The condition system is responsible for exception handling in Common Lisp. It provides conditions, handlers and restarts. Conditions are objects describing an exceptional situation (for example an error). If a condition is signaled, the Common Lisp system searches for a handler for this condition type and calls the handler. The handler can now search for restarts and use one of these restarts to repair the current problem. As part of a user interface (for example of a debugger), these restarts can also be presented to the user, so that the user can select and invoke one of the available restarts. Since the condition handler is called in the context of the error (without unwinding the stack), full error recovery is possible in many cases, where other exception handling systems would have already terminated the current routine. The debugger itself can also be customized or replaced using the *DEBUGGER-HOOK* dynamic variable.
In the following example (using Symbolics Genera) the user tries to open a file in a Lisp function test called from the Read-Eval-Print-LOOP (REPL), when the file does not exist. The Lisp system presents four restarts. The user selects the Retry OPEN using a different pathname restart and enters a different pathname (lispm-init.lisp instead of lispm-int.lisp). The user code does not contain any error handling code. The whole error handling and restart code is provided by the Lisp system, which can handle and repair the error without terminating the user code.
Command: (test ">zippy>lispm-int.lisp") Error: The file was not found. For lispm:>zippy>lispm-int.lisp.newest LMFS:OPEN-LOCAL-LMFS-1 Arg 0: #P"lispm:>zippy>lispm-int.lisp.newest" s-A,Read more about this topic: Common Lisp
Famous quotes containing the words condition and/or system:
“Undoubtedly we have not questions to ask which are unanswerable. We must trust the perfection of the creation so far, as to believe that whatever curiosity the order of things has awakened in our minds, the order of things can satisfy. Every man’s condition is a solution in hieroglyphic to those inquiries he would put. He acts it as life, before he apprehends it as truth.”
—Ralph Waldo Emerson (1803–1882)
“Loving feels lonely in a violent world,
irrelevant to people burning like last year’s weed
with bellies distended, with fish throats agape
and flesh melting down to glue.
We can no longer shut out the screaming
That leaks through the ventilation system ...”
—Marge Piercy (b. 1936)