handler-case

(handler-case expression (type ([var]) body...)... [(:no-error ([var]) body...)])

Evaluates expression; when an error is signaled during it, control transfers to the first clause whose condition type matches the signaled condition, with var (optional) bound to the condition object, and the clause body's value becomes the value of the whole form. When no clause matches, the error propagates outward (an enclosing handler-case may still catch it). The clause type is any typecase specifier, including condition classes defined by define-condition and the built-in hierarchy (condition > serious-condition > error, warning); an error signaled without a condition object (a plain (error "...") or a runtime failure inside a built-in) is caught as a simple-error whose slot 1 carries the message. The :no-error clause runs on normal completion with var bound to the (primary) value, outside the handler. Non-local exits (return/return-from) pass through uncaught, and an unwind-protect inside the expression runs its cleanup before the handler.

handler-case is supported on the interpreter and the JVM backend; the WASM compilers reject it (a WASM error is an uncatchable trap). Handlers are per thread of control, so concurrent rontolisp:http-handler requests do not interfere. Lite: handler-bind/restart-case restarts are not available (.todo/116 Phase 4).

Typed conditions dispatch through the class hierarchy, first matching clause wins: