facts-db/cl-facts

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

– Ludwig Wittgenstein, Tractatus Logico Philosophicus

cl-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

Requirements

You will need :

• https://git.kmx.io/facts-db/cl-compare
• https://git.kmx.io/facts-db/cl-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).

Variables are prefixed with a question mark symbol “?” and are wildcards, matching everything. Nested queries get their variables expanded, giving pattern matching abilities. For instance :

(with ((?s ?p ?o)
  (format t "~&~S ~S ~S~&" ?s ?p ?o))

=>
"Blade Runner" :ACTOR "Harison Ford"
"Blade Runner" :ACTOR "Rutger Hauer"
"Blade Runner" :DIRECTOR "Ridley Scott"
"Blade Runner" :IS-A :MOVIE
"Snow White" :DIRECTOR "David Hand"
"Snow White" :DIRECTOR "William Cottrell"
"Snow White" :IS-A :MOVIE

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))

Negative facts specifications will remove matching facts from the results.

(with ((?s ?p ?o)
       (:not ?s :actor "Harison Ford"))
  (format t "~&~S ~S ~S~&" ?s ?p ?o))

=>
"Snow White" :DIRECTOR "David Hand"
"Snow White" :DIRECTOR "William Cottrell"
"Snow White" :IS-A :MOVIE

TODO

• binding of negation : :not resulting from another binding
• barriers / hooks : functions that will be called when a spec is added or removed.

### 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.