README

Lazarus (free pascal) implementation of Lox language

From Crafting Interpreters book.

Implementing bytecode virual machine variant of Lox interpereter.

Currently completed chapters:

• Virtual machine;
  • No global states like in book, all VM data is wrapped in classes;
• Scanning on demand;
  • Implemented constant indexes greather than 255 with separate instruction;
• Compiling expressions;
  • Compiler is a class too, makes things simpler;
• Types of values;
• Strings;
• Hash tables;
  • Implemented hash set using "separate chaining" method for string interning;
  • Implementing SipHash (siphash-1-3)
  • Probe sequence is a bit different from linear probe;
• Global variables;
• Local variables;
• Jumping back and forth;
  • The book say that implementation is Turing complete at this point, so I decided to publish it to github;
  • Added tests from original Lox repository and implemented a way to easily load them into the app;
• Calls and functions;
  • Added arity check for native function calls;
  • "A really inefficient recursive Fibonacci function" is running around 5 times slower than equivalent program in Python 3.12.2;
    • I have not tried Python 3.7.3, but I doubt it will run that much slower than 3.12.2;
    • I have a guess this is because many things done with macro in clox I had to remake with actual functions and procedures;
    • Adding inline directive did increase the speed, but not much, now it's just 4 times slower than Python;
    • Adding local IP variable to run() didn't give much speed increase (there is no "register" equivalent in pascal);
• Closures;
• Garbage collection;
  • This chapter exposed a very nasty bug in my code, it looked really small, but could potentially do a lot of harm (even if it's a toy language), I'm glad it's squshed;
    • There was other bugs with hash tables, but those were very easy to spot just looking at call stack;
  • Back to the performance problem, I found that I was using 32 bit Lazarus 4.6 while on 64 bit Windows 10, so together with an upgrade to Lazarus 4.8, I switched to 64 bit compiler;
    • Fib.lox runs nearly 2 times faster than 32 bit variant, still not as fast as Python, but the difference is just 2 times slower now (down from 5 times slower a few chapters back);
  • Implementing this chapter required a lot of workarounds because free pascal don't allow cyclic dependency in interface section, meaning I can't for example add function like markValue to the interface of memory.pas;
    • If I'm going to make real compiler/interpreter using pascal, I'll need to think about where to put GC from the very start;
• Classes and instances;
  • Added hasField native function to check if instance has a field;
  • Added a way for native functions to trigger runtime error;
• Methods and initializers;
• Superclasses;
• Optimization;
  • Implemented NaN boxing;
  • Implemented faster hash table search, including hash set for string interning;

After the book:

• Implementing helpers, native functions that accept arguments can be invoked using dot syntax on value that becomes its first argument;
  pascalCopy

  class Foo {}
  var foo = Foo();
  // next lines do same job
  hasField(foo, "bar");
  foo.hasField("bar");

• Add ternary operator;

Implemented natives:

• clock() -> double return performance counter value in seconds;
• hasField(instance, string) -> bool return true if instance has field named string
• hasMethod(instance or class, string) -> bool return true if instance or class has method named string
• math fuinctions:
  • sqr(double) -> double square
  • sqrt(double) -> double square root
  • ln(double) -> double natural logarithm
  • logN(double, double) -> double logarithm with first double parameter as base

License

For files in "test" directory see test/LICENSE file.

Each other file in this repository falls under GPL 3.0 or later license.

Copyright (C) 2026 Ivan Markov (TangorCraft)

This program is free software: you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation, either version 3 of the License, or (at your option) any later version.

This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details.

You should have received a copy of the GNU General Public License along with this program. If not, see https://www.gnu.org/licenses/.