pascal-rs — A Modern Pascal Compiler & Interpreter in Rust

pascal-rs is a Pascal compiler and tree-walking interpreter written in Rust. It supports standard Pascal and a growing subset of Object Pascal (classes, exceptions, inheritance, virtual dispatch), backed by 216 automated tests and 10 runnable example programs.

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Status

Metric	Value
Tests	216 passing (115 unit + 101 integration)
Language	Standard Pascal + Object Pascal subset
Interpreter	Full-featured tree-walking execution
Compiler	x86-64 assembly generation with optimizations
Edition	Rust 2024

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What's New

Object Pascal in the Interpreter

The interpreter now supports core Object Pascal features at the AST/runtime level:

• Classes — Value::Object with field access, constructors, destructors, method dispatch
• Inheritance — single inheritance with field/method merging
• Virtual/Override — vtable-style dispatch via find_method_in_hierarchy
• is / as — runtime type checks walking the inheritance chain
• inherited — call parent class methods
• Properties — read/write resolution infrastructure
• Exceptions — try/except/finally, raise, on E: Type do matching, re-raise

Interpreter Enhancements

• Arrays — SetLength, indexing with bounds checking, high(), low(), length()
• Records — field access and assignment via dot notation
• String indexing — s[i] (1-indexed, Pascal style)
• exit — early return from functions/procedures, with optional return value (exit(42))
• break — loop termination
• with statement — pushes object/record fields into scope
• uses clause — loads and imports .pas unit files
• Nested functions — proper scoping for functions declared inside functions

Parser Improvements

• Source location tracking — error messages now include at line N, column M
• Error recovery — consume_or_skip and synchronize for resilient parsing
• exit/break tokens — properly parsed as statements (not silently skipped)

Bug Fixes

• Recursive function scoping — set_local_variable prevents recursive calls from clobbering parent scope return variables
• exit inside if/begin..end — EarlyReturn now propagates correctly through all control flow

10 Example Programs

Validated end-to-end (source → lexer → parser → interpreter):

#	File	Features
01	01_basics.pas	Variables, arithmetic, if/else, while, for, repeat/until, case
02	02_functions.pas	Factorial, Fibonacci, IsPrime, GCD, procedures
03	03_strings.pas	Reverse, palindrome, char counting, copy, pos, upcase
04	04_arrays.pas	SetLength, indexing, high/low/length
05	05_classes.pas	Shape area calculations with functions
06	06_exceptions.pas	try/except, try/finally, nested exceptions, raise
07	07_nested_functions.pas	exit() with return values, early return patterns
08	08_math_algorithms.pas	GCD, Collatz, digit sum, fast exponentiation
09	09_class_hierarchy.pas	Polymorphic dispatch, string comparisons
10	10_comprehensive.pas	Recursion, primes, strings, case, exceptions combined

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Why pascal-rs?

Compared to Alternatives

	pascal-rs	Free Pascal (FPC)	Delphi	GNU Pascal
Implementation language	Rust (memory-safe)	Object Pascal/C	C++	C
Interpreter mode	Built-in tree-walker	No	No	No
Test suite	216 automated tests	Large but external	Proprietary	Minimal
Object Pascal	Subset (classes, exceptions, virtual)	Full	Full	Partial
Trait-based design	Yes — testable, extensible	No	No	No
Error messages	Line/column, colored	Basic	Good	Basic
x86-64 codegen	Yes, with optimizations	Mature, multi-target	Mature	Basic
Active	Yes	Yes	Yes	No (2006)
Cross-platform	Yes (Rust targets)	Yes	Windows-focused	Yes
License	Apache-2.0	LGPL	Commercial	GPL

When to Use pascal-rs

• Learning compiler construction — clean Rust codebase, trait-based architecture, well-tested
• Running Pascal programs quickly — pascal run program.pas with no assembly/linking step
• Research & experimentation — easy to extend with new optimizations or language features
• Small projects & algorithms — full standard Pascal with functions, recursion, strings, arrays

When to Use Something Else

• Large production codebases — FPC or Delphi have decades of maturity
• Full Delphi compatibility — pascal-rs covers a subset of Object Pascal
• GUI applications — FPC/Lazarus or Delphi have mature widget libraries
• Multi-target compilation — FPC supports ARM, MIPS, PowerPC, WebAssembly, etc.

