Parser Implementation Strategy
Comparative Study
Section titled “Comparative Study”| Feature | Megaparsec | Parsec | Custom Parser |
|---|---|---|---|
| Type | Modern Monadic Parser Combinator | Legacy Parser Combinator | Manual String Processing |
| Error Reporting | Excellent. Detailed, annotated error bundles. | Basic. Often vague source positions. | Poor. Requires manual implementation of tracking. |
| Performance | High. Optimized for speed and memory. | Moderate. Older optimization techniques. | Variable. Depends entirely on implementation quality. |
| Infix Operators | Native. makeExprParser handles precedence automatically. | Native (buildExpressionParser), but older API. | None. Requires manual Shunting-yard algorithm. |
| Boilerplate | Low. Rich standard library of lexers. | Moderate. Requires more setup for tokens. | High. Must reinvent basic tokenizing logic. |
| Maintainability | High. Actively maintained standard. | Low. Mostly in maintenance mode. | Low. High technical debt for future teams. |
1. Custom Parser
Section titled “1. Custom Parser”- Pros: Complete understanding of every line of code; zero dependencies.
- Cons: Reinventing the wheel. We would have to manually implement backtracking, error position tracking (line/column), and operator precedence logic. This is time-consuming and prone to bugs, risking the tight deadlines of the Glados project.
2. Parsec
Section titled “2. Parsec”- Pros: The historical standard for Haskell parsing. Stable and widely documented.
- Cons: Dated. It lacks the modern error reporting capabilities of Megaparsec. The API is less ergonomic for complex lexing tasks compared to its successor.
3. Megaparsec
Section titled “3. Megaparsec”- Pros: The modern industry standard. It combines the flexibility of Parsec with significantly better error messages and performance. It includes robust helper modules (
Lexer) for handling whitespace, comments, and indentation automatically.
Why We Chose Megaparsec
Section titled “Why We Chose Megaparsec”We selected Megaparsec because the Glados project requirements (Part 2) demand features that are difficult to implement manually.
1. Handling Operator Precedence (Part 2)
Section titled “1. Handling Operator Precedence (Part 2)”The project specification explicitly requires:
“Infix arithmetic operators with the expected priorities” and “Infix user defined operators with user defined priority”
Implementing correct operator precedence (e.g., * binds tighter than +) manually requires implementing algorithms like Shunting-yard.
- With Megaparsec: We utilize
Control.Monad.Combinators.Expr. We simply define a table of operators and their precedence, and the library handles the parsing logic automatically.
2. Accessibility & Error Messages
Section titled “2. Accessibility & Error Messages”The project emphasizes accessibility:
“Your language must be easy to read and understand.”
A key part of a language’s usability is helpful error messages when the user makes a typo.
- Megaparsec provides “Error Bundles” out of the box. Instead of crashing or printing
Parse Error, it generates underlined, contextual error messages pointing to the exact line and column of the syntax error.
3. Separation of Concerns
Section titled “3. Separation of Concerns”Megaparsec encourages a clean separation between the Lexer (tokens, whitespace) and the Parser (grammar). This aligns with the architectural requirement to separate the frontend from the AST evaluation, ensuring code modularity.
Conclusion: Megaparsec allows us to focus on the semantics of our language (the VM and Compiler) rather than spending weeks debugging the syntax parsing mechanics.