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From Henry Story <henry.st...@gmail.com>
Subject Re: [scala-user] Solution: taking context seriously was: A scala anti pattern -- looking for a name
Date Sun, 24 Jul 2011 10:02:36 GMT

Hi Lex, 

   thanks for your feedback. I think you must have missed a bit of the discussion, and are
lacking some of the context, which needs filling in a little both to remove the sting present
it is true in the discussion, but also to allow us to see what the discussion has accomplished
and why it was a fruitful one for the scala-dev list.

What brought me here was that I was asked by Reto to verify on the scala-dev mailing list
that 

  import ez1._
  b_("reto") -- FOAF.homepage --> "http://bblfish.net/".uri

  import ez2._
  b_("hjs") -- FOAF.homepage --> "http://bblfish.net/".uri

 was an anti-pattern [1].  We are two to be developing enthusiastically with Scala in  Clerezza,
so the problem is that if we have a disagreement of this kind it is very difficult to resolve
it there. It is true that I found it so evidently to be a problem, that I thought that this
was a bit disingenuous  to ask me to come to this list initially. 

Anyway, so I came to the list, to ask the question, and the answer was very clear: I don't
think there was any support for the proposed way of doing things from anyone here. 

But was that therefore supporting some supposed way I had of doing things? Was this an issue
of my patch versus his patch?
No, very clearly not. The solution I had developed a while ago [2] was clearly over-engineered,
and I am not suggesting to go back to that. The discussion on this list helped me look very
carefully at Reto's arguments for his solution above. And you will see me taking those very
seriously in quite a few of the e-mails to this list [3]. In fact what I took out of this
was exactly why my original solution was problematic. But as I will show, that part of the
solution is still present within the code that Reto proposed. 

The problem with "my" original code, is to put it as simply as possible is that you need the
context to cover the whole of the DSL. Ie. in my original solutions one would write

  (   graph.node a FOAF.Person
               -- name --> "Lex"
               -- knows --> ( graph.bnode("hjs") -- name --> "Henry Story")
               -- knows --> ( reto -- knows -- graph.bnode("hjs"))
  )

  
Notice that following the first  -- knows --> arrow one MUST have a reference back to the
original graph object
with the code graph.bnode("hjs") if one wants that to be used again in the last statement
of the DSL above -
the one where it is claimed that Reto knows the same person you do. 

It is in order to improve this that Reto rewrote the code I had written and came up with the
solution I have claimed
is an anti-pattern. (It relies much too strongly on implicits.) Reto solved the above very
cleverly by using subclassing
and an import statement [4] it works miracles so long as you have only one graph. What you
do is you import an implicit conversion from bnodes or Uris to a GraphNode backed by a instance
of a Graph. This then allows you to write the above as

  val graph = new EzMGraph()
  import graph._
  (   node a FOAF.Person
               -- name --> "Lex"
               -- knows --> ( bnode("hjs") -- name --> "Henry Story")
               -- knows --> ( reto -- knows -- bnode("hjs"))
  )

Which as you will see removes all the references back to the original graph in the DSL. (
No more graph.xxx )
How this works I explained in detail in the bug report  CLEREZZA-603

So I think I explained on this list very clearly why Reto's solution was so attractive for
him, and why to that extent it was an improvement on the code I had written, and indeed  this
is why I did not oppose it being introduced into the Clerezza subversion repository. 

But of course the problem is that it makes use of this anti pattern that we are discussing
here, and that is visible when you start using multiple graphs together, at which point that
notation becomes very confusing. (Now it took me some time to see spot this myself, btw.)

  Reto claims that we never need more than one graph - and I fundamentally disagree. A very
large part of the work I need to do in creating distributed social networks, is going to be
writing, comparing and playing with multiple graphs. In fact if the semantic web brings one
thing to us it brings this ability to work with multiple points of view, without needing to
assert anyone as the only truth [5]. So this is a very important tool that I wish to use,
and I believe it will be more and more important as Clerezza builds apps.

  So accepting the anti-pattern - or whatever you wish to call it - and accepting that Reto's
solution contains some interesting insights, you will notice that I was not banging on my
chest - as you say - trying to get my own old solution back in, but rather I sought out a
new solution that could satisfy both the point that Reto had made without requiring the anti-pattern.
This is the solution I developed in the part of this thread entitled "taking context seriously",
and which I have been developing on the github branch of clerezza I am working on at https://github.com/bblfish/clerezza/
in the zz-issues branch, and soon the bblfish branch. How does this end up looking? 

Well with the proposed changes [6] one just makes the context clear by writing

 new context {(   
     node a FOAF.Person
               -- name --> "Lex"
               -- knows --> ( bnode("hjs") -- name --> "Henry Story")
               -- knows --> ( reto -- knows -- bnode("hjs"))
  )}

Here the DSL is protected as an inner class by the context, so that one never moves out of
one graph into another by mistake, which would have many unclear consequences. I have been
using this on my github branch, and it does work very nicely on the whole. It also avoids
subclassing a graph object, which is dangerous as that tends to give code more access to the
internals of how a graph works that is relevant to the coder writing such triples.

Henry



[1] Reto wrote on the clerezza mailing list 
 "I think it's important to identify anti-pattern. Post a link here if you document the anti-pattern
you think you've discovered on the scala mailing list" 
[2] see the older version 1141579 of EzMGraph 
[3] My reply dated 21 July 2011 to Reto's 5 questions among others
[4]  version 1144633 of EzMGraph and version 1144633 of RichGraphNode
[5] See my mail on blogs.sun.com Beatnik: change your mind
[6] see the new context class I proposed on my branch in github

Social Web Architect
http://bblfish.net/


On 24 Jul 2011, at 06:58, Lex wrote on scala-dev:

> Does this really belong on the scala list? 

yes

> So your patch was not accepted, not a big deal. I

This was not about patches being accepted or not. 

> f you feel a strong disagreement with the
> author, just fork the project.

I have a branch on github, where I have been developing this.


> If you are unwilling to maintain the fork, then think about how much effort it takes
to start a project and
> keep it going. Maybe that will make you more respectful and
> professional about the subject.

It is a lot of work, that is why I am trying to make sure that Clerezza does the right thing.
I would rather the right thing were done without my having to discuss it, as I have a lot
of more
urgent things to do. But we are breaking new ground here, and we are discovering the power
of 
Scala - which one has to be careful not to abuse.

> Banging on the chest and pointing to your way of doing things as
> superior, while calling the original "anti-pattern" wont get you
> anywhere.

I don't think I pointed to my way of doing things as being better anywhere on this thread.
I just asked whether or not the solution was an anti-pattern. It is important to agree on
that,
because otherwise there is no reason to find a better solution.

This discussion helped us find a solution, so I think it is all to the good.





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