Re: Closures versus objects

On Feb 20, 2007, at 2:49 AM, Pascal Costanza wrote:
> On 20 Feb 2007, at 04:58, Dave Roberts wrote:
>> Which brings up a good point. Given this observation, why wasn't  
>> LENGTH
>> made a GF when CL was specified? Was CLOS such a late addition and
>> everybody was afraid of the possible performance impact? Surely,  
>> there
>> was not technical reason that it couldn't have been made generic,  
>> as you
>> say; all old code would have continued working, no?
>>
>> Over the past couple years, I have really learned to appreciate  
>> CLOS and
>> this is one of those examples where it seems like a good idea wasn't
>> taken far enough.
>
> Here is a quote from http://groups.google.com/group/comp.lang.lisp/ 
> msg/3cad948789e116e9
>
> "The reason many operators are not generic [in Common Lisp] is that  
> deciding how to do
> them generically is tricky and people disagree on the definition. I
> recall the NIL project (JonL White, Jonathan Rees, and Rick Bryan) ran
> afoul of a generic definition for LENGTH enough that let us worry.  
> Consider:
>  (defmethod length ((x cons)) (+ 1 (length (cdr x))))
>  (defmethod length ((x null)) 0)
>  (defmethod length ((x string)) (array-dimension x 0))
> Now think about
>  (length '(a b . "foo"))
> The point is that often independent definitions invite what I'll  
> call the
> "modularity problem" where definitions look ok in isolation but  
> surprising
> effects happen in combination.  Similar to what
>  (+ 1 2 "foo")
> might do if we allowed + to be generic."

This seems like a bit of a strange argument. Consider that length, as  
currently defined, does indeeed work on lists and strings. An obvious  
implementation not using generic functions would be:

(defun length (x)
  (typecase x
   (null 0)
   (cons (+ 1 (length (cdr x))))
   (string (array-dimension x 0))))

which, of course, has the exact same behavior as the generic length  
above. That choice of behavior is simply a choice in specification,  
and really has very little to do with whether you use a generic  
function or not.

James