A Julia package for Prolog-style logic programming.
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December 2019


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A Julia package for Prolog-style logic programming.


Enter the package manager by pressing ] at the Julia REPL, then run:

add Julog

The latest development version can also be installed by running:

add <link to this git repository>



Terms and Horn clauses can be expressed in Prolog-like syntax using the @julog macro:

# This creates a term
@julog teacher(bodhidharma, huike)
# This creates a fact (a term which is asserted to be true)
@julog teacher(bodhidharma, huike) <<= true
# This creates a definite clause
@julog grandteacher(A, C) <<= teacher(A, B) & teacher(B, C)

The @julog macro can be also applied to a list of clauses to create a knowledge base. We use the traditional Zen lineage chart as an example:

clauses = @julog [
  ancestor(sakyamuni, bodhidharma) <<= true,
  teacher(bodhidharma, huike) <<= true,
  teacher(huike, sengcan) <<= true,
  teacher(sengcan, daoxin) <<= true,
  teacher(daoxin, hongren) <<= true,
  teacher(hongren, huineng) <<= true,
  ancestor(A, B) <<= teacher(A, B),
  ancestor(A, C) <<= teacher(B, C) & ancestor(A, B),
  grandteacher(A, C) <<= teacher(A, B) & teacher(B, C)

With the resolve function, we can query the knowledge base via SLD resolution (the form of backward-chaining proof search used by Prolog):

# Query: Is Sakyamuni the dharma ancestor of Huineng?
julia> goals = @julog [ancestor(sakyamuni, huineng)]; # List of terms to query or prove
julia> sat, subst = resolve(goals, clauses);
julia> sat

# Query: Who are the grandteachers of whom?
julia> goals = @julog [grandteacher(X, Y)];
julia> sat, subst = resolve(goals, clauses);
julia> subst
4-element Array{Any,1}:
  {Y => sengcan, X => bodhidharma}
  {Y => daoxin, X => huike}
  {Y => hongren, X => sengcan}
  {Y => huineng, X => daoxin}

Forward-chaining proof search is supported as well, using derive. We can also compute the list of n-step derivations with derivations(clauses, n):

# Facts derivable from one iteration through the rules
julia> derivations(clauses, 1)
16-element Array{Clause,1}:
 teacher(bodhidharma, huike)
 ancestor(sakyamuni, huike)

# The set of all derivable facts (i.e. the closure / fixed-point)
julia> derivations(clauses, Inf)
30-element Array{Clause,1}:
 teacher(bodhidharma, huike)
 ancestor(sakyamuni, huineng)

More examples can be found in the test folder.


Julog uses syntax very similar to Prolog. In particular, users should note that argument-free terms with initial capitals are parsed as variables, whereas lowercase terms are parsed as constants:

julia> typeof(@julog(Person))
julia> typeof(@julog(person))

However, several important operators differ from Prolog, as shown by the examples below:

Julog Prolog Meaning
human(socrates) <<= true human(socrates). Socrates is human.
mortal(X) <<= human(X) mortal(X) :- human(X). If X is human, X is mortal.
!mortal(gaia) \+mortal(gaia) Gaia is not mortal.
mortal(X) <<= can_live(X) & can_die(X) mortal(X) :- can_live(X), can_die(X) X is mortal if it can live and die.

In words, <<= replaces the Prolog turnstile :-, <<= true or ' replaces . when stating facts, ! replaces \+ for negation, there is no longer a special operator for cut, & replaces , in the bodies of definite clauses, and there is no or operator like the ; in Prolog.

In addition, when constructing Prolog-style linked-lists, the syntax @julog list[a, b, c] should be used when the list is not nested within any other compound term. This is because the @julog [a, b, c] syntax is reserved for creating a Julia list of Julog objects, such as a list of Julog clauses. Lists which are nested within other term, e.g., member(b, [a, b, c]), are parsed in the same way as Prolog. (Note that list predicates like member are not built-in predicates, and have to be manually defined as a clause.)

If Prolog syntax is preferred, the @prolog macro and parse_prolog functions can be used to convert Prolog strings directly to Julog constructs, while write_prolog converts a list of Julog clauses to a Prolog string. However, this conversion cannot presently handle all of Prolog syntax (e.g., nested infix operators or comparison operators such as =:=), and should be used with caution.


