get-diplay-size' and get-display-left-top-inset' use #f for failure

Since the number of monitors can change at any time, reliable
use of these functions requires handling failure in some way.
Handling #f results is easier (and less likely to mask other
problems) than catching exceptions.

collects/mred/private/wx/cocoa/procs.rkt

@@ -89,7 +89,7 @@
 
 (define (check-for-break) #f)
 
-(define (display-origin xb yb all? num)
+(define (display-origin xb yb all? num fail)
   (if (or all? (positive? num))
       (unless (atomically
                (with-autorelease
@@ -108,13 +108,13 @@
                                                     (NSSize-height (NSRect-size f0)))))))
                         #t)
                       #f))))
-        (error 'get-display-left-top-inset "no such monitor: ~v" num))
+        (fail))
       (set-box! xb 0))
   (when (zero? num)
     (set-box! yb 0))
   (set-box! yb (+ (unbox yb) (get-menu-bar-height))))
 
-(define (display-size xb yb all? num)
+(define (display-size xb yb all? num fail)
   (unless (atomically
            (with-autorelease
             (let ([screens (tell NSScreen screens)])
@@ -134,7 +134,7 @@
                                               (get-menu-bar-height)]))))
                     #t)
                   #f))))
-    (error 'get-display-size "no such monitor: ~v" num)))
+    (fail)))
 
 
 (define (display-count)

collects/mred/private/wx/gtk/frame.rkt

@@ -265,7 +265,7 @@
                 (send p get-size sw-box sh-box)
                 (set-box! sx-box (send p get-x))
                 (set-box! sy-box (send p get-y)))
-              (display-size sw-box sh-box #t 0)))
+              (display-size sw-box sh-box #t 0 void)))
         (let* ([sw (unbox sw-box)]
                [sh (unbox sh-box)]
                [fw (unbox w-box)]
@@ -497,21 +497,21 @@
 (define-gdk gdk_screen_get_monitor_geometry (_fun _GdkScreen _int _GdkRectangle-pointer -> _void))
 (define-gdk gdk_screen_get_n_monitors (_fun _GdkScreen -> _int))
 
-(define (monitor-rect who num)
+(define (monitor-rect num fail)
   (let ([s (gdk_screen_get_default)]
 	[r (make-GdkRectangle 0 0 0 0)])
     (unless (num . < . (gdk_screen_get_n_monitors s))
-      (error who "no such monitor: ~v" num))
+      (fail))
     (gdk_screen_get_monitor_geometry s num r)
     r))
 
-(define (display-origin x y all? num)
+(define (display-origin x y all? num fail)
-  (let ([r (monitor-rect 'get-display-left-top-inset num)])
+  (let ([r (monitor-rect num fail)])
     (set-box! x (- (GdkRectangle-x r)))
     (set-box! y (- (GdkRectangle-y r)))))
 
-(define (display-size w h all? num)
+(define (display-size w h all? num fail)
-  (let ([r (monitor-rect 'get-display-size num)])
+  (let ([r (monitor-rect num fail)])
     (set-box! w (GdkRectangle-width r))
     (set-box! h (GdkRectangle-height r))))
 

collects/mred/private/wx/win32/frame.rkt

@@ -78,12 +78,12 @@
 			 #f)
     (reverse rects)))
 
-(define (display-size xb yb all? num)
+(define (display-size xb yb all? num fail)
   (cond
    [(positive? num)
     (let ([rs (get-all-screen-rects)])
       (unless (num . < . (length rs))
-	(error 'get-display-size "no such monitor: ~v" num))
+        (fail))
       (let ([r (list-ref rs num)])
 	(set-box! xb (- (caddr r) (car r)))
 	(set-box! yb (- (cadddr r) (cadr r)))))]
@@ -99,12 +99,12 @@
       (set-box! xb (- (RECT-right r) (RECT-left r)))
       (set-box! yb (- (RECT-bottom r) (RECT-top r))))]))
 
