unCLog

I like conditions. I like them a lot. I like to use them to describe bad program states instead of strings. (e.g., (assert (ok-p state) nil 'it-is-not-ok-condition :state state) instead of (assert (ok-p state) nil "It is not ok")). I like them because you can easily test whether or not they happened programmatically and because using them helps to centralize error messages -- and thus gives me hope that I can make them consistent. There is a catch though: define-condition is a prolix form. Thus Metatilities has long had defcondition to make writing them a bit easier and yesterday I wrote a sibling macro (with the same name) to handle my most common case. The macro is simplicity itself:

(defmacro defcondition (name (&rest super-conditions) 
			slot-specs format &body args)
  (flet ((massage-slot (slot-spec)
	   (cond ((atom slot-spec)
		  `(,slot-spec 
		    :initarg ,(read-from-string (format nil ":~a" slot-spec))))
		 (t
		  slot-spec)))
	 (massage-format-arg (arg)
	   (cond ((atom arg)
		  `(slot-value condition ',arg)) 
		 (t
		  arg))))
    `(progn
       (export '(,name))
       (define-condition ,name ,super-conditions
	 ,(mapcar #'massage-slot slot-specs)
	 (:report (lambda (condition stream)
		    (format stream ,format 
			    ,@(mapcar #'massage-format-arg args))))))))

and this lets us write

(define-condition record-number-too-large-error
    (invalid-record-number-error)
  ((record-count :initarg :record-count))
  (:report 
   (lambda (condition stream)
     (format stream 
	     "Record number ~a is too large for this store. Store size is ~a."
	     (slot-value condition 'record-number)
	     (slot-value condition 'record-count)))))

(defcondition record-number-too-large-error
    (invalid-record-number-error)
    (record-count)
    "Record number ~a is too large for this store. Store size is ~a."
  record-number record-count)

(the record-number slot is inherited). It's not much but its enough to make writing simple condition like this almost as easy as writing an error message as string. Programming environments are (in part) about reducing impedance and about making it easy to do the right thing.

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