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Re: Hash Surprises with Fixnum, #hash, and #eql?

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On Thu, Apr 7, 2011 at 4:49 PM, Phillip Gawlowski
<cmdjackryan@googlemail.com> wrote:
> On Thu, Apr 7, 2011 at 4:28 PM, Robert Klemme
> <shortcutter@googlemail.com> wrote:
>>
>> Well, even with technical inheritance ("kind of") sub often add state
>> (i.e. member variables) but do only restrict valid values of
>> superclass state if at all.  The cannot do otherwise because then
>> superclass methods may break.  Silly example: superclass holds an
>> index which must be >= 0.  All superclass methods use that index for
>> some kind of lookup.  Assuming a sub class would suddenly set that
>> value to -13 the superclass contract would be violated.  Now, if you
>> let Complex inherit from Real (trying to avoid "irrational" :-)) you
>> would add another field for imaginary part.  So far so good, but
>> method to_f would sometimes throw an exception in Complex which it
>> would never do in Real.  So suddenly Complex breaks Real's contract.
>
> But that's a failure of implementation, isn't it?

That too, but the root cause lies in the area of the incompatibility
of the design with the properties of numerical classes.

> If I were to implement my own class Complex, I'd have to deal with the
> edge-cases that my sub-class has and can produce.
>
> Thus, I either undefine #to_f, or redefine it so that it throws an
> Exception. the value of inheritance is, after all, generalization, so
> that I don't have to reimplement the wheel all the time, instead
> making the wheel bigger or smaller, as the implementation requires.
> That conversely also means that that more specialized sub-classes
> derived from a generic-er super-class, *has* to implement an interface
> that works, and works consistently.
>
> To stay with Complex as an example:
> #to_f would require an additional argument to work properly: Either
> convert the real, or the imaginary part into a Float, and so would
> anything derived from Complex, whatever that may be, if it has the
> same properties.

But we were talking about the case of Complex inheriting Float.  Your
arguments do not make sense with a Float class because there is no
imaginary part yet you would need them in order to be able to provide
a meaningful to_f in the subclass Complex.

>> That's why I prefer to look at inheritance as "is a" relationship:
>> after all OO is about better abstraction capabilities and to be able
>> to hide implementation details behind a clearly defined clean
>> interface.  If you let yourself get dragged too much into technical
>> issues chances are that the design comes out awful.  Only languages
>> which allow to inherit without publishing all features of the
>> inherited class (private inheritance e.g. in Eiffel) do not
>> necessarily suffer from these issues.  But then, inheritance is just
>> an implementation detail in such cases.
>
> But isn't it always?

If you treat it as such it is.  However then you using a powerful
feature for abstraction and modeling.

> Regarding technical issues: Design is a bit of an art; knowing when to
> stop abstracting is important. ;)

In my experience far too many people in our profession have the other
problem: they dive into details too fast and do not think on an
abstract level.  That's the reason why so much code I get to see has
issues.  And since design flaws are generally much more costly to
repair than mere technical issues I'd rather say people should learn
to _start_ abstracting. :-)

Thank you for the interesting discussion!

Cheers

robert


PS: I just read that there was another earthquake in Japan and there
is a tsunami warning.  I hope the best for everybody in that area and
I hope these catastrophes end rather sooner than later.

-- 
remember.guy do |as, often| as.you_can - without end
http://blog.rubybestpractices.com/

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Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-07 14:01 +1000
  Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 04:19 -0500
    Re: Hash Surprises with Fixnum, #hash, and #eql? Phillip Gawlowski <cmdjackryan@googlemail.com> - 2011-04-07 06:20 -0500
      Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 06:53 -0500
        Re: Hash Surprises with Fixnum, #hash, and #eql? Phillip Gawlowski <cmdjackryan@googlemail.com> - 2011-04-07 08:21 -0500
          Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 08:43 -0500
            Re: Hash Surprises with Fixnum, #hash, and #eql? Phillip Gawlowski <cmdjackryan@googlemail.com> - 2011-04-07 09:02 -0500
              Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 09:28 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Phillip Gawlowski <cmdjackryan@googlemail.com> - 2011-04-07 09:49 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-07 10:11 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Phillip Gawlowski <cmdjackryan@googlemail.com> - 2011-04-07 10:31 -0500
              Re: Hash Surprises with Fixnum, #hash, and #eql? Brian Candler <b.candler@pobox.com> - 2011-04-07 11:06 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Vincent Manis <vmanis@telus.net> - 2011-04-07 17:42 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-08 01:55 -0500
    Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-08 17:29 +1000
      Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-08 05:12 -0500
        Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-09 11:21 +1000
          Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-10 19:02 -0500
            Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-11 13:17 +1000
              Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-12 04:09 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-13 12:02 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-13 00:51 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-13 16:32 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-13 07:23 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-14 07:38 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-14 00:36 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-14 15:55 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-19 17:41 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-21 12:49 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-21 06:28 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-21 22:32 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-21 08:51 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? John W Higgins <wishdev@gmail.com> - 2011-04-21 11:11 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-27 00:33 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-27 01:44 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-27 16:22 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-04-27 00:23 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-30 17:43 +1000
                Re: Hash Surprises with Fixnum, #hash, and #eql? Charles Oliver Nutter <headius@headius.com> - 2011-05-01 23:01 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Robert Klemme <shortcutter@googlemail.com> - 2011-04-13 02:08 -0500
                Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-14 07:36 +1000
      Re: Hash Surprises with Fixnum, #hash, and #eql? Xavier Noria <fxn@hashref.com> - 2011-04-08 19:00 -0500
        Re: Hash Surprises with Fixnum, #hash, and #eql? Clifford Heath <no@spam.please.net> - 2011-04-09 11:22 +1000
  Re: Hash Surprises with Fixnum, #hash, and #eql? Mazi Ayışığı <mazi.ayisigi@gmail.com> - 2011-04-07 17:16 -0700

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