Separate Mechanism From Policy
Condition handling moreover provides a separation of mechanism from policy. Restarts provide various possible mechanisms for recovering from error, but do not select which mechanism is appropriate in a given situation. That is the province of the condition handler, which (since it is located in higher-level code) has access to a broader view.
An example: Suppose there is a library function whose purpose is to parse a single syslog file entry. What should this function do, if the entry is malformed? There is no one right answer, because the same library could be deployed in programs for many different purposes. In an interactive log-file browser, the right thing to do might be to return the entry unparsed, so the user can see it—but in an automated log-summarizing program, the right thing to do might be to supply null values for the unreadable fields, but abort with an error, if too many entries have been malformed.
That is to say, the question can only be answered in terms of the broader goals of the program, which are not known to the general-purpose library function. Nonetheless, exiting with an error message is only rarely the right answer. So instead of simply exiting with an error, the function may establish restarts offering various ways to continue—for instance, to skip the log entry, to supply default or null values for the unreadable fields, to ask the user for the missing values, or to unwind the stack and abort processing with an error message. The restarts offered constitute the mechanisms available for recovering from error; the selection of restart by the condition handler supplies the policy.
Read more about this topic: Exception Guarantees, Exception Handling in Software, Condition Systems
Famous quotes containing the words separate, mechanism and/or policy:
“Heaven has its business and earth has its business: those are two separate things. Heaven, that’s the angels’ pasture; they are happy; they don’t have to fret about food and drink. And you can be sure that they have black angels to do the heavy work like laundering the clouds or sweeping the rain and cleaning the sun after a storm, while the white angels sing like nightingales all day long or blow in those little trumpets like they show in the pictures we see in church.”
—Jacques Roumain (1907–1945)
“A mechanism of some kind stands between us and almost every act of our lives.”
—Sarah Patton Boyle, U.S. civil rights activist and author. The Desegregated Heart, part 3, ch. 2 (1962)
“Carlyle said “a lie cannot live.” It shows that he did not know how to tell them. If I had taken out a life policy on this one the premiums would have bankrupted me ages ago.”
—Mark Twain [Samuel Langhorne Clemens] (1835–1910)