handle out-of-bounds writes nicely

canvas.lisp

@@ -1,6 +1,6 @@
 ;;;; canvas.lisp
 ;;;;
-;;;; Copyright (c) 2013 Robert Smith
+;;;; Copyright (c) 2013-2018 Robert Smith
 ;;;;
 ;;;; A simple notion of a "canvas" on which we can draw formulas.
 
@@ -28,10 +28,37 @@
   "Obtain the character (X, Y) in the canvas CANVAS."
   (aref (canvas-data canvas) y x))
 
+(defvar *error-on-out-of-bounds-write* nil
+  "Error when attempting to write out of bounds on a canvas.")
+
+(defvar *warn-on-out-of-bounds-write* t
+  "Warn when attempting to write out of bounds on a canvas.")
+
 (defun canvas-set (canvas x y new-data)
   "Set the character at (X, Y) in the canvas CANVAS to the value NEW-DATA."
-  (setf (aref (canvas-data canvas) y x)
-        new-data))
+  (destructuring-bind (width height) (canvas-dimensions canvas)
+    (cond
+      ((and (<= 0 x (1- width))
+            (<= 0 y (1- height)))
+       (setf (aref (canvas-data canvas) y x)
+             new-data))
+      (t
+       (cond
+         (*error-on-out-of-bounds-write*
+          (cerror "Ignore write."
+                  "Attempting to write ~S out of bounds at ~
+                  position (~D, ~D) for canvas ~A."
+                  new-data
+                  x
+                  y
+                  canvas))
+         (*warn-on-out-of-bounds-write*
+          (warn "Attempted to write ~S out of bounds at ~
+                 position (~D, ~D) for canvas ~A."
+                new-data
+                x
+                y
+                canvas)))))))
 
 (defsetf canvas-ref canvas-set)
 
@@ -55,21 +82,21 @@
     (terpri stream)
     (destructuring-bind (width height)
         (canvas-dimensions canvas)
-      (loop :initially (write-char #\+ stream) 
+      (loop :initially (write-char #\+ stream)
             :repeat width
             :do (write-char #\- stream)
             :finally (progn
                        (write-char #\+ stream)
                        (terpri stream)))
-      
+
       (dotimes (y height)
         (write-char #\| stream)
         (dotimes (x width)
           (write-char (canvas-ref canvas x y) stream))
         (write-char #\| stream)
         (terpri stream))
-      
-      (loop :initially (write-char #\+ stream) 
+
+      (loop :initially (write-char #\+ stream)
             :repeat width
             :do (write-char #\- stream)
             :finally (progn

package.lisp

@@ -7,11 +7,14 @@
 
   ;; canvas.lisp
   (:export
-   #:canvas
+   #:canvas                             ; TYPE/STRUCTURE
    #:canvasp
    #:make-canvas
    #:canvas-dimensions
-   #:canvas-ref)
+   #:*error-on-out-of-bounds-write*     ; VARIABLE
+   #:*warn-on-out-of-bounds-write*      ; VARIABLE
+   #:canvas-ref                         ; FUNCTION, SETF
+   )
 
   ;; boxes.lisp
   (:export