— procedure: with-simple-restart name reporter thunk

Invokes thunk in a dynamic environment created by adding a restart named name to the existing named restarts. Reporter may be used during the execution of thunk to produce a description of the newly created restart; it must either be a procedure of one argument (a port) or a string. By convention, the description generated by reporter should be a short complete sentence, with first word capitalized and terminated by a period. The sentence should fit on one line with a little room to spare (see the examples below); usually this means that the sentence should be 70 characters or less in length.

If the restart created by with-simple-restart is invoked it simply aborts the computation in progress by returning an unspecified value from the call to with-simple-restart. Otherwise with-simple-restart returns the value computed by thunk.

     (with-simple-restart 'george "This restart is named george."
       (lambda () 3)) => 3
     
     (with-simple-restart 'george "This restart is named george."
       (lambda ()
         (invoke-restart (find-restart 'george)))) => unspecific
     
     (with-simple-restart 'george "This restart is named george."
       (lambda () (car 3)))
     ;The object 3, passed as the first argument to car,
     ; is not the correct type.
     ;To continue, call RESTART with an option number:
     ; (RESTART 3) => Specify an argument to use in its place.
     ; (RESTART 2) => This restart is named george.
     ; (RESTART 1) => Return to read-eval-print level 1.

— procedure: with-restart name reporter effector interactor thunk

Invokes thunk in a dynamic environment created by adding a restart named name to the existing named restarts. Reporter may be used during the execution of thunk to produce a description of the newly created restart; it must either be a procedure of one argument (a port) or a string. Effector is a procedure which will be called when the restart is invoked by invoke-restart. Interactor specifies the arguments that are to be passed to effector when it is invoked interactively; it may be either a procedure of no arguments, or #f. If interactor is #f, this restart is not meant to be invoked interactively.

The value returned by with-restart is the value returned by thunk. Should the restart be invoked by a condition handler, however, the effector will not return back to the handler that invoked it. Instead, the effector should call a continuation created before the condition-signalling process began, and with-restart will therefore not return in the normal manner.

     (define (by-george! thunk)
       ; This code handles conditions that arise while executing thunk
       ; by invoking the GEORGE restart, passing 1 and 2 to the restart's
       ; effector code.
       (bind-condition-handler '() ; All conditions
        (lambda (condition)
          (invoke-restart (find-restart 'george) 1 2))
        thunk))
     
     (define (can-george! thunk)
       ; This code provides a way of handling errors: the GEORGE restart.
       ; In order to GEORGE you must supply two values.
       (lambda ()
         (call-with-current-continuation
          (lambda (kappa)
            (with-restart
             'george                         ; Name
             "This restart is named george." ; Reporter
             (lambda (a b)                   ; Effector
               (kappa (list 'george a b)))
             values                          ; Interactor
             thunk)))))                      ; Thunk
     
     (by-george! (can-george! (lambda () -3))        => -3
     (by-george! (can-george! (lambda () (car 'x)))) => (george 1 2)