A Toy Prolog Interpreter in C++

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Introduction

One of my old mentors told me that he thought you were never a real computer programmer until you’ve written a compiler. I was still at high school at that time and a few years later, I had the opportunity to implement a compiler as part of a university paper. That idea has stuck with me and a few years back, I got interested in the inner workings of prolog. I had used prolog briefly at university but hadn’t ever attempted to understand how that was implemented and set about implementing an interpreter rather than a compiler, just to understand the details and problems that come about when you try to execute prolog.

In this article, I’m going to give you a brief tour of some of the parts of an implementation of a logic and inference algorithm. The kinds of logic used in this program can be used as a basis of a logic programming language, or a type inference system. Obviously, a real prolog implementation would have many more features.

Download Link

If all you want is to peek at my source code, feel free to download it from the link at the top of this post.

Usage

The program is split into two main parts, the parser, and the interpreter. First you parse the program, and then process the clauses into compressed clauses that use integers for identifiers rather than strings, and then you can execute the clauses. Here’s a sample main:

int main(int argc, char* argv[])
{
   std::string program = ""
   "parent(great_grandad, nana).\n"
   "parent(great_nana, nana).\n"
   "parent(great_pop, grandad).\n"
   "parent(great_grandma, grandad).\n"
   "parent(grandad, dad).\n"
   "parent(nana, dad).\n"
   "parent(grandad, uncle).\n"
   "parent(nana, uncle).\n"
   "parent(dad, miss).\n"
   "parent(dad, master).\n"
   "ancestor(X, Y) :- parent(X, Y).\n"
   "ancestor(X, Y) :- parent(X, Z), ancestor(Z, Y).\n";

   runtime rt;
   std::stringstream ssprogram(program);
   parser p;
   execute_program(p.parse_program(ssprogram), rt);
   rt.dump_program();

   run_predicate("ancestor(dad,B)", rt);
   enumerate_predicate("ancestor(Ancestor, master)", rt);
   return 0;
}

This code creates a runtime and parses the program and then executes it in the runtime. This will install the clauses into the runtime. The very next thing the sample does is it dumps the program out to the console, this helps me see what is actually loaded.The final two things that the program does are running a predicate to get one answer using run_predicate, and then running another predicate to list all of the answers using enumerate_predicate.

How It Works

Prolog clauses are made up of a left side, and a right side. The left side might be considered a goal that you might be wanting to prove, and the right side would be one possible list of things that if true would mean that the left side is true. Clauses can have variables, beginning with a capital that can match any value.

#if You're happy and you know it clap your hands
clap_your_hands(You) :- happy(You), know_it(You).

One of the first problems you encounter when executing prolog, is how when you attempt to prove a clause and find a solution, you may need to attempt several possible candidate clauses to check which of those might lead to an answer. In the example above, there might be several reasons to be happy, but you might not know it.

Unification of Values

Solving a clause with two predicates on the right side might mean delving deep into one predicate before you can know the parameters that can be passed into the second predicate, especially in the case that the clause specifies variables rather than values. Basically what we need to do is specify some things that we want to do for every possible match, and we can do that with a lambda.

Prolog’s unification mechanism, means that you can specify a variable at one point in a clause, which will bind to the first value you specify, and which later when you use that variable again the same value will be used.

The following process will attempt to unify either two values, or two variables. If either variable or both are not bound, then the unification passes, otherwise the values must be identical. Note the function thenwhat,
which is used to continue execution once values are unified. Once execution is finished, the unifications are rolled back so that another one can be attempted.

The point I’m trying to stress to you here, is that it’s more difficult than returning yes/no answer to a question, you might also have other things you need to take care of before you know the actual answer.

bool unify_variants(
   int base,
   variable_handle v1,
   variable_handle v2,
   const std::function<bool()>& thenwhat)
{
   variable_handle v1_root = obtain_root_index(v1);
   variable_handle v2_root = obtain_root_index(v2);

   if(v1_root == v2_root)
   {
      return thenwhat(); //already are
   }

   variable* v1_value = v1_root.get_value(stack);
   variable* v2_value = v2_root.get_value(stack);

   bool result = false;
   if(v1_value == nullptr)
   {
      auto bound_variable =
         v2_root.get_bound_variable(stack);
      v1_root.set_value(stack, bound_variable);
      result = thenwhat();
      v1_root.unlink(stack);
   }
   else if(v2_value == nullptr)
   {
      auto bound_variable =
         v1_root.get_bound_variable(stack);
      v2_root.set_value(stack, bound_variable);
      result = thenwhat();
      v2_root.unlink(stack);
   }
   else
   {
      if(variant::compare(
         ((value_variable*)v1_value)->value.get(),
         ((value_variable*)v2_value)->value.get()))
      {
         result = thenwhat();
      }
   }
   return result;
}

Unification of Parameters

In order to solve a clause, you first need to be able to pass arguments into the formal parameters of the clause. Some of the arguments might be constrained, and some might not.

