Details on Boolean Expressions

Clara boolean expressions essentially provide a more concise way to represent logic that would otherwise require multiple rules. For example, the following are equivalent:

(defrule bad-weather-rule
 [Temperature (< temperature 20)]
 [:or [WindSpeed (> speed 20)]
      [Rainfall (> height 20)]]
 =>
 (insert! (->BadWeather)))

(defrule bad-weather-rule-1
 [Temperature (< temperature 20)]
 [WindSpeed (> speed 20)]
 =>
 (insert! (->BadWeather)))

(defrule bad-weather-rule-2
 [Temperature (< temperature 20)]
 [Rainfall (> height 20)]
 =>
 (insert! (->BadWeather)))

An important consequence of this equivalence is that an :or in a Clara boolean expression, unlike the Clojure(Script) “or” function, is not short-circuiting.

Similarly, the following are equivalent as well:

(defrule good-weather-rule
[Temperature (> temperature 30)]
[Sunny]
=>
(insert! (->GoodWeather)))

(defrule good-weather-rule
[:and
  [Temperature (> temperature 30)]
  [Sunny]]
=>
(insert! (->GoodWeather)))

A key point here is that the conditions in a Clara rule have an implicit :and. An explicit :and is useful when it is nested inside another boolean condition.

The file in clara-examples for boolean expressions contains some more examples: clara-examples boolean expressions

Facts for the preceding examples:

(defrecord Temperature [temperature])
(defrecord WindSpeed [speed])
(defrecord Rainfall [height])
(defrecord BadWeather [])
(defrecord Sunny [])
(defrecord GoodWeather [])