This page uses BNF notation. Non-terminal names are bracketed by enclosed in angle brackets. Meta symbols are ::= indicating the definition, square brackets indicating that what is within is optional, and the vertical bar indicating options or possible definitions of the non-terminal to the left of ::=. Uppercase non-terminal names indicate something that is not yet defined, or which is left undefined, or defined by hand-waving and/or verbiage. The proposed grammar is presented in pieces, hierarchically, with each piece as self-contained as possible, and further refined in subsequent sections.
The grammar used in the node pico engine can be perused at https://github.com/Picolab/pico-engine/blobtree/master/packages/krl-parser/src/grammar.ne, and is authoritative where it differs from the presentation here.
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Rulesets are identified by RID, which and must be unique per within each pico engine. A RID, like an identifier, can be made up of alphanumeric characters and the underscore character, but in addition can contain periods and dashes. We recommend using the Java convention of using reversing the reversed name of a domain that you control, followed by a name that is unique within your organization. For example, we use the RID io.picolabs.wrangler for one of our internal rulesets. Anyone in the world following this convention will never choose a RID that exactly matches this one. Someone at IBM, for example, might want to define a "wrangler" ruleset, but they would name it com.ibm.wrangler. Thus the convention makes globally unique RID choice easier.
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This block is optional, but is generally present. The global block is a sequence of expressions which are evaluate evaluated as the ruleset is installed into a pico. The value of each expression is then bound to a name (the identifier on the left hand side of the equal sign). Names defined in the global block are in scope from their point of definition throughout the ruleset. There is no syntax (nor is there run-time support) for modification of the value bound to these names. This is similar to final in Java.
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Finally a ruleset may contain a sequence of rule definitions. Each rule definition follows this grammar:
| Code Block | ||
|---|---|---|
| ||
<rule> ::= rule <ID> {
select when <eventex> [within <time expr>]
[foreach <EXPR> setting(<ID>[,<ID>])]
[pre { [<bindings>] }]
[[if <BOOL EXPR> then ]<action>]
[<postlude>]
}
<time expr> ::= <POS INT> <period>
<period> ::= seconds | minutes | hours | days |weeks |
A rule is identified by a simple identifier, which must be unique within the ruleset, and whose only use is to refer to the rule within the pico engine log files.
Each rule may react to incoming events. Which rules actually do is determined by the rule selector, which consists of an event expression (either simple or complex), subject to an optional time limit for all of the needed events to come in. The syntax of event expressions is described later on.
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If a prelude is present, it binds names to computed values. These names are in scope from where they are bound for the remainder of the rule.
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The action is the heart of a rule. An action has effects on things outside of the pico: other picos on this engine, other picos on other engines, other systems, and whatever system sent the original event.
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The postlude is where effects to inside this same pico are specified.
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The simplest form is simply two identifers, the event domain <ID 1> and the event type <ID 2>. Event attributes can be matched against regular expressions. All of the matches must succeed for the rule to be selected for evaluation. Portions of each attribute thus matched can be captured and named using the setting clause. The number of identifiers given should match the total number of captures in all of the regular expressions for the simple event expression. If fewer are given, extra capture groups will not be retained. If more are given, the extra names will be bound to the null value.
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While the action has effects on things outside of the pico, the postlude can change the state of the pico or add select additional rules to for the event evaluation currently in progress (see the Event Loop Introduction page for more information).
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