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From Daniel Fagerstrom <dani...@nada.kth.se>
Subject Re: Schema of block.xml
Date Tue, 29 Mar 2005 12:00:58 GMT
Reinhard Poetz wrote:

> Daniel Fagerstrom wrote:
>
>> Reinhard Poetz wrote:
>>
<snip/>

>> This is not just symmetry thinking, we are using a simple form of 
>> sitemap extesions in the webapps at my company, and polymorphism is a 
>> must. Question is if it should be done by default or be an option. I 
>> would suggest optional as you probably want to design what should be 
>> polymorph and that the overidable services is part of the (non 
>> existing) interaface.
>>
>>>  - services:
>>>    * components
>>
>>
>> A component "foo" is taken from the blocks own CM with "self:foo" or 
>> just "foo" if we have that behaviour as default. A component will be 
>> found polymorphically by "polymorph:foo". In the polymorph case where 
>> A extends B and B extends C, if "poymorph:foo" is used in C the block 
>> manager will first ask the CM in A for "foo" then the one in B and at 
>> last in C.
>>
>> For the case where A extends B,C,D,E, for "self:foo" in A the block 
>> manager will first ask the CM in A and if it is not there we must 
>> impose a search order on B,C,D and E, e.g. the order in which they 
>> are declared in the extensions list in block.xml for A.
>
>
> I'm not sure if we really need this. Sounds like FS to me but maybe 
> you can give
> an example where this would be prefered over delegation (blockA 
> requires blockB,
> blockC, blockD and blockE and if you need a component in blockA and 
> you don't know whether it is in blockB or blockC you have to call it 
> explicitly by
>
> comp = manager.getComponent("blockB:componentXYZ");
> if(comp == null) {
>   comp = manager.getComponent("blockC:component:XYZ");
> }
>
> This also makes it possible to use different orders and you don't 
> stick to a
> single order declared in an extensions list in block.xml.

Sure, that works well for *using* stuff from another block, but if you 
want to let the componentXYZ be part of what blockA exports you have to 
build and export a component that does what you describe above. 
Otherwise you don't get any polymorphism.

But if we step up from the technical details, the main reason that I 
want multiple inheritance is that I want to make it easy to build 
webapps by extending and partly overiding a couple of orthogonal blocks. 
When you build your webapp block it might extend e.g. a Forrest block 
for documentation structure and skinning, a Lenya block for CMS 
functionality a user handling block for user adminstration etc. If you 
use extension you get a default behaviour from the beginning and then 
you can a step at the time overide the resources you want to modify.

If you use a block instead of extending it, it is much more complicated 
to modify its behaviour. Take a look at Forrest, e.g. To make it 
extendible you have to use all kinds of global variables to simulate 
polymorphism. You must explicitly tell where it should search for 
different resources in your Forrest conf. As I have said before, I have 
practical experience of building webapps based on sitemap polymorphism 
and multiple inheritance, and it is much more convenient than having 
Forrest like conigurations for every block you want to extend.

<snip/>

>>> ------------------------------------------------------------------------------

>>>
>>> Improvement #3: block inheritance
>>>
>>> The third step is to allow blocks to extends other blocks.
>>>
>>> The idea is to be able to wrap a block with another one, creating an 
>>> 'overloading' mechanism similar to the one used by OOP inheritance 
>>> where methods are 'fall back' to the extended class if the extending 
>>> class doesn't implement them.
>>>
>>> Let us supposed we have the following block (very simple):
>>>
>>> block "A" implements "skin"
>>>
>>>          /stylesheets/changes2document.xslt
>>>          /stylesheets/faq2document.xslt
>>>          /stylesheets/document2html.xslt
>>>          /resources/logo.gif
>>>
>>> and let us suppose that we want to change the look and feel of that 
>>> block. The first two stylesheets provide simply a way to adapt from 
>>> more specific markup to the Document DTD. So, my block would need to 
>>> change only the last two resources 'document2html.xslt' and 'logo.gif'.
>>>
>>> The best solution is to allow my block to explicitly "extend" that 
>>> block and inherits the resources that it doesn't contain.
>>>
>>> block "B" extends block "A"
>>>
>>>          /stylesheets/document2html.xslt
>>>          /resources/logo.gif
>>>
>>> but then block B still is considered implementing behavior "skin" 
>>> because the rest is inherited.
>>>
>>> This mainly:
>>>
>>>     * reduces block development and maintanance costs because 
>>> changes and bugfixes are directly inherited by all the extending 
>>> blocks, thus allowing better SoC between the two groups mainaining 
>>> the different blocks
>>>
>>>     * easy customization: blocks can be adapted for personal 
>>> specific needs simply with a wrapper around and without the need to 
>>> repackaging.
>>
>>
>>
>> Ok, allready discussed that above. I think we need to be more 
>> explicit about what behaviour we want. If we just write:
>>
>> <map:transform src="stylesheets/document2html.xslt"/>
>>
>> that means normally the same as:
>>
>> <map:transform src="context:/stylesheets/document2html.xslt"/>
>>
>> and I don't think it is a good idea for isolation between block to be 
>> able to overide what is in the current context, only things that are 
>> exposed through the sitemap should IMO be overidable:
>>
>> <map:transform src="block:polymorph:/stylesheets/document2html.xslt"/>.
>
>
> I'm not sure if Stefano and you mean the same here. IIUC in Stefano's 
> example
> you have e.g. blockA and blockB. blockA implements the behaviour skin and
> extends blockB:
>
>
> <block id="http://cocoon.apache.org/blocks/A/1.0.0">
>  <extends>
>   <block id="http://cocoon.apache.org/interface/B/1.0.0"/>
>  </extends>
>  <implements>
>    <interface id="http://cocoon.apache.org/interface/skin/1.0"/>
>  </implements>
> </block>
>
> A sitemap snippet of blockA:
>
> <map:match pattern="stylesheets/document2html.xslt">
>   <map:read src="{0}"/>
> </map:match>
>
> A sitemap snippet of blockB:
>
> <map:match pattern="stylesheets/changes2.html.xslt">
>   <map:read src="{0}"/>
> </map:match>
> <map:match pattern="stylesheets/document2html.xslt">
>   <map:read src="{0}"/>
> </map:match>
>
>
> Another block, e.g. blockC now uses blockA for its styling:
>
> <block id="http://cocoon.apache.org/blocks/C/1.0.0">
>  <requirements>
>   <requires block="http://cocoon.apache.org/interface/skin/1.0"
>    default="http://cocoon.apache.org/blocks/A/1.0.0"
>    name="skin"/>
>  </requirements>
> </block>
>
> Here a sitemap snippet of blockC:
>
> <map:match pattern="myPipeline1">
>   <map:generate src="bla"/>
>   <map:transform src="blocks:skin:/stylesheets/document2html.xslt"/>
>   <map:serialize/>
> </map:match>
> <map:match pattern="myPipeline2">
>   <map:generate src="bla"/>
>   <map:transform src="blocks:skin:/stylesheets/changes2html.xslt"/>
>   <map:serialize/>
> </map:match>
>
> myPipeline1 gets the stylesheet from blockA and myPipeline2 from 
> blockB (no
> stylesheet available in blockA --> fallback to super block blockB)
>
> Does this make sense for you?

