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Message from discussion Confused by augment and inner
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From: Gregory Woodhouse <gregwoodho...@me.com>
Message-id: <3B12168F-A5D1-401A-ADB0-0DA0211B5...@me.com>
Date: Sun, 23 Sep 2012 23:31:48 -0700
To: Racket Mailing List <us...@racket-lang.org>
Subject: [racket] Confused by augment and inner
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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.

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<html><head><meta http-equiv=3D"Content-Type" content=3D"text/html =
charset=3Dus-ascii"></head><body style=3D"word-wrap: break-word; =
-webkit-nbsp-mode: space; -webkit-line-break: after-white-space; ">I am =
trying to understand how augmentable methods (as described in section =
13.5 of The Guide) work.<div><br></div><div><span style=3D"font-family: =
serif; text-align: left; background-color: rgb(255, 255, 255); ">Between =
the extremes of allowing arbitrary overriding and disallowing overriding =
entirely, the class system also supports Beta-style&nbsp;</span><span =
style=3D"font-family: serif; text-align: left; background-color: =
rgb(255, 255, 255); font-style: italic; ">augmentable</span><span =
style=3D"font-family: serif; text-align: left; background-color: =
rgb(255, 255, 255); ">&nbsp;methods [</span><a =
href=3D"file:///Applications/Racket%20v5.3/doc/guide/doc-bibliography.html=
#(cite._.Goldberg04)" pltdoc=3D"x" style=3D"font-family: serif; =
text-align: left; background-color: rgb(255, 255, 255); =
">Goldberg04</a><span style=3D"font-family: serif; text-align: left; =
background-color: rgb(255, 255, 255); ">]. A method declared =
with&nbsp;</span><span class=3D"RktSym" style=3D"font-family: monospace; =
color: rgb(38, 38, 128); text-align: left; background-color: rgb(255, =
255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._pubment))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">pubment</a></span><span style=3D"font-family: serif; =
text-align: left; background-color: rgb(255, 255, 255); ">&nbsp;is =
like&nbsp;</span><span class=3D"RktSym" style=3D"font-family: monospace; =
color: rgb(38, 38, 128); text-align: left; background-color: rgb(255, =
255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._public))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">public</a></span><span style=3D"font-family: serif; text-align: =
left; background-color: rgb(255, 255, 255); ">, but the method cannot be =
overridden in subclasses; it can be augmented only. A&nbsp;</span><span =
class=3D"RktSym" style=3D"font-family: monospace; color: rgb(38, 38, =
128); text-align: left; background-color: rgb(255, 255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._pubment))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">pubment</a></span><span style=3D"font-family: serif; =
text-align: left; background-color: rgb(255, 255, 255); ">&nbsp;method =
must explicitly invoke an augmentation (if any) using&nbsp;</span><span =
class=3D"RktSym" style=3D"font-family: monospace; color: rgb(38, 38, =
128); text-align: left; background-color: rgb(255, 255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._inner))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">inner</a></span><span style=3D"font-family: serif; text-align: =
left; background-color: rgb(255, 255, 255); ">; a subclass augments the =
method using&nbsp;</span><span class=3D"RktSym" style=3D"font-family: =
monospace; color: rgb(38, 38, 128); text-align: left; background-color: =
rgb(255, 255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._augment))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">augment</a></span><span style=3D"font-family: serif; =
text-align: left; background-color: rgb(255, 255, 255); ">, instead =
of&nbsp;</span><span class=3D"RktSym" style=3D"font-family: monospace; =
color: rgb(38, 38, 128); text-align: left; background-color: rgb(255, =
255, 255); "><a =
href=3D"file:///Applications/Racket%20v5.3/doc/reference/createclass.html#=
(form._((lib._racket/private/class-internal..rkt)._override))" =
class=3D"RktStxLink" pltdoc=3D"x" style=3D"text-decoration: none; color: =
black; ">override</a></span><span style=3D"font-family: serif; =
text-align: left; background-color: rgb(255, 255, 255); =
">.</span><br><div><br></div><div><br></div><div>=46rom 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</div></div><div><br></div><div><div>#lang =
racket</div><div><br></div><div>(define a%</div><div>&nbsp; (class =
object%</div><div>&nbsp; &nbsp; (super-new)</div><div>&nbsp; &nbsp; =
(define/pubment (hello)</div><div>&nbsp; &nbsp; &nbsp; (printf =
"Hello~n")</div><div>&nbsp; &nbsp; &nbsp; (inner =
0))))</div><div><br></div><div>(define b%</div><div>&nbsp; (class =
a%</div><div>&nbsp; &nbsp; (super-new)</div><div>&nbsp; &nbsp; =
(define/augment (hello)</div><div>&nbsp; &nbsp; &nbsp; ;Call super =
here?</div><div>&nbsp; &nbsp; &nbsp; (printf ", =
world!~n"))))</div></div><div><br></div><div><br></div><div>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.</div></body></html>=

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