Error Handling and Conditions

Common Lisp does not have a bare exception mechanism; it has a condition system that separates the act of signaling a problem from the act of deciding how to respond to it. When code calls (error "disk full") or (signal 'my-condition), the runtime walks up the stack looking for a handler, but crucially it does this without unwinding the stack first. That means a handler can inspect the exact state where the problem occurred and choose to resume execution there, not just log and abandon the computation the way a thrown exception in most languages forces you to.

Conditions are ordinary CLOS classes defined with define-condition, typically subclassing error, warning, or condition itself. (error 'my-condition :slot value) both creates the instance and signals it, and if no handler intervenes, it drops into the debugger. warn signals a condition of type warning and, if unhandled, prints a message and continues — it never enters the debugger by default. This three-way split (error, warning, plain signal) lets a library author express severity precisely: a malformed but recoverable input can warn, while a corrupted file handle should error.

handler-case is the convenient, exception-like form: it unwinds the stack back to the handler-case form before running its clause, so it's safe and simple but you lose access to the original dynamic state. handler-bind, by contrast, runs its handler function in the exact dynamic environment where the condition was signalled, without unwinding — this is what makes restart invocation possible, because the stack frames that established the restarts are still live. As a rule of thumb, reach for handler-case when you just want to catch-and-recover like a normal exception, and handler-bind when you need to inspect state or invoke a restart before deciding to unwind.

A restart, established with restart-case or restart-bind, is a named recovery strategy that a signalling function offers to whoever ends up handling its condition — it does not decide how to recover, only what the available options are. Callers invoke one with invoke-restart, or use the built-ins use-value and store-value for the common pattern of substituting a bad value and retrying. This inversion of control is the condition system's signature idea: the low-level code that detects the problem publishes possible fixes, while the high-level code (or even a human at the interactive debugger) picks which one applies.