[rules-dev] RFC: declaration syntax

[Edson Tirelli]

   All,

BACKGROUND:
   As Drools moves forward, we need to be able to declare several new
structures, as well as make existing structures more robust for compile time
analysis, error reporting and tooling support. For instance, as it is today,
entry points are implicit declared by simply using them, i.e., if someone
writes a pattern:

X() from entry-point Y

   The engine will create entry-point Y implicitly. This is easy to use, but
this makes it impossible for the engine to detect typos and report during
compilation time. So, the idea is that we will allow the user to declare all
the entry points he wants to use in his application and we will have a
configuration option to enforce or not that list of declared entry points.
If the configuration is set to enforce and the engine finds a rule that is
using an entry point that is not part of the list, the engine can raise a
compile time error point out the user's mistake. E.g., using pseudo code:

declare entry-point X, Y
declare entry-point Z

   Drools also needs a way to declare named windows for re-use on multiple
rules. For instance, using pseudo code:

declare window X
    @type( tumbling )
    @keep( last 10 )
    StockTick( symbol == "RHT" ) from entry-point Y
end

  And in the rule:

rule Z
when
    StockTick() from window:named X
then
    ...
end

QUESTION:
   As we can see above, we will need several new constructs in order to
support features in our roadmap. We have the choice of making them all top
level constructs, making the error recovery in the parser a lot more
complicated, or we can re-use our declare construct to define these new
structures as I presented in the pseudo-code above. Any pros/cons you can
see in such approach?

OPTION 1 (more verbose, but simpler and more stable):
<epDeclaration> ::= declare entry-point <epName> [, <epName]*
<windowDeclaration> ::= declare window <windowName>
<restOfTheWindowDeclaration> end
<typeDeclaration> ::= declare [type] <typeName> <restOfTheTypeDeclaration>
end

OPTION 2 (makes error recovery harder, but it is less verbose):
<epDeclaration> ::= entry-point <epName> [, <epName]*
<windowDeclaration> ::= window <windowName> <restOfTheWindowDeclaration> end
<typeDeclaration> ::= declare <typeName> <restOfTheTypeDeclaration> end

OPTION 3:
<epDeclaration> ::= someOtherKeyword entry-point <epName> [, <epName]*
<windowDeclaration> ::= someOtherKeyword window <windowName>
<restOfTheWindowDeclaration> end
<typeDeclaration> ::= declare [type] <typeName> <restOfTheTypeDeclaration>
end


   Edson

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