README

Tiny BASIC Interpreter (Pascal)

A lightweight, feature-rich BASIC interpreter written in Free Pascal. Supporting classic syntax, floating-point math, and character-based animations.

User Guide

How to Build

Requires Free Pascal Compiler (fpc).

fpc basic.pas

How to Run

Run the executable directly for an interactive REPL:

.\basic.exe

Or pipe a script file:

type anim.bas | basic.exe

Supported Commands

• PRINT: Output text or values. Use ; for 8-char tab stops and , for a single space.
• LET: Variable assignment (e.g., LET X = 10 or just X = 10).
• IF ... THEN: Conditional execution.
• FOR ... TO ... STEP ... NEXT: Loop constructs (supports nested loops and expressions).
• GOTO: Jump to a line number.
• LOAD "file.bas" / SAVE "file.bas": File persistence.
• SLEEP <ms>: Pause execution.
• CLS: Clear the screen.
• LIST, RUN, NEW, BYE: Environment control.

Project Structure

• basic.pas: The core interpreter source code.
• anim.bas, multi_char_vars.bas, user_loop.bas: High-level program examples and demos.
• tests/: Extensive suite of technical tests and feature verification scripts.

Technical Details (Internal Architecture)

🤖 LLM Implementation Notes

This interpreter is designed for modularity and correctness in parsing complex expressions and control flow.

Core Components

• Scanner/Tokenizer: Keyword-aware scanner that handles "spaceless" syntax (e.g., LETA=10) by preferring keywords unless they are prefixes of longer identifiers. Case-insensitive.
• Variable Storage: Uses TStringList for dynamic Name-Value mapping. Variables are stored as strings but treated as Double for calculations.
• Expression Parser: Recursive descent parser supporting operator precedence (* / > + - > = > < <> <= >=).
• Control Flow:
  • Line-based memory: Program is stored as a linked list of lines.
  • Statement-level jumps: Supports jumping back to specific statements within a multi-command line (separated by :).
  • Loop Stack: Handles nested FOR loops with a context stack storing the target line and the specific statement content to resume from.

Key Features

• Floating-Point: Internal numeric type is Double.
• Trigonometry: Built-in SIN() function.
• Multi-Statement: : separator allows multiple commands per line.
• Clean REPL: No prompt symbols (>) and minimal status messages for sleek output compatibility.

Constraints & Compatibility

• Compiled with fpc version 3.2.2+.
• Cross-platform CLS support (ANSI escape codes on POSIX, cmd /c cls on Windows).
• Fixed-size loop stack (10 levels deep).
• No line number range limits (uses Integer for line numbers).