harald-lang / datalog.js

A trivial Datalog with top-down and bottom up evaluation written in Javascript to learn how Datalog evaluation works.

Datalog.js

A trivial Datalog with top-down and bottom up evaluation written in Javascript to learn how Datalog evaluation works.

Features:

• Implements both (naive) bottom-up and top-down evaluation for Datalog
• Traces the history for the top-down evaluation if successful

Caveats:

• No parser: you need to type the expressions in as JSON
• No validation (e.g. no sanity checking on the edb or idb contents)
• No negation or aggregate operators
• No repeated goal detection for the top-down evaluation
• Only tested with a simple transitive closure / graph reachability problem

Bottom-up evaluation

Works by inferring new facts until no new facts can be inferred.

expand(edb, idb): returns the db with all possible facts.

Example:

var expand = require('datalog.js').expand,
    edb = [
      { edge: [ 'a', 'b' ] },
      { edge: [ 'a', 'c' ] },
      { edge: [ 'b', 'd' ] },
      { edge: [ 'c', 'd' ] },
      { edge: [ 'd', 'e' ] }
    ],
    idb = [
    {
      head: { path: [ 'X', 'Y'] }, // :-
      body: [ { edge: [ 'X', 'Y' ]} ]
    },
    {
      head: { path: [ 'X', 'Y'] }, // :-
      body: [ { path: [ 'X', 'Z' ]}, { path: [ 'Z', 'Y' ]} ],
    }
  ];

/*
  Graph used in the example:

    /-> b \
  a        -> d -> e
    \-> c /

  Paths:
    (a,b), (a,c), (b,d), (c,d), (d,e)
    (a,d), (b,e), (c,e)
    (a,e)
*/

console.log(expand(edb, idb));

Output:

[ { edge: [ 'a', 'b' ] },
  { edge: [ 'a', 'c' ] },
  { edge: [ 'b', 'd' ] },
  { edge: [ 'c', 'd' ] },
  { edge: [ 'd', 'e' ] },
  { path: [ 'a', 'b' ] },
  { path: [ 'a', 'c' ] },
  { path: [ 'b', 'd' ] },
  { path: [ 'c', 'd' ] },
  { path: [ 'd', 'e' ] },
  { path: [ 'a', 'd' ] },
  { path: [ 'b', 'e' ] },
  { path: [ 'c', 'e' ] },
  { path: [ 'a', 'e' ] } ]

Top-down evaluation

Note that the top-down evaluation currently stops at the first instance that unifies.

establish(edb, idb, goal, {}, []): tries to establish the goal, given an edb and idb. Returns true if successful.

Example:

var establish = require('datalog.js').establish,
    idb = ...,
    edb = ...;

// edge: a, b
console.log('\n\n?edge(a, b)');
establish(this.edb, this.idb, { edge: [ 'a', 'b' ]}, {}, []);

// edge: b, a
console.log('\n\n?edge(b, a)');
establish(this.edb, this.idb, { edge: [ 'b', 'a' ]}, {}, []);

// path: a, b
console.log('\n\n?path(a, b)');
establish(this.edb, this.idb, { path: [ 'a', 'b' ]}, {}, []);

// path: a, d
console.log('\n\?path(a, d)');
establish(this.edb, this.idb, { path: [ 'a', 'd' ]}, {}, []);

Output:

?edge(a, b)
  Original goal: edge(a, b)
  DONE - Unified fact: edge(a, b) and final goal: edge(a, b)


?edge(b, a)
  Original goal: edge(b, a)
  Failed to find any match


?path(a, b)
  Original goal: path(a, b)
  Unified head of path(X, Y) => edge(X, Y) and goal: path(a, b)
  OK - unification is successful, substitutions: {"X":"a","Y":"b"}
  New goal: edge(a, b)
  DONE - Unified fact: edge(a, b) and final goal: edge(a, b)

?path(a, d)
  Original goal: path(a, d)
  Unified head of path(X, Y) => path(X, Z) & path(Z, Y) and goal: path(a, d)
  OK - unification is successful, substitutions: {"X":"a","Y":"d"}
  New goals: path(a, Z) & path(Z, d)
  Unified head of path(X, Y) => edge(X, Y) and goal: path(a, Z)
  OK - unification is successful, substitutions: {"X":"a","Y":"Z"}
  New goals: edge(a, Z) & path(Z, d)
  Unified fact: edge(a, b) and goal: edge(a, Z)
  OK - unification is successful, substitutions: {"Z":"b"}
  Remaining goal: path(b, d)
  Unified head of path(X, Y) => edge(X, Y) and goal: path(b, d)
  OK - unification is successful, substitutions: {"X":"b","Y":"d"}
  New goal: edge(b, d)
  DONE - Unified fact: edge(b, d) and final goal: edge(b, d)