I agree that this doesn't look possible within a single class, but I
don't see any problem with the pattern (aside from being awkward to
write). As far as I can tell, it would make sense for `class` to
support new a case that fuses your `overment` plus `augride` steps.
Internally, that would create a new index for the method while wiring
`super` in subclasses directly to the superclass implementation.
Are there other useful variants that are not currently supported (at
least directly)? Why wasn't this one a blank space in the table of
possibilities that we sketched in the super+inner paper? I don't know.
At Sat, 17 Jul 2021 00:10:10 -0500, Alexis King wrote:
> Hello,
>
> While doing some OOP in Racket today, I found myself in a situation that
> would benefit from two seemingly contradictory things:
>
>1.
>
>I want to be able to override a superclass method, and I want to be
>certain that I get to handle the method before any of my subclasses do.
>This suggests I want to use inner.
>2.
>
>At the same time, I want my subclasses to be able to override this
>method, not augment it. If I call inner and my subclass calls super,
>control should jump to *my superclass*.
>
> In other words, I want to get a sort of “first try” at handling the method
> so that if I choose to, I can neglect to call my subclass’s implementation
> altogether. But if I decide *not* to handle it, then I want super-style
> dispatch to proceed as if my class were never there at all.
>
> At first, I thought this wasn’t possible using Racket’s class system, since
> if I override my superclass’s method using overment, the subclass
> necessarily cannot use super, violating requirement 2. Yet if I use override,
> I don’t get the “first try” I want, violating requirement 1. However, after
> some thought, I realized it’s possible if I’m willing to use *two* classes
> rather than one:
>
> (define my-superclass%
> (class object%
> (define/public (m x) `(foo ,x))
> (super-new)))
>
> (define my-class%
> (let ()
> (define-local-member-name super-m)
> (class (class my-superclass%
> (define/public (super-m x)
>(super m x))
> (define/overment (m x)
>(if (not x)
>'skip
>(inner (error "impossible") m x)))
> (super-new))
> (inherit super-m)
> (define/augride (m x)
> (super-m x))
> (super-new
>
> The trick here is twofold:
>
>1.
>
>First, I override m using overment, which ensures method dispatch will
>call my implementation first.
>2.
>
>Next, I augment my own implementation of m using augride, which makes
>the method overridable again in subclasses. To satisfy the other half of
>requirement 2, my augmentation calls my-superclass%’s implementation of m
>via a sort of secret “side channel,” kept private using
>define-local-member-name.
>
> Using this trick, subclasses of my-class% can still override m, and as long
> as x is non-#f, my sneaky interposition doesn’t seem to have any effect.
> But if x *is* #f, I can short-circuit the computation immediately:
>
> (define my-subclass%
> (class my-class%
> (define/override (m x)
> `(baz ,(super m x)))
> (super-new)))
>
> (define obj (new my-subclass%))
> (send obj m #t) ; => '(baz (foo #t))
> (send obj m #f) ; => 'skip
>
> I think this is kind of cute, since it makes it possible to effectively
> conditionally interpose on method dispatch. However, it’s rather awkward to
> write. This brings me to my question: is there any simpler way to do this?
> And are there any hidden gotchas to my technique?
>
> Thanks,
> Alexis
>
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