binding::= (type handler)

handler - a form; evaluated to produce a handler-function.

handler-function - a designator for a function of one argument.

forms - an implicit progn.

results - the values returned by the forms.

Each handler should evaluate to a handler-function, which is used to handle conditions of the given type during execution of the forms. This function should take a single argument, the condition being signaled.

If more than one handler binding is supplied, the handler bindings are searched sequentially from top to bottom in search of a match (by visual analogy with typecase). If an appropriate type is found, the associated handler is run in a dynamic environment where none of these handler bindings are visible (to avoid recursive errors). If the handler declines, the search continues for another handler.

If no appropriate handler is found, other handlers are sought from dynamically enclosing contours. If no handler is found outside, then signal returns or error enters the debugger.

 (handler-bind ((unbound-variable #'(lambda ...))
                (error #'(lambda ...)))
   ...)

If any other kind of error is signaled, the second function is called. In either case, neither handler is active while executing the code in the associated function.

 (defun trap-error-handler (condition)
   (format *error-output* "~&~A~&" condition)
   (throw 'trap-errors nil))

 (defmacro trap-errors (&rest forms)
   `(catch 'trap-errors
      (handler-bind ((error #'trap-error-handler))
        ,@forms)))
 
 (list (trap-errors (signal "Foo.") 1)
       (trap-errors (error  "Bar.") 2)
       (+ 1 2))
 Bar.
 (1 NIL 3)

Note that "Foo." is not printed because the condition made by signal is a simple condition, which is not of type error, so it doesn't trigger the handler for error set up by trap-errors.