[plt-scheme] inheritance of private fields?

Take a look at the paper at

  http://www.ccs.neu.edu/scheme/pubs/#aplas06-fff

It shows how to generate "private" names that you can share in a way  
that's analogous to protected in Java but you can choose the  
restriction policy via lexical scope.


On Feb 29, 2008, at 7:45 AM, Tom Sgouros wrote:

>
> Hello all:
>
> I want to define a general class with some private fields that can be
> used by subclass.  Here's a dumb example of what I thought would be  
> the
> right way to do this, but apparently isn't:
>
> (define size%
>   (class* object% ()
>     (public print-name print-age print-weight)
>
>     (init-field (name 'Fred) (bday 070661)))
>
>     (define age (calculate-age bday))
>     (define weight 0)
>
>     (define print-name (display name))
>     (define print-age (display age))
>     (define print-weight (display weight))
>
>     (super-new)))
>
> Now I want to be able to define a sub-class of this, so that age and
> weight are available to definitions in that sub-class.  I haven't been
> able to do that, so feel I must be missing something.
>
> In the example age is a calculated value that depends on other
> init-field values.  I'd make it public by putting it into the init- 
> field
> sequence, if there's a guarantee that the init-field entries are
> processed in order (that it's a let* and not a let).  I can't find any
> such guarantee in the documentation.  Failing that, what should I  
> do to
> make it available?
>
> The weight private field should have no default value, but is intended
> to be given one by the sub-class.  In my class-naif view of these
> things, I should really make size% an interface and force the sub- 
> class
> to fill in the weight.  But I can't seem to figure out how to do that,
> either.
>
> Can someone explain to me the proper way to deal with these  
> issues?  Any
> advice welcome.
>
> Many thanks,
>
>  -tom
>
> -- 
>  ------------------------
>  tomfool at as220 dot org
>  http://sgouros.com
>  http://whatcheer.net
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