Similar to string interpolation and expression interpolation in Julia, you can interpolate Julia expressions when constructing Julog terms using the @julog macro. Julog supports two forms of interpolation. The first form is constant interpolation using the $ operator, where ordinary Julia expressions are converted to Consts:

julia> e = exp(1)
julia> term = @julog irrational($e)
julia> dump(term)
  name: Symbol irrational
  args: Array{Term}((1,))
    1: Const
      name: Float64 2.718281828459045

The second form is term interpolation using the : operator, where pre-constructed Julog terms are interpolated into a surrounding Julog expression:

julia> e = Const(exp(1))
julia> term = @julog irrational(:e)
julia> dump(term)
  name: Symbol irrational
  args: Array{Term}((1,))
    1: Const
      name: Float64 2.718281828459045

Interpolation allows us to easily generate Julog knowledge bases programatically using Julia code:

julia> people = @julog [avery, bailey, casey, darcy];
julia> heights = [@julog(height(:p, cm($(rand(140:200))))) for p in people]
4-element Array{Compound,1}:
 height(avery, cm(155))
 height(bailey, cm(198))
 height(casey, cm(161))
 height(darcy, cm(175))

Custom Functions

In addition to standard arithmetic functions, Julog supports the evaluation of custom functions during proof search, allowing users to leverage the full power of precompiled Julia code. This can be done by providing a dictionary of functions when calling resolve. This dictionary can also accept constants (allowing one to store, e.g., numeric-valued fluents), and lookup-tables. An example is shown below:

funcs = Dict()
funcs[:pi] = pi
funcs[:sin] = sin
funcs[:cos] = cos
funcs[:square] = x -> x * x
funcs[:lookup] = Dict((:foo,) => "hello", (:bar,) => "world")

@assert resolve(@julog(sin(pi / 2) == 1), Clause[], funcs=funcs)[1] == true
@assert resolve(@julog(cos(pi) == -1), Clause[], funcs=funcs)[1] == true
@assert resolve(@julog(lookup(foo) == "hello"), Clause[], funcs=funcs)[1] == true
@assert resolve(@julog(lookup(bar) == "world"), Clause[], funcs=funcs)[1] == true

See test/custom_funcs.jl for more examples.

Unlike Prolog, Julog also supports extended unification via the evaluation of functional terms. In other words, the following terms unify:

julia> unify(@julog(f(X, X*Y, Y)), @julog(f(4, 20, 5)))
Dict{Var, Term} with 2 entries:
  Y => 5
  X => 4

However, the extent of such unification is limited. If variables used within a functional expression are not sufficiently instantiated at the time of evaluation, evaluation will be partial, causing unification to fail:

julia> unify(@julog(f(X, X*Y, Y)), @julog(f(X, 20, 5))) === nothing

Built-in Predicates

Julog provides a number of built-in predicates for control-flow and convenience. Some of these are also part of ISO Prolog, but may not share the exact same behavior.

  • cons and cend are reserved for lists. [x, y, z] is equivalent to cons(x, cons(y, cons(z, cend())).
  • true and false operate as one might expect.
  • and(A, B, C, ...) is equivalent to A & B & C & ... in the body of an Julog clause.
  • or(A, B, C, ...) is equivalent to A ; B ; C ; ... in Prolog-syntax.
  • not(X) / !X is true if X cannot be proven (i.e. negation as failure).
  • unifies(X, Y) / X ≐ Y is true if X unifies with Y.
  • exists(Cond, Act) is true if Act is true for at least one binding of Cond.
  • forall(Cond, Act) is true if Act is true for all possible bindings of Cond (beware infinite loops).
  • imply(Cond, Act) / Cond => Act is true if either Cond is false, or both Cond and Act are true.
  • call(pred, A, B, ...), the meta-call predicate, is equivalent to pred(A, B, ...).
  • findall(Template, Cond, List) finds all instances where Cond is true, substitutes any variables into Template, and unifies List with the result.
  • countall(Cond, N) counts the number of proofs of Cond and unifies N with the result.
  • fail causes the current goal to fail (equivalent to false).
  • cut causes the current goal to succeed and suppresses all other goals. However, this does not have the same effects as in Prolog because Julog uses breadth-first search during SLD-resolution, unlike most Prolog implementations, which use depth-first search.

See test/builtins.jl for usage examples.

Conversion Utilities

Julog provides some support for converting and manipulating logical formulae, for example, conversion to negation, conjunctive, or disjunctive normal form:

julia> formula = @julog and(not(and(a, not(b))), c)
julia> to_nnf(formula)
and(or(not(a), b), c)
julia> to_cnf(formula)
and(or(not(a), b), or(c))
julia> to_dnf(formula)
or(and(not(a), c), and(b, c))

This can be useful for downstream applications, such as classical planning. Note however that these conversions do not handle the implicit existential quantification in Prolog semantics, and hence are not guaranteed to preserve equivalence when free variables are involved. In particular, care should be taken with negations of conjunctions of unbound predicates. For example, the following expression states that "All ravens are black.":

@julog not(and(raven(X), not(black(X))))

However, to_dnf doesn't handle the implied existential quantifier over X, and gives the non-equivalent statement "Either there are no ravens, or there exist black things, or both.":

@julog or(and(not(raven(X))), and(black(X)))

Related Packages

There are several other Julia packages related to Prolog and logic programming:

  • HerbSWIPL.jl is a wrapper around SWI-Prolog that uses the Julog parser and interface.
  • Problox.jl is a lightweight Julia wrapper around ProbLog, a probabilistic variant of Prolog.


This implementation was made with reference to Chris Meyer's Python interpreter for Prolog, as well as the unification and SLD-resolution algorithms presented in An Introduction to Prolog by Pierre M. Nugues.

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