A ruleset is the primary means of program organization in KRL.  A ruleset is a collection of rules. Each rule is designed to respond to a specific event. 

Rulesets are the primary means of organizing KRL programs. Modules and key repositories are simply rulesets that have been created using specific idioms. 

Each ruleset has a Ruleset ID (RID). The RID is an important identifier used when referring to your ruleset. For example, your ruleset's RID is used when importing that ruleset as a module. It is also used in the engine and displayed in debug logs.  

Each ruleset can have the following sections. 


KRL is a rule language which is evaluated by the pico engine within the context of a specific pico.  The pico engine accepts events for picos and executes corresponding rules to change the state of a pico. 

There are many things to know about KRL, in general, KRL is:

  • event-based rule language. 
  • executed in a pico engine
  • executed in a system where identity is pervasive; all events are raised on behalf of a specific entity for a specific pico.
  • pico-specific persistent storage; there is no need for external databases.

The result of these properties is a programming model that more closely resembles programming cloud-based persistent objects than anything else. We call these persistent computational objects "picos"

Rule Evaluation

The pico engine allows unique channel identifiers to be created for each pico.  Channels create http event URL's.  The pico engine accepts http events on event URL's and evaluates any installed rules which select on the accepted event. 

For a rule to be executed in a pico on an event, a ruleset must be

  • registered with the pico engine with a unique ruleset ID or RID
  • installed in the pico with the corresponding event channel 

Ruleset Structure

This is a more in depth ruleset example with in line comments.

ruleset rulesetExample{// "rulesetExample" is the RID, and must be unique in the engine
  //The meta block is where module declarations, ruleset names/descriptions, and logging declarations occur
    name "Rule Example"
    description <<
      An example of how to build a ruleset
    author "CS462 TA"

    //You can use other modules (a.k.a. rulesets) that have functions or defactions that you want to use in this ruleset.
    //To use a function/defaction in another ruleset, call the module: io.picolabs.pico:someProvidedFunction(<pass in params here>)
    //The optional alias keyword shortens the above function call to: wrangler:someFunction(<pass in params here>)
    use module io.picolabs.pico alias wrangler

    //to use more than one module, do not separate with commas. Simply add a new line:
    use module someRulesetName

    //shares <give function/defaction names separated by commas>
    //the "shares" keyword means that anyone can call the shared function on this pico, so long as they have an authorized channel
    shares foo

    //provides <give function/defaction names separated by commas>
    //the "provides" keyword allows other rulesets that use this one (rulesetExample) as a module to call
    //the stated functions/defactions.
    provides foo, defFoo

    //NOTICE: This ruleset provides, but does not share defFoo, which is a defaction. To clarify, this means that any
    //ruleset using this ruleset (rulesetExample) as a module may call it, but outside sources querying this pico
    //do NOT have access to it.

    //historicaly used to enable and disable logs printing to the terminal where pico engine is running. This may be depricated
    logging on
  } //end meta

  //The place to declare global variables and functions
    //functions can be used in pre-blocks or called by queries on authorized channels to this pico
    foo = function(someValue, anotherValue){//pass any arbitrary number of parameters by separating with commas
      aReturnValue = someValue + " " + anotherValue;//just do whatever processing you might want. Put semicolons on every statement execution except the return statement
      aReturnValue //the last expression in a function is what will be returned. It can be a map, array, string, number etc, but not a function

    //defactions can be called in action-blocks, and as such have slightly different syntax
    defFoo = defaction(someParam, anotherParam){
      //perform any needed pre-processing by simply writing code
      aProcessedBinding = someParam + " " + anotherParam;

      //then perform all action calls in an every block, including event:send(), send_directive() etc.
        //every action has a semicolon at the end of it save for the last action statement
        event:send(/* put params here */);
        send_directive("An Example Directive", { "processedValue": aProcessedBinding })
      //You can also return values from a defaction with an optional returns statement
      //put the value you want to return here... it could be an array, map, number, string etc.
      "A temporary placeholder value."

  } //end global

  //Then begin to declare rules
  //Rules are primarily made up of a name, select statement, a pre-block, an action-block, and a post-block
  rule exampleRule{//First: give the name
    //Second: the select statement tells the engine software when to run the following code. When events (NOT QUERIES)
    //are received, the engine matches up the domain name and type name with those given in the select statements of
    //rules on installed rulesets and queues them up to run. Rules in the same ruleset that select on the same domain and type
    //will be queued up from top going down. This garentees order of exacution within a single ruleset.
    select when exampleDomain exampleType

    //the pre-block is used for pre-processing, and is where you would want to call functions
      resultOfFunction = foo("Hello,", "World!")

    //the action-block performs defaction calls and other action calls, like send_directive() or event_send()
    //For the following, in place of "true", you can have any valid krl expression or boolean value. It follows
    //javascript truthy rules, meaning that any expression that evaluates to 0 will be false, other number values
    //will evaluate as true

    if true then //alternate ex: could use "if x > 5 then"
    every{//perform action calls separated by semicolons save for the last expression, just like in a defaction
      defFoo("Hello,", "World!") setting(returnedValue)//grab the result of a defaction using setting

    //the post-block is used to set entity variables and raise new events on this pico in response to what happened in this event
    //A fired block will run if an action was executed (if the conditional statement above was true). A fired block may
    //be chained with an else block, which will run if the condition failed. Alternative post-blocks include "always{}".
    //A common pitfall includes writing a fired block but leaving out an action block (action blocks are optional). This
    //will ALWAYS fail, since no action block is run, and therefore the fired block will not run. Use an always block instead.

      ent:someEntityVar := "Assign an entity variable like this!";
      raise exampleDomain event "exampleRuleSucceeded"
        attributes {};
      ent:someEntityVar := {};
      raise exampleDomain event "exampleRuleFailed"
        attributes event:attrs();

  }//end exampleRule