Common Lisp the Language, 2nd Edition

29.3.7. Named Restarts

In addition to anonymous restarts, one can have named restarts, which can be invoked by name from within code. As a trivial example, one could write

(restart-case (invoke-restart 'foo 3) 
  (foo (x) (+ x 1)))

to add 3 to 1, returning 4. This trivial example is conceptually analogous to writing:

(+ (catch 'something (throw 'something 3)) 1)

For a more realistic example, the code for the function symbol-value might signal an unbound variable error as follows:

(restart-case (error "The variable ~S is unbound." variable) 
  (continue () 
      :report 
        (lambda (s)     ;Argument s is a stream 
          (format s "Retry getting the value of ~S." variable)) 
    (symbol-value variable)) 
  (use-value (value) 
      :report 
        (lambda (s)     ;Argument s is a stream 
          (format s "Specify a value of ~S to use this time." 
                  variable)) 
    value) 
  (store-value (value) 
      :report 
        (lambda (s)     ;Argument s is a stream 
          (format s "Specify a value of ~S to store and use." 
                  variable)) 
    (setf (symbol-value variable) value) 
    value))

If this were part of the implementation of symbol-value, then it would be possible for users to write a variety of automatic handlers for unbound variable errors. For example, to make unbound variables evaluate to themselves, one might write

(handler-bind ((unbound-variable 
                 #'(lambda (c)     ;Argument c is a condition 
                     (when (find-restart 'use-value) 
                       (invoke-restart 'use-value 
                                       (cell-error-name c)))))) 
  body)