Datalog

A toy implementation of naive-evaluated Datalog in Elixir, based on the blog post The Essence of Datalog. The Datalog program given in the blog post is included in the Datalog.Example module.

DO NOT USE THIS IN PRODUCTION, THIS IS FOR EDUCATIONAL PURPOSES ONLY.

Some differences from the Haskell version

• No abstract types. I use structs for Rule (%Datalog.Rule{}) and Atom (%Datalog.Atom{}), and two-tuples for the two variants of Term: Var and Sym.

• No monadic do-notation. In a few places, I had to unpack the implicit iteration and flat-mapping that do-notation induces and make it explicit. The code looks a little strange as a result, with some ++ and Enum.concat/1 to get the desired level of list-nesting. See eval_atom/3 for an example of this.

• No inline named-functions. When the blog author uses Haskell where clauses to define local functions inside a function scope, I've substituted for comprehensions or functions of the same name but different arities. Example: unify/2 takes two atoms, and unify/1 (local function go in the blog post) works over the zipped list of terms to unify from the inputs of unify/2.

• No free-to-use fixpoint combinator. I have to tail-recurse, comparing the inputs to the outputs for equality. See solve/1.

• Pretty-printed data-structures. I wanted the inspect output to look like a Datalog program, so I implemented the Inspect protocol for the included structs.

Executing the example

$ iex -S mix
Erlang/OTP 22 [erts-10.7.2.1] [source] [64-bit] [smp:12:12] [ds:12:12:10] [async-threads:1] [hipe]

Interactive Elixir (1.10.3) - press Ctrl+C to exit (type h() ENTER for help)
iex(1)> Datalog.Example.ancestor
[
  adviser("Andrew Rice", "Mistral Contrastin").,
  adviser("Dominic Orchard", "Mistral Contrastin").,
  adviser("Andy Hopper", "Andrew Rice").,
  adviser("Alan Mycroft", "Dominic Orchard").,
  adviser("David Wheeler", "Andy Hopper").,
  adviser("Rod Burstall", "Alan Mycroft").,
  adviser("Robin Milner", "Alan Mycroft").,
  academicAncestor(X, Y) :- adviser(X, Y).,
  academicAncestor(X, Z) :- adviser(X, Y), academicAncestor(Y, Z).,
  query1(Intermediate) :-
    academicAncestor("Robin Milner", Intermediate),
    academicAncestor(Intermediate, "Mistral Contrastin")
  .,
  query2() :- academicAncestor("Alan Turing", "Mistral Contrastin").,
  query3() :- academicAncestor("David Wheeler", "Mistral Contrastin").
]
iex(2)> Datalog.solve(v(1))
[
  adviser("Andrew Rice", "Mistral Contrastin"),
  adviser("Dominic Orchard", "Mistral Contrastin"),
  adviser("Andy Hopper", "Andrew Rice"),
  adviser("Alan Mycroft", "Dominic Orchard"),
  adviser("David Wheeler", "Andy Hopper"),
  adviser("Rod Burstall", "Alan Mycroft"),
  adviser("Robin Milner", "Alan Mycroft"),
  academicAncestor("Andrew Rice", "Mistral Contrastin"),
  academicAncestor("Dominic Orchard", "Mistral Contrastin"),
  academicAncestor("Andy Hopper", "Andrew Rice"),
  academicAncestor("Alan Mycroft", "Dominic Orchard"),
  academicAncestor("David Wheeler", "Andy Hopper"),
  academicAncestor("Rod Burstall", "Alan Mycroft"),
  academicAncestor("Robin Milner", "Alan Mycroft"),
  academicAncestor("Andy Hopper", "Mistral Contrastin"),
  academicAncestor("Alan Mycroft", "Mistral Contrastin"),
  academicAncestor("David Wheeler", "Andrew Rice"),
  academicAncestor("Rod Burstall", "Dominic Orchard"),
  academicAncestor("Robin Milner", "Dominic Orchard"),
  academicAncestor("David Wheeler", "Mistral Contrastin"),
  academicAncestor("Rod Burstall", "Mistral Contrastin"),
  academicAncestor("Robin Milner", "Mistral Contrastin"),
  query1("Dominic Orchard"),
  query1("Alan Mycroft"),
  query3()
]
iex(3)> Datalog.query("query1", v(1))
[
  [{{:var, "Intermediate"}, {:sym, "Dominic Orchard"}}],
  [{{:var, "Intermediate"}, {:sym, "Alan Mycroft"}}]
]
iex(4)> Datalog.query("query2", v(1))
[]
iex(5)> Datalog.query("query3", v(1))
[[]]
iex(6)>