[racket] Confused by augment and inner

[Gregory Woodhouse]

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]

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

#lang racket

(define a%

  (class object%

    (super-new)

    (define/pubment (hello)

      (printf "Hello ")

      (inner 0)

      (printf "!~n "))))

(define b%

  (class a%

    (super-new)

    (define/augment (hello greg)

      ;Call super here?

      (printf " ~a ~n" greg))))

(send (new b%) hello "Bob")

-> Hello Bob!

Is this right?

Cheers, 

Stephen

--

 

--

Stephen De Gabrielle

stephen.d...@acm.org

Telephone +44 (0)20 85670911

Mobile        +44 (0)79 85189045

http://www.degabrielle.name/stephen

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[Stephen De Gabrielle]

wow - i got that wrong. 

check this example from the reference

http://docs.racket-lang.org/reference/createclass.html#(form._((lib._racket/private/class-internal..rkt)._augment))

#lang racket

(define buzzer%

  (class object%

    (super-new)

    (define/pubment (buzz)

      (displayln "bzzzt")

      (inner (void) buzz))))

(define loud-buzzer%

  (class buzzer%

    (super-new)

    (define (buzz)

      (displayln "BZZZZZZZZZT"))

    (augment buzz)))

(send (new buzzer%) buzz)

;bzzzt

(send (new loud-buzzer%) buzz)

;bzzzt

;BZZZZZZZZZT

s.

[Robby Findler]

Your understanding seems correct to me. You had a syntax error.

#lang racket

(define a%
(class object%
(super-new)
(define/pubment (hello)
(printf "Hello~n")

(inner 0 hello))))

(define b%
(class a%
(super-new)
(define/augment (hello)
;Call super here?
(printf ", world!~n"))))

(send (new b%) hello)

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