From: Philipp Meier <meier@...>
Date: 2002-09-13T18:52:13+09:00
Subject: Re: not grasping the method overloading/multi-dispatch thing


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On Fri, Sep 13, 2002 at 08:41:33AM +0900, Patrick May wrote:
> Philipp Meier <meier@meisterbohne.de> wrote in message news:<200209121311=
50.GA25625@o-matic.net>...
> Is this a good example of the types of dispatches you often do?
>=20
> Dispatching on type smells like there is a need for some polymorphic
> method.  For example:
>=20
> def description( build_target )
>     case build_target.class
>     when Compile
>         "compile libraries"
>     when InstallDependencies
>         "install dependencies"
>     else
>=20
>=20
>         "unknown target"
>     end
> end
>=20
> (I doubt that code in the wild would be this obvious)
>=20
> To me, the point of OO programming is to have one conditional that
> picks the type.  If you find conditionals on type downstream from your
> factories, then I think it's a sign of methods that should be on the
> classes, i.e. Compile.description, InstallDependencies.description,
> etc.

This generally true. I had the need for this kind of polymorphism when I
wrote some kind of generic Object to XML filter for the WebWork
framework. The general idea was to plug in adapter classes which
have the know how of converting a certain class to a meaningful xml
document. The pseudo-code of the adapter looked like this

class Adapter
	def adapt(node, object, attr_name)
		adapter =3D adapters.find(object.type)
		adapter.adapt(document, object, attr_name)
	end
end

class ObjectAdapter
	def adapter(adapter, node, object, attr_name)
		new_node =3D object.appendChild(attr_name)
		object.attributes.each do |a|
			adapter.adapt(new_node, object.send a, a)
		end
	end
end

class StringAdapter
	def adapter(adapter, node, value, attr_name)
		new_node =3D object.appendChild(attr_name)
		new_node.appendText(value)
	end
end

class ArrayAdapter
	def adaptObject(adapter, node, array, attr_name)
		new_node =3D node.appandChild(attr_name)
		array.each do |e|=20
			adapter.adaptObject(child_node, e, "item");
		end
	end
end

Here, i implemented the polymorphism in the method Adapter#find which
tries to find an Adapter that matches a specific type as possible.

-billy.

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