[racket] Confused by augment and inner

From: Stephen De Gabrielle (stephen.degabrielle at acm.org)
Date: Mon Sep 24 03:15:03 EDT 2012

The way I read it was the new function gets called instead of inner;

#lang racket

(define a%
  (class object%
    (define/pubment (hello)
      (printf "Hello ")
      (inner 0)
      (printf "!~n "))))

(define b%
  (class a%
    (define/augment (hello greg)
      ;Call super here?
      (printf " ~a ~n" greg))))

(send (new b%) hello "Bob")
-> Hello Bob!

Is this right?


On Monday, September 24, 2012, Gregory Woodhouse wrote:

> I am trying to understand how augmentable methods (as described in section
> 13.5 of The Guide) work.
> Between the extremes of allowing arbitrary overriding and disallowing
> overriding entirely, the class system also supports Beta-style augmentable methods
> [Goldberg04]. A method declared with pubment is like public, but the
> method cannot be overridden in subclasses; it can be augmented only. A
> pubment method must explicitly invoke an augmentation (if any) using inner;
> a subclass augments the method using augment, instead of override.
> From this I conclude that if, for example, I have a class a% and a
> subclass b% and I want to augment a method of a% in b% I need to call inner
> in the definition of the method in a%, not b%. Something like this
> #lang racket
> (define a%
>   (class object%
>     (super-new)
>     (define/pubment (hello)
>       (printf "Hello~n")
>       (inner 0))))
> (define b%
>   (class a%
>     (super-new)
>     (define/augment (hello)
>       ;Call super here?
>       (printf ", world!~n"))))
> but this gives me an error (not really a surprise). My assumption is that
> super should be used in the subclass when overriding a method, but if a
> method is to be augmented any potential code in an augmenting class should
> be invoked from the superclass and super is not used in the subclass.
> Obviously, I'm missing something.


Stephen De Gabrielle
stephen.degabrielle at acm.org
Telephone +44 (0)20 85670911
Mobile        +44 (0)79 85189045
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