How slots can contribute named values and simplify implications

Support for slots has been introduced in NxBRE v2.5. Since the release of patch for bug 1470721 , NxBRE has gain the capacity to have slots contribute named values in an implication body (as requested by RFE 1483072 ).

This works for fixed value individuals like:

   <Ind>min(150000,EUR)</Ind>

or dynamic ones like:

   <Ind uri="nxbre://expression">{ind}.StartsWith("hello")</Ind>
   <Ind uri="nxbre://operator">GreaterThan(25)</Ind>
   <Ind uri="nxbre://binder">isStartingWith(hello)</Ind>

How it was before

Let's take a simple rule as an example: "If a measure has a value greater than 25, a warning showing the value must be emitted".

Our immediate approach would be to filter with an operator on the individual, like this:

   <Implies>
      <Atom>
         <Rel>measure</Rel>
         <Ind uri="nxbre://operator">GreaterThan(25)</Ind>
       </Atom>
       <Atom>
          <Rel>warning</Rel>
          <Var>what can we possibly use here?</Var>
        </Atom>
    </Implies>

And the answer to the question above is: nothing! The atom in the body part correctly selects matching facts but do not produce any named value that the head part could reference in a variable.

So, the classical workaround consisted in bringing all the facts and compare them to the threshold with a function-based atom:

   <Implies>
       <And>
         <Atom>
           <Rel>measure</Rel>
           <Var>value</Var>
         </Atom>
         <Atom>
           <Rel uri="nxbre://operator">GreaterThan</Rel>
           <Var>value</Var>
           <Data xsi:type="xs:int">25</Data>
         </Atom>
       <And>
       <Atom>
          <Rel>warning</Rel>
          <Var>value</Var>
        </Atom>
    </Implies>

This was functional but complex and inefficient.

How it is now

Using a slot to name the individual used for selecting the facts allows the following syntax:

   <Implies>
      <Atom>
         <Rel>measure</Rel>
         <slot>
           <Ind>value</Ind>
           <Ind uri="nxbre://operator">GreaterThan(25)</Ind>
         </slot>
       </Atom>
       <Atom>
          <Rel>warning</Rel>
          <Var>value</Var>
        </Atom>
    </Implies>

Neat and efficient!

It also works in expression

Sloted values are also available in expressions like variables:

    <Implies>
        <Atom>
          <Rel>flight</Rel>
          <Var>customer</Var>
          <slot>
            <Ind>distance</Ind>
            <Ind uri="nxbre://operator">GreaterThan(999)</Ind>
          </slot>
        </Atom>
        <Atom>
          <Rel>miles</Rel>
          <Var>customer</Var>
          <Ind uri="nxbre://expression">{var:distance}/10</Ind>
        </Atom>
    </Implies>

Note that variables supersede slots, i.e. if in the body part of an atom both a variable and a slot have the same name, the variable will contribute values instead of the slot. Of course, having variables or slots with the same name is a pretty bad idea''