From: Philipp Meier <meier@...>
Date: 2002-09-13T19:18:39+09:00
Subject: Re: not grasping the method overloading/multi-dispatch thing


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On Fri, Sep 13, 2002 at 02:22:25PM +0900, Patrick May wrote:
> Dale Martenson <dmartenson@multitech.com> wrote in message news:<20C86D26=
20F6D411A199009005DC0102016D1830@exchange serve.multitech.prv>...

> > 2. It allows for "cleaner" solutions where overloading is the "best" op=
tion.
> > It could be argued that overloading is needed to satisfy POLS. If Ruby =
is
> > truly the best OO language, how can it not have overloading!!  :-)
>=20
> Overloaded methods are the same as a conditional on type.  To me, this
> is never the best option, not when you can put methods on the class to
> convert it to the form you need.
>=20
> I'd be curious to see code that
>=20
> a. calls for overloading on type
> b. couldn't be handled by pushing code up to the classes of the
> argument objects

Because of Rubys dynamic nature this is IMHO always possible. But is it
good design? You will agree, that, as an example, code that formats a
business object for reporting certainly does not belong into the business
object class but in a reporting class. A solution I proposed before is
extracting this kind of methods into a seperate classes which go along
the classes you propose the methods belong into. The polymorphic
dispatching logic then dispatch by type (untested):

class PolyTest
	@@receivers =3D { :to_html =3D> { String =3D> StringAdapter,
		                      Integer =3D> IntegerToHTML },
	                :to_rtf  =3D> { String =3D> StringAdapter,
		                      Integer =3D> IntegerToRTF }
			}

	def dispatch(method, index, *args)
		r =3D @@receivers[method][args[index].type]
		r.send(method, self, *args)
	end

	def to_html(doc, object)
		dispatch(:to_html, 1, doc, object)
	end

	def to_rtf(doc, object)
		dispatch(:to_rtf, 1, doc, object)
	end

	def test
		to_html("", "Test") # =3D> calls StringAdapter.to_html("", "Test")
		to_rtf ("", "Test") # =3D> calls StringAdapter.to_rtf("", "Test")
		to_html("", 1)      # =3D> calls IntegerToHTML.to_html("", 1)
		to_rtf ("", 1)      # =3D> calls IntegerToRTF.to_rtf("", 1)
	end
end

class StringAdapter
	def to_html(doc, string)
		doc << "<b>String:</b> #{string}<br>"
	end
	def to_rtf(doc, string)
		...
	end
end

class IntegerToHTML
	def to_html(doc, i)
		doc << "<i>Integer:</i> #{i}<br>"
	end
end

class IntegerToRTF
	def to_rtf(doc, i)
		...
	end
end

-billy.

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