Facts

1.1 The world is the totality of facts, not of things.

– Ludwig Wittgenstein, Tractatus Logico Philosophicus

Facts is a small in-memory graph database for Common Lisp. It features :

• a triple store based on unlabelled skip lists
• a very simple query language (facts:with)
• transactions using rollback functions
• logging and replay of transactions to/from disk
• dumping and loading the database to/from disk

https://git.kmx.io/thodg/cl-facts

Requirements

You will need :

• https://github.com/thodg/lessp
• https://github.com/thodg/rollback

Usage

FACTS:ADD &rest SPECS

Adds facts (triples) to the database. Triples can be grouped by subject.

(facts:add ("Blade Runner" :is-a :movie
                           :director "Ridley Scott"
                           :actor "Harison Ford"
                           :actor "Rutger Hauer")
           ("Snow White" :is-a :movie
                         :director "William Cottrell"
                         :director "David Hand"))

(facts:add (?movie :is-a :movie
                   :title "Blade Runner"
                   :director "Ridley Scott"
                   :actor "Harrison Ford"
                   :actor "Rutger Hauer"))

(facts:add (?movie :is-a :movie
                   :title "Snow White and the Seven Dwarfs"
                   :director "William Cottrell"
                   :director "David Hand"))

The second version with ?movie will generate an anonymous symbol prefixed with movie-. It is considered a more clean and efficient way to abstract identifiers.

FACTS:RM &rest SPECS

(facts:rm (?movie :actor "Harison Ford"))

FACTS:WITH SPECS &body BODY

To follow Wittgenstein’s view of the world, all queries get turned into testing the presence or absence of triples (facts).

(defun movie-title (movie)
  (facts:with ((movie :is-a :movie
                      :title ?title))
    (return ?title)))

is equivalent to

(defun movie-title (movie)
  (facts:with ((movie :is-a :movie)
               (movie :title ?title))
    (return ?title)))

which is itself equivalent to

(defun movie-title (movie)
  (facts:with ((movie :is-a :movie))
    (facts:with ((movie :title ?title))
      (return ?title))))

Multiple queries on the same subject can be grouped together easily :

(facts:with ((?movie :is-a :movie
                     :title ?title
                     :director ?director))
  (format t "~A directed ~A~%" ?director ?title))

FACTS:*DB*

The current facts database.

FACTS:CLEAR-DB

Clears the database from every facts.

FACTS:SAVE-DB &key INTO (READABLY T)

Dump the database facts into filespec INTO.

FACTS:LOAD-DB SRC

Load the facts from SRC into *db*.

FACTS:WITH-TRANSACTION &body BODY

Enclose BODY database operations into a transaction.

A transaction ensures that all database operations will succeed or be reverted using their respective rollback functions.

Transactions can be nested safely.

Examples

• https://github.com/RailsOnLisp/rol-server/blob/master/resource.lisp
• https://github.com/lowh/mentats