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Quick Start

Installation

git clone https://github.com/yingkitw/pascal-rs.git
cd pascal-rs
cargo build --release

Run a Program

# Interpret directly (no assembly step)
./target/release/pascal run examples/01_basics.pas

# Compile to x86-64 assembly
./target/release/pascal compile examples/01_basics.pas -S

Run All Tests

cargo test

test result: ok. 216 passed; 0 failed; 0 ignored

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Usage

Commands

Command	Description	Example
run	Interpret a Pascal program	pascal run prog.pas
compile	Compile to assembly	pascal compile prog.pas -S
info	Inspect a PPU file	pascal info module.ppu
clean	Remove build artifacts	pascal clean

Options

Option	Description
-o, --output <DIR>	Output directory
-O, --optimize <LEVEL>	Optimization level (0–3)
-v, --verbose	Verbose output
-S, --assembly	Generate assembly output
-d, --debug	Debug information
-I, --include <DIR>	Add unit search path
--no-cache	Disable PPU caching

Interpreter Mode

The interpreter runs Pascal programs directly without an assembly/linking step:

pascal run program.pas        # Run
pascal run program.pas -v     # Run with trace output

Compilation Mode

pascal compile program.pas          # Compile
pascal compile program.pas -S       # Emit assembly
pascal compile program.pas -O2      # Optimize
pascal compile program.pas -o build # Output to directory

Unit System

# Compile a unit
pascal compile MathUtils.pas -v

# Use it from a program
pascal run Calculator.pas    # uses MathUtils;

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Examples

Recursive Fibonacci with Exit

program Fibonacci;
var
  i: integer;

function Fib(n: integer): integer;
begin
  if n <= 1 then
    exit(n);
  Fib := Fib(n - 1) + Fib(n - 2);
end;

begin
  for i := 0 to 10 do
    writeln('Fib(', i, ') = ', Fib(i));
end.

Prime Sieve with Early Return

program Primes;
var
  i, count: integer;

function IsPrime(n: integer): boolean;
var
  i: integer;
begin
  if n < 2 then
  begin
    IsPrime := false;
    exit;
  end;
  i := 2;
  while i * i <= n do
  begin
    if n mod i = 0 then
    begin
      IsPrime := false;
      exit;
    end;
    inc(i);
  end;
  IsPrime := true;
end;

begin
  count := 0;
  for i := 2 to 100 do
    if IsPrime(i) then
      inc(count);
  writeln('Primes under 100: ', count);  { Output: 25 }
end.

String Operations

program Strings;
var
  s, rev: string;
  i, len: integer;

function ReverseStr(s: string): string;
var
  i, n: integer;
  r: string;
begin
  n := length(s);
  r := '';
  for i := n downto 1 do
    r := concat(r, copy(s, i, 1));
  ReverseStr := r;
end;

begin
  s := 'Hello, Pascal!';
  writeln('Original: ', s);
  writeln('Length: ', length(s));
  writeln('Upper: ', upcase(s));
  writeln('Reversed: ', ReverseStr(s));
  writeln('Substring: ', copy(s, 8, 6));  { Pascal }
  writeln('Char 1: ', s[1]);              { H }
end.

Exception Handling

program Exceptions;
begin
  try
    writeln('Before raise');
    raise Exception.Create('something went wrong');
    writeln('SHOULD NOT PRINT');
  except
    on E: Exception do
      writeln('Caught: ', E.Message);
  end;

  try
    raise Exception.Create('error');
  finally
    writeln('Finally always runs');
  end;
end.

Math Algorithms

program Math;
var
  a, b: integer;

function GCD(a, b: integer): integer;
begin
  while b <> 0 do
  begin
    a := a mod b;
    if a = 0 then
    begin
      GCD := b;
      exit;
    end;
    b := b mod a;
  end;
  GCD := a;
end;

function Power(base, exp: integer): integer;
var
  result: integer;
begin
  result := 1;
  while exp > 0 do
  begin
    if exp mod 2 = 1 then
      result := result * base;
    base := base * base;
    exp := exp div 2;
  end;
  Power := result;
end;

begin
  writeln('GCD(48, 18) = ', GCD(48, 18));    { 6 }
  writeln('2^10 = ', Power(2, 10));           { 1024 }
end.

