[rules-dev] else

[Edson Tirelli]

   Mark,

   The [] syntax for the labels will clash with the sequencing syntax we've
been discussing. Possibly {} or a unique separator:

{else1} A()

else1 := A()

else1 ?= A()

   Considering that Patterns can also take bindings, probably {} is more
distinct:

{else1} a : A()

   My vote:

when
    {else1} Person( name == "darth" ) // works on patterns
    A()
    {else2} B()
then
   ....
otherwise.else1
...
otherwise.else2
...
end

   Will we support unlabeled "else" as well?

when
    A() and B()
then
   ...
otherwise
   ...
end

   If so, what will be the semantics of it? What happens if an A() is
inserted but not B()? vice-versa? What happens if C() is inserted?

   Regarding inline "consequences", at the moment I am not really a fan of
it. I think it complicates the syntax unnecessarily at this point but I can
be convinced. The support to else by itself is a big step forward as you
know users frequently ask for that.

   My .02c.

   Edson


2011/8/18 Mark Proctor <mproctor at codehaus.org>

> We have been looking into designs around else, so here are our initial
> brain storming ideas, which aims at doing more than just else, but
> handling signal processing like situations. "else" is always triggerd by
> the failure of a left propagation. In effect an named "else" block is
> just another terminal node that will result in an activation on the
> agenda. It will have access to declarations prior to the failure of
> propagation in the network.
>
> // Possible syntaxes
> [name] ( CE+ ) // no symbol
> [name] | ( CE+ )
> [name] < ( CE+ )
>
> 1)
> when
>     [name1] < Person( name == "darth" ) // works on patterns
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> 2)
> when
>     $p : Person( )
>     [name1] < eval( $p.name == "darth" ) // works on evals
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> 3)
> when
>     [name1] < ( Person( name == "darth" ) and Address( city == "death
> star" ) // works on groups
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> This could actuall be extended to have inline "then" too. In this case
> when their is a success propagation on that node it will result in an
> activation placed on the agenda that has access to all the prior bound
> declarations.
>
> 1)
> when
>     Person( name == "darth" ) > [name1]  // works on patterns
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> 2)
> when
>     $p : Person( )
>     eval( $p.name == "darth" ) > [name1] // works on evals
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> 3)
> when
>    ( Person( name == "darth" ) and Address( city == "death star" ) >
> [name1]  // works on groups
>     A()
> then
>    ....
> then.name1
> ...
> end
>
> This can be used with 'or'
> when
>     ( A() > [a1] or
>       B() > [b1] or
>       C() > [c1] )
>    D()
> then
> ...
> then.a1
> ....
> then.b1
> ....
> then.c1
> ...
> end
>
> It's a little tricker but in theory we can do this before/afer the 'or' too
> This can be used with 'or'
> when
>     [x1] < ( A() > [a1] or
>                  B() > [b1] or
>                  C() > [c1] )
>                  D()
> then
> ...
> then.a1
> ....
> then.b1
> ....
> then.c1
> ...
> then.x1
> ....
> end
>
> We could allow [name] as just an inline creation to an activation that
> always passes, which with 'or' could provide a "default".
> when
>     [x1] < ( A() > [a1] or
>                  B() > [b1] or
>                  C() > [c1] or
>                   [default] )
>                  D()
> then
>
> Of course both could be supported at the same time
> [afailed] < A() > [asuccess]
>
>
> We could further allow just an inline code block, isntead of an inline
> reference to a block {...code here...} instead of [name1].
>
> We can also use this to do switch like operations, for erlang style
> signal processing, although i'd like to see an improvemet to the syntax
> here, just not sure what it would be...
> $o : Object() from stream
> ( A() > [a] from $o or
>   B() > [b] from $o or
>   C() > [c] from $o )
>
> Where as 'or' currently works like java's "|" single operator, i.e. all
> logical branches are tested. We could add a short cut or operationr
> 'sor' that would work like "||", so once the first CE matches in an 'or'
> block the rest are igored. We could even consider an 'xor' ....
>
> Finally there is no reason why we couldn't allow other CE's after the <.
> Which would provide for very rich signal processing. For instance. If
> A() fails, it'll propagate to B, if B() fails it'll activate [a1]
> [a1] < B() < A()
> This can be nested and using using parenthesis to show groupings.
> ( [a1] < B() > [b2] ) < A()
>
> Anyway more food for thought, enjoy :)
>
> Mark
>
>
>
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>



-- 
  Edson Tirelli
  JBoss Drools Core Development
  JBoss by Red Hat @ www.jboss.com
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