A JSON-based rule language

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Contents

The first Example

Rule: For all elements with class note having as first child a ul element, change the first child background color to blue. 

{
 "id":"rule101",
 "appliesTo": ["http://www.yahoo.com", "http://www.google.com/"],
 "condition":
   "
    $X:Element( class==’note’,$Y:firstChild)
    &&
   ($Y.nodeName == ’ul’)
   ",
 "actions":
   ["changeBackground($Y, ’blue’)"]
}

The Language

UML model of JSON Rule

The Condition Language

A rule condition is always a conjunction of atoms. Empty conditions are interpreted as true conditions.

  • JavaScriptBooleanCondition the simplest conditional atom. Any JavaScript boolean expression is allowed.
  • Description: related to the syntax of Drools pattern conditional.
  • XPathCondition

Atom types (Conditions)

JavaScriptBooleanCondition

  • the simplest conditional atom;
  • any JavaScript boolean expression is allowed.

Example: 

window.find(’rule’)
document.getElementById(’id’).value==10

Descriptions

  • related to the syntax of Drools pattern conditional;
  • offers two types of constraints:
    1. PropertyRestriction: describes a set of value restrictions to properties of the JSONTerm that are bound to it.
    2. PropertyBinding: performs a variable binding of a property belonging to the related JSONTerm.

Descriptions - UML Model

XPathCondition

Example:

Consider a HTML page that contains two tables:

T1:row 1,cell 1 T1:row 1,cell 2
T1:row 2,cell 1 T1:row 2,cell 2
T2:row 1,cell 1 T2:row 1,cell 2
T2:row 2,cell 1 T2:row 2,cell 2

A condition checking that the content of the first row of the second table is "T2:row1, cell 1", might be: 

{"nodeName":"tr",
  "firstChild":{"nodeName":"td",
                "textContent":"T2:row1, cell 1"
               }
} in "html//table//tr"

XPathCondition - UML Model

Actions

  • standard actions from OMG Production Rule Representation (PRR);
  • any user-defined JavaScript functions can be called in the rule actions part.
PRR Standard Actions JSON Rules
AssignExp change properties of an element
InvokeExp any JavaScript function call
AssertExp insert a DOM node
RetractExp remove a DOM node
UpdateExp update a DOM node

EventExpression

  • JSON event expressions are related to the Event interface specification in W3C DOM Level 2 Events
  • the properties of these expressions have the same meaning as in the W3C specification
  • properties of event expressions are matched against the incoming Dom events at the runtime. Their values can be processed in the rule conditions and actions.

EventExpression Example

Rule: When click event is raised, if the event cames from an a element, and if the href property matches a specific regular expression[1] then call append() with the message subject as parameter. 

{"id":"rule102",
  "appliesTo":["http://mail.yahoo.com/"],
  "eventExpression": { "eventType": "click",
                       "eventTarget": "$X"
                     },
  "condition":" ($X.nodeName == ’a’,
                 $X.href==’match(showMessage\?fid=Inbox)’)",
  "actions":["append($X.textContent)"]
}

EventExpression - UML Model


  1. ↑ rudimentary check that the link is an inbox Yahoo message link
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