The following functions take arguments and attempt to unify those arguments with the parameters of a clause. Performing the unifications recursively allows the interpreter to keep track of the variables and unifications already applied.

bool unify_parameter(
   int base,
   variable_handle argument,
   parameter parameter,
   const std::function& thenwhat)
{
   variable_handle new_argument =
      parameter.instantiate_argument(base, stack);
   bool result = unify_variants(
      base, argument, new_argument, thenwhat);
   stack.pop_back();
   return result;
}

bool unify_parameters(
   int base, int parameter_index,
   const std::vector<parameter>& parameters,
   const std::vector<variable_handle>& arguments,
   const std::function<bool()>& thenwhat)
{
   if(parameter_index == parameters.size())
   {
      if(parameter_index == arguments.size())
      {
         return thenwhat();
      }
      else
      {
         return false; //argument list too long
      }
   }
   else if(parameter_index == arguments.size())
   {
      return false; //argument list too short
   }
   else
   {
      return unify_parameter(
         base,
         arguments[parameter_index],
         parameters[parameter_index],
         [&](){
            return unify_parameters(
               base,
               parameter_index + 1,
               parameters,
               arguments,
               thenwhat);
         });
   }
}

Proving Conjunctions

In my implementation, parameters specify where in the clause or in the value database a variable or value should come from. The following code pushes values onto the stack for each formal parameter and then unifies the arguments with the parameters as above.

The unify_conjunction method will instantiate all arguments, and then attempt to prove each clause in the conjunction.

std::vector<variable_handle>
   instantiate_predicate_arguments(
      int base,
      const std::vector<parameter>& parameters)
{
   std::vector<variable_handle> arguments;
   for(auto p : parameters)
   {
      arguments.push_back(
         p.instantiate_argument(base, stack));
   }
   return arguments;
}

std::vector<variable_handle>
   instantiate_predicate_arguments(
      const compiled_predicate& predicate)
{
   return instantiate_predicate_arguments(
      stack.size(), predicate.parameters);
}

void unstack_arguments(
   const std::vector<variable_handle>& arguments)
{
   for(int n = 0; n < arguments.size(); n++)
   {
      stack.pop_back();
   }
}

bool unify_conjunction(
   int base,
   int index,
   const conjunction& conj,
   const std::function<bool()>& thenwhat)
{
   if(index == conj.size())
   {
      return thenwhat();
   }
   else
   {
      std::vector<variable_handle> arguments =
         instantiate_predicate_arguments(
            base, conj[index].parameters);

      bool result = unify_clause(
         conj[index].predicate_index,
         arguments,
         [&]{
            return unify_conjunction(
               base,
               index + 1,
               conj, thenwhat);
         });

      unstack_arguments(arguments);
      return result;
   }
}

Proving Clauses

The last piece of the interpretation puzzle is unification of clauses. In order to prove a clause, you need to first check that the arguments passed to the goal match, and then you need to prove the conjunction. The following code goes through each clause in the clause dictionary and attempts to match the arguments with the parameters of the clause. If they match, then the next thing it does is attempt to prove the conjunction.

bool unify_clause(
   int predicate,
   const std::vector<variable_handle>& arguments,
   const std::function<bool()>& thenwhat)
{
   int base = stack.size();
   for(const auto& clause: clause_db[predicate])
   {
      if(unify_parameters(
         base,
         0,
         clause.goal_parameters,
         arguments,
         [&](){
            return unify_conjunction(
               base,
               0,
               clause.requirements,
               thenwhat);
         }))
      {
         return true;
      }
   }
   return false;
}

Summary

In this article, I’ve given you a tour of some of the complicated parts of a toy implementation of the inference part of prolog. Hopefully, you found something of use in here, and if you have any comments or questions, let me know either by commenting here or by sending me an email at phillipvoyle@hotmail.com.

Thanks for reading.

This blog originally appeared here: https://dabblingseriously.wordpress.com/2015/11/15/a-toy-prolog-interpreter-in-c/