Unit System

MathUtils.pas:

unit MathUtils;

interface

function Add(a, b: integer): integer;
function IsEven(n: integer): boolean;

implementation

function Add(a, b: integer): integer;
begin
  Add := a + b;
end;

function IsEven(n: integer): boolean;
begin
  IsEven := (n mod 2) = 0;
end;

end.

Main.pas:

program Main;
uses MathUtils;
var
  x: integer;
begin
  x := Add(10, 5);
  writeln('10 + 5 = ', x);
  writeln('Is even: ', IsEven(x));
end.

See the examples/ directory for all 10 validated example programs.

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Architecture

Source (.pas) → Lexer (logos) → Parser (recursive descent) → AST
                                                              │
                                    ┌─────────────────────────┼──────────────────┐
                                    ▼                         ▼                  ▼
                              Interpreter              Optimizer           Type Checker
                           (tree-walking)          (const fold, DCE)    (basic validation)
                                    │                     │
                                    ▼                     ▼
                              Direct Output        Code Generator → Assembly (.s)

Project Structure

pascal-rs/
├── src/
│   ├── lexer.rs                # Lexical analysis (logos-based, 100+ tokens)
│   ├── parser/                 # Recursive descent parser
│   │   ├── mod.rs              # Parser core with source location tracking
│   │   ├── expression.rs       # Expression parsing (precedence climbing)
│   │   ├── statement.rs        # Statement parsing (if, while, for, try, exit)
│   │   └── decl.rs             # Declaration parsing (var, type, class, function)
│   ├── ast.rs                  # AST node definitions
│   ├── interpreter.rs          # Tree-walking interpreter (Object Pascal support)
│   ├── type_checker.rs         # Type validation
│   ├── optimizer.rs            # Optimization passes
│   ├── unit_codegen.rs         # x86-64 code generation
│   ├── resolver.rs             # Symbol resolution
│   └── traits/                 # Trait abstractions for testability
├── tests/                      # 101 integration tests across 8 test files
├── examples/                   # 10 validated example programs
├── ARCHITECTURE.md
├── SPEC.md
└── TODO.md

Built-in Functions & Procedures

Math: abs, sqr, sqrt, sin, cos, ln, exp, round, trunc

String: length, concat, copy, pos, upcase, lowercase, inttostr, strtoint

Ordinal: ord, chr, odd, succ, pred, inc, dec

Array: length, high, low, setlength

I/O: write, writeln, readln

Control: exit, break, halt, random

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Testing

cargo test                                        # All 216 tests
cargo test --lib                                  # 115 unit tests
cargo test --test run_example_tests               # 19 example pipeline tests
cargo test --test run_integration_tests           # 10 integration tests
cargo test --test run_compiler_tests              # 10 codegen tests
cargo test --test run_complex_validation_tests    # 9 complex validation tests
cargo test --test run_interpreter_tests           # 11 interpreter tests
cargo test --test run_simple_compiler_tests       # 18 parser tests
cargo test --test run_simple_interpreter_tests    # 13 simple interpreter tests
cargo test --test run_type_checker_tests          # 10 type checker tests

Test Breakdown

Library (unit tests)        115
Example pipeline tests       19  ← NEW: validates 10 examples end-to-end
Compiler codegen tests       10
Complex validation            9
Integration tests            10
Interpreter tests            11
Simple compiler (parser)     18
Simple interpreter           13
Type checker                 10
Basic                         1
─────────────────────────────────
Total                       216

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Documentation

• ARCHITECTURE.md — module design and data flow
• SPEC.md — language feature matrix (parser/interpreter/codegen status)
• TODO.md — development roadmap and completed phases
• TRAIT_ARCHITECTURE.md — trait-based design guide

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Current Limitations

• Inline class method bodies — parser doesn't yet support method bodies inside type class declarations (interpreter supports classes via AST)
• Array element assignment — arr[i] := val not yet supported
• Multi-dimensional arrays — single dimension only in interpreter
• File I/O — not implemented
• Generics/templates — not implemented
• GUI framework — not included (use FPC/Lazarus for GUI apps)

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License

Apache-2.0

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Made with Rust — 216 tests passing | Standard Pascal + Object Pascal subset | Compiler + Interpreter