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 > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email > to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/CAA8dsae1HR2CVUNq3aC4nc2vpPU6L9f- > ENor-074LyZnWXgKYg%40mail.gmail.com. -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/20210719100827.35e%40sirmail.smtps.cs.utah.edu.