It does.

> How does block:polymorph apply in this usecase?

In your example block A extends block B, then in block B we can have a 
sitemap rule like:

<map:match pattern="**.html">
  <map:generate src="{1}.xml"/>
  <map:transform src="block:polymorph:/stylesheets/document2html.xslt"/>
  <map:serialize/>
</map:match>

The block:polymorphism makes that stylesheets/document2html.xslt is 
taken from A rather than B. Say that you use that stylesheet in several 
rules, then you change multiple behaviours by overriding it.


>>> ---------------------------------------------------------------------------------

>>>
>>>
>>> I'm not sure about multiple-block inheritance. For me it's some kind 
>>> of an anti-pattern but maybe I'm too Java-minded.
>>
>> Multiple inheritance is not an antipattern in itself. The idea that 
>> it should be is IMO mainly marketing BS from Sun to make the stupid 
>> idea of single implementation inheritance seem like a feature.
>>
>> But there are a number of antipattern that was popular in OO maybe 
>> 10-20 years ago that was based on multiple inheritance. But that is a 
>> different thing. People tended to model the world with classes 
>> instead of interfaces, and used deep an multiple inheritance that got 
>> a lot of garbage from all the to full featured base classes. Also as 
>> the extension relation in Java (and many other OO languages) is much 
>> "harder" than using through interface, extension was considered less 
>> flexible.
>>
>> But as discussed above we don't need extension to be as hardcoded as 
>> in OO languages. And also implementation inheritance is a rather 
>> natural and efficient way to put things together.
>
> I think that modelling applications using interfaces and implementing 
> them using
> delegation is better than extension ...

If you want to convince me I'm afraid that you have to tell why you 
think it as well ;)

Using extension as it happen to be implemented in C++ and Java can be an 
anti pattern under certain circumstances. But saying that something is 
an antipattern without giving a context is close to meaningsless. For 
blocks extension give us polymorphism and if we want to extend and 
override several different concern areas in our webapp, we need multiple 
inheritance.

If we instead using interfaces they are for the moment just an URI an 
maybe some documentation. And for the delegation, it means that you have 
to explicitly code the usage pattern for every block service.

> The question for blocks now is what
> multi-block inheritance really buys us.

Extend and override from multiple blocks based on polymorphism.

> As I said above I think we should go
> through all 3 block services (pipelines, components, flows) and describe
> scenarios and compare pros and cons of multi-block inheritance 
> compared to single-block inheritance.

The pros with multi-block inheritance is that your block can be vertical 
frameworks with default behaviour that you can modify by overiding 
certain pipeline rules, components and flow functions. The cons is that 
we have to spend some thinking to get the design right and to get the 
behaviour intuitive enough.

Single block inheritance is possibly good in the way that it is easy to 
see where you get things from and it is bad in that you have to do a lot 
of coding if you want to use several vertical frameworks that takes care 
of different concern areas in your webapp.

Frankly I can't see much advantages in single implementation inheritance 
at all. If delegation always is better we should only have interface 
inheritance and not even single implementation inheritance. And if 
implementation inheritance is good, and for webapps I'm quite certain 
that it is, it seem like a very arbitrary limitation to just allow for 
inheriting implementation for one concern area.

/Daniel


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