Re: namespaces, goops, etc.

[Michael Livshin]

"Lars J. Aas" <address@hidden> writes:

> On Fri, Nov 03, 2000 at 08:13:19PM +0200, Michael Livshin wrote:
> : "Lars J. Aas" <address@hidden> writes:
> : > Another thing I've noticed is that generic goops methods has to be defined
> : > in the toplevel namespace.  Was there a reason for this design decision, 
> other
> : > than perhaps making the goops module easier to implement (global 
> variables and
> : > stuff?)?  Just wondering - it's not a feature I miss or anything, 
> although I
> : > can imagine it could be useful with local generic methods in certain
> : > circumstances...
> : 
> : this design decision surely looks right to me.  except you are
> : speaking specifically about `define-method', because you can add and
> : remove methods anytime, by more "imperative" means.
> 
> I'm not sure I understand exactly how/what you mean.
> 
> Can I for instance create a generic function (with polymorphism) that I can 
> store
> in an alist and retrieve later to execute with some arguments?  I actually 
> found out
> I could really use that a few minutes ago...

oops, I took your words at face value without checking.  but I see
your problem now.

`define-method' is a magic syntax.  the magic is that when the generic 
function you specialize doesn't already exist, it's defined in place.

for some hairy implementational reasons the code emitted by
`define-method' for non-existing generic is not legal for placing in
any place other then top level.

so, this works:

(define-generic foo)

(define (hiho)
  ;; the generic already exists, so this works:
  (define-method foo ((x <top>)) x))

this doesn't work:

(define (hiho)
  ;; if no generic named "foo" exists by the time we are called for
  ;; the first time, there's trouble:
  (define-method foo ((x <top>) x)))

but the following works, but has different semantics (the semantics
you actually asked for):

(define (hiho)
  ;; define the generic locally:
  (define-generic foo)
  ;; now happily specialize it:
  (define-method foo ((x <top>) x)))

-- 
May all your PUSHes be POPped.