-(define (display-origin xb yb avoid-bars? num)
+(define (display-origin xb yb avoid-bars? num fail)
   (cond
    [(positive? num)
     (let ([rs (get-all-screen-rects)])
       (unless (num . < . (length rs))
-	(error 'get-display-left-top-inset "no such monitor: ~v" num))
+	(fail))
       (let ([r (list-ref rs num)])
 	(set-box! xb (- (car r)))
 	(set-box! yb (- (cadr r)))))]
@@ -435,7 +435,7 @@
 	  [wh (box 0)]
 	  [wx (box 0)]
 	  [wy (box 0)])
-      (display-size sw sh #f 0)
+      (display-size sw sh #f 0 void)
       (if wrt
 	  (begin
 	    (send wrt get-size ww wh)

collects/mred/private/wxtop.rkt

@@ -35,19 +35,22 @@
     (lambda ([full-screen? #f] #:monitor [monitor 0])
       (unless (exact-nonnegative-integer? monitor)
 	(raise-type-error 'get-display-size "exact non-negative integer" monitor))
+      (let/ec esc
         (let ([xb (box 0)]
               [yb (box 0)])
-	(wx:display-size xb yb full-screen? monitor)
+          (wx:display-size xb yb full-screen? monitor
-	(values (unbox xb) (unbox yb)))))
+                           (lambda () (esc #f #f)))
+          (values (unbox xb) (unbox yb))))))
 
   (define get-display-left-top-inset
     (lambda ([advisory? #f] #:monitor [monitor 0])
       (unless (exact-nonnegative-integer? monitor)
 	(raise-type-error 'get-display-left-top-inset "exact non-negative integer" monitor))
+      (let/ec esc
         (let ([xb (box 0)]
               [yb (box 0)])
-	(wx:display-origin xb yb advisory? monitor)
+          (wx:display-origin xb yb advisory? monitor (lambda () (esc #f #f)))
-	(values (unbox xb) (unbox yb)))))
+          (values (unbox xb) (unbox yb))))))
 
   (define get-display-count
     (lambda ()

collects/scribblings/gui/global-draw-funcs.scrbl

@@ -29,7 +29,8 @@ Returns the depth of the main display (a value of 1 denotes a monochrome display
 
 @defproc[(get-display-left-top-inset [avoid-bars? any/c #f]
                                      [#:monitor monitor exact-nonnegative-integer? 0])
-         (values exact-nonnegative-integer? exact-nonnegative-integer?)]{
+         (values (or/c exact-nonnegative-integer? #f)
+                 (or/c exact-nonnegative-integer? #f))]{
 
 When the optional argument is @racket[#f] (the default), this function
  returns the offset of @racket[monitor]'s origin from the
@@ -46,13 +47,15 @@ When the optional @racket[avoid-bars?] argument is true, for @racket[monitor]
  monitor @racket[0], the result is always @racket[0] and @racket[0].
  For monitors other than @racket[0], @racket[avoid-bars?] has no effect.
 
-If @racket[monitor] is not less than the current number of available monitors, the
+If @racket[monitor] is not less than the current number of available
- @racket[exn:fail] exception is raised.}
+ monitors (which can change at any time), the results are @racket[#f]
+ and @racket[#f].}
 
 
 @defproc[(get-display-size [full-screen? any/c #f]
                            [#:monitor monitor exact-nonnegative-integer? 0])
-         (values exact-nonnegative-integer? exact-nonnegative-integer?)]{
+         (values (or/c exact-nonnegative-integer? #f)
+                 (or/c exact-nonnegative-integer? #f))]{
 
 @index["screen resolution"]{Gets} the physical size of the specified @racket[monitor] in
  pixels.  On Windows, this size does not include the task bar by
@@ -62,8 +65,9 @@ If @racket[monitor] is not less than the current number of available monitors, t
 On Windows and Mac OS X, if the optional argument is true and @racket[monitor] is @racket[0], then
  the task bar, menu bar, and dock area are included in the result.
 
-If @racket[monitor] is not less than the current number of available monitors, the
+If @racket[monitor] is not less than the current number of available
- @racket[exn:fail] exception is raised.}
+ monitors (which can change at any time), the results are @racket[#f]
+ and @racket[#f].}
 
 
 @defproc[(is-color-display?)