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Alex Dixon edited this page Jul 28, 2017 · 1 revision

Table of Contents

Quick examples

Rules

(rule clear-cart-on-command ; <-- rule name
  {:group :action} ; <-- salience group (optional)
  [[_ :clear-cart]] ; <-- tuple expression with ignored first slot
                    ;     and value slot omitted
  [?item <- [_ :cart-item/product-id]] ; <-- expr. with fact binding (?item)
  => ; <-- separator
  (retract! ?item)) ; <-- right-hand side / consequence
(rule add-item-to-cart-when-not-in-cart-on-command
  {:group :action}
  [[_ :add-to-cart ?product-id]] ; <-- variable binding in value slot
  [:not [_ :cart-item/product-id ?product-id]] ; <-- boolean operator
  =>
 (api/add-to-cart {:db/id (random-uuid) ; <-- entity map
                   :cart-item/product-id ?product-id
                   :cart-item/quantity 1}))

Facts

Vector representation:

[[id :cart-item/product-id 123]
 [id :cart-item/quantity 1]])

Map representation:

{:db/id id
 :cart-item/product-id 123
 :cart-item/quantity 1}

Record representation (internal):

[(->Tuple id :cart-item/product-id 123 1)
 (->Tuple id :cart-item/quantity 1 2)]

Specification

Variables

?<symbol-name>

  • First reference: binds a value to the variable
  • Subsequent references: variable contains the value

Example:

; ?e binds to eid of a fact with attribute :color, value "blue"
[[?e :color "blue"]] 
; ?e has been assigned a value. Expression uses the value 
; captured from the previous expression when finding a match
[[?e :shape "circle"]]

Special forms and symbols

_ : symbol

[[_ keyword? ?any]]

Ignores e or v slot within a tuple expression. May not be used to ignore an attribute.

<- : symbol

[<variable> <- <expression>]

Binds result of expression to variable.

Usage:

[?my-color-fact <- [_ :color "blue"]]
[?all-color-facts <- (acc/all) :from [_ :color]]

<- : macro

[(<- <variable> <special-form>)]

Binds a variable to the result of a special form

Usage:

(:require-macros [precept.dsl :refer [<- entity]])
...
[[?e :color "blue"]]
[(<- ?blue-entity (entity ?e)]]

entity : macro

[(<- <unbound-variable> (entity <bound-variable=eid>))]

Accumulates all facts for a given entity id.

(:require-macros [precept.dsl :refer [<- entity]])
...
[[?e :color "blue"]]
[(<- ?blue-entity (entity ?e)]]

entities : macro

[(<- <unbound-variable> (entities <bound-variable=[eids]>))]

Accumulates all facts from a collection of entity ids.

(:require-macros [precept.dsl :refer [<- entities]])
...
[?eids <- (acc/all :e) :from [_ :color "blue"]]
[(<- ?blue-entities (entities ?eids)]]

Expressions

Tuple expression

Positionally match all or some of a fact's fields.

EAV

[any? keyword? any?]

Attribute only

[keyword?] | [_ keyword?]

Value omitted

[any? keyword?] | [_ keyword?]

Fact binding expression

[<unbound-variable> <- <tuple-expression | accumulator-expression>]

Binds a fact to a variable. Binds a list of facts if expression is an accumulator.

Example:

[?my-fact <- [_ :color "blue"]]

:test expression

[:test <predicate>]

Test expressions may be used to specify an arbitrary predicate as part of the rule's condition.

Example:

[[_ :high-temp/yesterday ?x]]
[[_ :high-temp/today ?y]]
[[_ :high-temp/tomorrow ?z]]
[:test (> (max ?x ?y ?z) 100)]

S-expression predicates

Arbitrary s-expressions predicates may be used in the value slot.

Examples:

[[?e :number (> 42 ?num)]]
(def magic-number 42)
(defn my-func [x] 
  (and (number? x)
       (> (+ 1 x) magic-number)))
...
[[?e :number (my-func ?num)]]

Accumulator expression

[<variable> <- <accumulator-function> :from <tuple-expression>]

See precept.accumulators namespace for list of predefined accumulator functions.

Example:

[?names-of-colors <- (acc/all :v) :from [_ :color]]

Boolean operations

:or

[:or <expression> <expression> ...]

Logical or.

May nest or be nested within other boolean expressions.

:and

[:and <expression> <expression> ...]

Logical and.

May nest or be nested within other boolean expressions.

Because expressions contain an implicit logical and, reserve for cases where a logical and must be explicit.

Usage:

[:or [:and [_ :color "blue"]
           [_ :shape "circle"]]
     [:and [_ :color "yellow"]           
           [_ :shape "triangle"]]]

:not

[:not <expression>]

Logical not.

May nest or be nested within other boolean expressions.

Usage:

[[?e :shape "circle"]]
[:not [?e :color "blue"]]
[:not [:and [_ :color "blue"]
            [_ :shape "circle"]]]

:exists

[:exists <expression>]

True if the wrapped expression has at least one match.

Usage:

[[?e :shape ?triangle]]
[:exists [?e :color "blue"]]

Inserting facts

insert!

(insert! <entity-vector | entity-map | Tuple>)

Argument may be a collection of one supported type.

For use in the consequence block of a rule only.

Equivalent of insert logical. Inserts facts that exist so long as the conditions under which they were inserted are true.

insert-unconditional!

(insert-unconditional! <entity-vector | entity-map | Tuple>)

Argument may be a collection of one supported type.

For use in the consequence block of a rule only.

Inserts facts that exist until they are manually removed.

insert

(insert! <session> <entity-vector | entity-map | Tuple>)

For use outside of a consequence block.

Inserts facts that exist until they are manually removed.

Takes a session as first argument. Second argument may be a collection of one supported type.

Retracting facts

retract!

(retract! <Tuple>)

Removes fact from session.

For use in the consequence block of a rule only.

Argument may be a collection of Tuple record instances. Requires fact instance to retract.

Example:

[?blue-fact <- [_ :color "blue"]]
=>
(retract! ?blue-fact)
[?blue-facts <- (acc/all) :from [_ :color "blue"]]
=>
(retract! ?blue-facts)

retract

(retract <session> <Tuple>)

Removes fact from session.

For use outside of a consequence block.

Second argument may be a collection of Tuple record instances.

Examples

Matching a value

When :color is "blue":

[[_ :color "blue"]]

Value slot can be anything.

Equality determined by Clojure's = function.

Binding a value to a variable

When a fact with attribute :color, bind its value to ?color:

[[_ :color ?color]]

Creates variable ?color and binds it to the value in the value slot.

Binding a fact to a variable

When a fact with attribute :color, bind the entire fact to ?my-fact:

[?my-fact <- [_ :color ?color]]
  • ?color is still available as a bound value
  • ?my-fact is returned as a Tuple record instance

Join on id

Obtains when an entity with the same eid has :color "blue" and :shape "circle".

[[?e :color "blue"]]
[[?e :shape "circle"]]

Join on value

Obtains when a :temperature/current fact and :temperature/record-high fact have the same value.

[[_ :current-temperature ?v]]
[[_ :really-hot-temperature ?v]]

Referential join

Fact values may hold references to other facts.

[[_ :node/parent ?node-id]]
[[?node-id :node/name ?name]]

Precept

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