A Proof of Concept for writing a Prolog Compiler

This guide provides instructions on setting up and using the Prolog compiler within the SWI-Prolog environment. The compiler is designed to transpile Prolog code into C++ executables, for a very small part of the Prolog language.

Prerequisites

• SWI-Prolog
• GNU Make
• GCC compatible C++ Compiler: clang++ (generates the fastest code) or gcc++
• Standard development tools (e.g., make)

Usage Guide

1. Installing Required Packages

Before starting, you need to install the dcg4pt package within SWI-Prolog:

?- pack_install(dcg4pt).

Having dcg4pt properly installed and working is essential before attempting to install or use additional tools like plammar.

2. Editing the dcg4pt.pl File (Required for Compatibility)

In order to ensure compatibility with newer versions of SWI-Prolog, you may need to edit the dcg4pt.pl file:

1. Open the dcg4pt.pl file for editing:

   <EDITOR> ~/.local/share/swi-prolog/pack/dcg4pt/prolog/dcg4pt.pl

2. Locate the two spots where an unquoted comma (,) is used. Replace the unquoted comma with a quoted one (',') to make the code compatible with SWI-Prolog.

3. Save the file and exit the editor.

3. Installing plammar

After making the necessary edits to dcg4pt, you can now proceed to install plammar:

?- pack_install(plammar).

4. Transpiling Prolog Files to C++

Start by launching SWI-Prolog with your custom files:

This command loads the interpreter.pl and compiler.pl files, which are required for the compilation process.

swipl -l interpreter.pl -l compiler.pl

To transpile a Prolog file to C++, use the compile/2 predicate. It takes two arguments: the file name and a test query as a string.

Example:

?- compile(file("nqueens.pl"), string("queens(4, Q).")).
true.

You can modify the file and query string as needed for different predicates. Here are some sample queries:

% Sample queries
compile(file("append.pl"),string("append([1,2],[3,4],X).")).
compile(file("append.pl"),string("append([1,2],X,[1,2,3,4]).")).
compile(file("append.pl"),string("append(X,[1],[2]).")).
compile(file("append.pl"),string("append(X,[3,4],[1,2,3,4]).")).
compile(file("append.pl"),string("append(X,Y,[1,2,3,4]).")).
compile(file("nqueens.pl"),string("queens(1,Q).")).
compile(file("nqueens.pl"),string("range(1,4,X).")).
compile(file("nqueens.pl"),string("queens_aux([1],[],Q).")).
compile(file("nqueens.pl"),string("selectx(X,[1],Y).")).
compile(file("nqueens.pl"),string("selectx(X,[1],Y).")).
compile(file("selectx.pl"),string("selectx(X,[1,2,3,4],Y).")).
compile(file("selectx.pl"),string("selecty(X,[1,2,3,4],Y).")).
compile(file("nqueens.pl"),string("queens(4,Q).")).

5. Generating the C++ File

The compilation process generates a C++ file named PrologGenerated.cpp, which contains the translated code.

To inspect the generated file and compile it, run:

cat PrologGenerated.cpp
make

6. Running the Compiled Binary

Once compiled, run the generated binary to see the output of your Prolog queries:

./test

7. Generating and Running a Debug Version

If you want to use a debugger like gdb, or want to track down an error, it is possible to build and run a debug version:

make debug
./test.debug

8. Tracing

To generate an execution trace, just change:

trace_mode :- fail.

to

trace_mode.

on about line 147 of compiler.pl , then reload and rerun the transpiler, make the binary and run it.

Common Issues

• Directory Requirements: Ensure that your files are in the correct directory structure as expected by the SWI-Prolog pack system, or adjust your environment accordingly.