16.4.1 Establishing Restart Code

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)