Enhancing Prolog: Adding Digit Grouping In Literals

Introduction: The Quest for Improved Readability

In the realm of programming, readability reigns supreme. Code that is easy to understand is easier to maintain, debug, and collaborate on. One area where readability can often suffer is in the representation of numerical literals, especially large numbers or those with many decimal places. The original issue identifies a key enhancement: the implementation of underscore digit grouping in decimal literals within Vibe-Prolog. This feature, already present in prominent Prolog implementations like SWI-Prolog and Scryer-Prolog, allows programmers to visually separate digits using underscores, making numbers like 1_000_000 or 3.1415_9265 far more legible than their ungrouped counterparts. This initiative not only aims to boost the ease of reading but also to align Vibe-Prolog with established industry practices, improving the portability of code. Let's delve into the specifics of this enhancement.

The Problem: Current Limitations

The fundamental challenge lies within the vibeprolog/parser.py file, specifically the NUMBER lexer rule. Currently, this rule isn't equipped to handle underscores except within the base'digits syntax. This restriction directly conflicts with the desired behavior of allowing underscores within decimal, floating-point, and scientific notation literals. The outcome is a failure to parse code that leverages this useful feature, hindering code portability. Resolving this issue means updating the parser to recognize and correctly interpret these underscore-separated digit groups, ensuring that Vibe-Prolog can handle a wider range of valid numerical representations.

The Goal: Implementing Underscore Digit Grouping

The central aim of this project is to equip Vibe-Prolog with the ability to correctly parse and interpret numerical literals that incorporate underscore digit grouping. This encompasses integers, floating-point numbers, and scientific notation. The addition of this feature will significantly enhance code readability and bring Vibe-Prolog closer to compatibility with other leading Prolog implementations. It's about empowering programmers to write cleaner, more understandable code while maintaining interoperability with codebases that already utilize this feature. The implementation will include parser updates, normalization logic, comprehensive tests, and updated documentation.

Technical Deep Dive: Implementation Steps

This section details the necessary technical steps to implement underscore digit grouping. Each part is crucial to ensure the feature functions correctly, is well-tested, and is properly documented.

1. Tokenizer and Grammar Updates: The Foundation

The first step focuses on updating the lexer and parser. The NUMBER token definition in vibeprolog/parser.py requires modification to accept underscores within decimal, floating-point, and scientific literal productions. This means modifying the grammar to correctly parse these new formats. It's essential that the existing handling of base'digits remains unchanged to preserve existing functionality. The placement of underscores must follow ISO-style rules: they can only appear between digits and not at the start or end of the number or adjacent to decimal points or exponents. This ensures both consistency and avoids ambiguity.

2. Numeric Normalization Logic: Processing the Input

After parsing, the Transformer.number helper needs an update to remove these underscores before calling int() or float(). This normalization step is crucial for converting the parsed string representation into the correct numerical value. Furthermore, this phase introduces validation to catch malformed patterns. These invalid patterns, like 1__0, _12, 12_, 1_.0, 1._0, or 1e_3, should trigger a PrologThrow with a syntax error, rather than silently removing the characters. A dedicated helper function, perhaps named _normalize_digit_groups(value: str) -> str, can be created to handle both underscore removal and validation. This helper can be reused across integers, floats, and scientific numbers, including the existing base'digits parsing, ensuring consistency across all numerical literal types.

3. Comprehensive Tests: Ensuring Correctness

Thorough testing is paramount. This phase requires the creation of extensive parser and interpreter tests to cover success and failure scenarios. Success tests should validate that the interpreter correctly parses and evaluates integers, floats, and scientific notations with underscore-grouped digits. Examples include queries like X = 1_000_000., X is 3.1415_9265., and X is 6.022_140_76e23.. Failure tests must assert that malformed patterns (like those mentioned in the validation section) raise syntax errors. Tests should be located in the tests/ directory, and each test module must instantiate a fresh PrologInterpreter per test to prevent state from carrying over between tests. This guarantees that any changes made do not break existing functionality and provides confidence in the correct interpretation of the new syntax.

4. Documentation: Guiding the User

Detailed documentation is crucial for user adoption. The docs/FEATURES.md file should be updated to reflect the new underscore grouping support, including any limitations. Review docs/SYNTAX_NOTES.md to add a subsection describing the new behavior, complete with examples and placement rules. The goal is to provide users with clear guidance on how to write portable code using this new feature. Mention the feature in other relevant syntax reference sections if needed. Clear, concise documentation is crucial for user understanding and adoption of the new feature.

5. Validation: Ensuring Quality and Stability

Before merging any changes, running uv run pytest (or the updated test modules) is critical to ensure there are no regressions. This validation step is essential to confirm that all tests pass, guaranteeing the stability and quality of the updated code. This ensures that the new feature integrates correctly without causing any unforeseen issues. Regular testing during development and before merging is a cornerstone of maintaining a robust and reliable codebase.

Acceptance Criteria: What Success Looks Like

Success is determined by several factors:

• Correct Parsing: Decimal literals like 1_000, 123_456.789_01, and 6.022_140_76e23 must parse and evaluate as their underscore-free equivalents. This confirms the core functionality works correctly.
• Error Handling: Invalid underscore placements must correctly generate ISO-style syntax errors. This guarantees that malformed inputs are handled gracefully.
• Comprehensive Documentation: Documentation must clearly explain the capability, and docs/FEATURES.md accurately reflects the support status. This ensures users understand and can effectively use the new features.
• Robust Testing: Tests must cover both valid and invalid cases to catch regressions immediately. This ensures the ongoing stability and reliability of the code.

Conclusion: A Step Towards Enhanced Prolog

The implementation of underscore digit grouping in decimal literals represents a significant advancement for Vibe-Prolog, improving both code readability and alignment with established Prolog standards. By following the outlined steps—updating the parser, implementing robust validation, creating comprehensive tests, and providing clear documentation—the Vibe-Prolog community can embrace a more readable and user-friendly programming experience. This enhancement empowers developers to write cleaner and more maintainable code, promoting better collaboration and broader adoption within the Prolog community.

For further details on Prolog and its functionalities, you can explore resources on the SWI-Prolog website.