SETF ; I think

[John G. Aspinall <JGA at MIT-MC>]

I'm sorry that I don't have a simpler example of this.

(load '((jga) ht))

this will load other files - notably JGA;PLOT, JGA;HOUSPL and JGA;GDEFS.
If none of those files are compiled, it runs fine - ignore the
wierd characters output to the tty - those will be understood by the
plotter this will eventually speak to.

However if JGA;PLOT and JGA;GDEFS are compiled, the same code bombs
out.  Some poking around reveals the problem happens in the function
houst:move but in a pretty strange manner.

(defun houst:move (x y)
  (declare (fixnum x y) (special current-tty-state current-viewport))
  (let ((dx (- x (view-pen-x)))
	(dy (- y (view-pen-y))))
    (declare (fixnum dx dy))
    (let ((adx (abs dx))
	  (ady (abs dy))
	  (sign-x (if (minusp dx) 40 100))
	  (sign-y (if (minusp dy) 40 100)))
      (declare (fixnum adx ady sign-x sign-y))
      (set-pen-position #\HOUST-PENUP)
      (mapc #'(lambda (vector)
		(set-vector-repeat (car vector))
		(+tyo (+ (cadr vector) sign-x) graphic-output)
		(+tyo (+ (caddr vector) sign-y) graphic-output))
	    (what-to-move adx
			  ady
			  (houst-x-step-size)
			  (houst-y-step-size)
			  (houst-max-multiplier)))))
  (setf (view-pen-x) x)
  (setf (view-pen-y) y)
  T)

When evaluating the setf at the end - (setf (view-pen-x) x) it finds x
is bound to the symbol STRUCT=INFO.  This symbol appears nowhere in
any of my code, however it does appear in the EXTSTR package.  The
setf and the structure reference (view-pen-x) expand correctly into
(store (arraycall fixnum ...)...) but x is now bound to this clearly
out-of-place symbol.  Furthermore, this happened sometime during the
evaluation of this function, because x is used at the beginning of the
let form that starts the function, and this does not give an error.

This looks as if it might be a missing comma, or something similar,
but I have exhausted the obvious avenues of approach.

John.