Re: Closures versus objects

Pascal Costanza wrote:
> 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."

And the end results of doing just that 'work' just fine.  Consider that 
Maple, Mathematica and MuPAD all have generic definitions of length and 
+, which are defined as in the above.

All those systems have a 1-level generic length function, and a 
fully-recursive generic length function, for the user to choose from.  + 
is done so that it adds up those pieces that are compatible, and returns 
the rest 'unevaluated'.  Actually, I am fairly sure that that is all 
really taken from Macsyma - which is Lisp par excellence!

Jacques