This paper details the formal verification of a garbage collector for a substantial subset of OCaml, including higher-order functions, algebraic data types, and mutable references. The collector, implemented and verified using the Coq proof assistant, employs a hybrid approach combining mark-and-sweep with Cheney's copying algorithm for improved performance. A key achievement is the proof of correctness showing that the garbage collector preserves the semantics of the original OCaml program, ensuring no unintended behavior alterations due to memory management. This verification increases confidence in the collector's reliability and serves as a significant step towards a fully verified implementation of OCaml.
Ohm is a parsing toolkit designed for creating parsers in JavaScript and TypeScript that are both powerful and easy to use. It features a grammar definition syntax closely resembling EBNF, enabling developers to express complex syntax rules clearly and concisely. Ohm's built-in support for semantic actions allows users to directly embed JavaScript or TypeScript code within their grammar rules, simplifying the process of building abstract syntax trees (ASTs) and performing other actions during parsing. The toolkit provides excellent error reporting capabilities, helping developers quickly identify and fix syntax errors. Its flexible architecture makes it suitable for various applications, from validating user input to building full-fledged compilers and interpreters.
HN users generally expressed interest in Ohm, praising its user-friendliness, clear documentation, and the power offered by its grammar-based approach to parsing. Several compared it favorably to traditional parser generators like PEG.js and nearley, highlighting Ohm's superior error messages and easier learning curve. Some users discussed potential applications, including building linters, formatters, and domain-specific languages. A few questioned the performance implications of its JavaScript implementation, while others suggested potential improvements like adding support for left-recursive grammars. The overall sentiment leaned positive, with many eager to try Ohm in their own projects.
Mukul Rathi details his journey of creating a custom programming language, focusing on the compiler construction process. He explains the key stages involved, from lexing (converting source code into tokens) and parsing (creating an Abstract Syntax Tree) to code generation and optimization. Rathi uses his language, which he implements in OCaml, to illustrate these concepts, providing code examples and explanations of how each component works together to transform high-level code into executable machine instructions. He emphasizes the importance of understanding these foundational principles for anyone interested in building their own language or gaining a deeper appreciation for how programming languages function.
Hacker News users generally praised the article for its clarity and accessibility in explaining compiler construction. Several commenters appreciated the author's approach of building a complete, albeit simple, language instead of just a toy example. Some pointed out the project's similarity to the "Let's Build a Compiler" series, while others suggested alternative or supplementary resources like Crafting Interpreters and the LLVM tutorial. A few users discussed the tradeoffs between hand-written lexers/parsers and using parser generator tools, and the challenges of garbage collection implementation. One commenter shared their personal experience of writing a language and the surprising complexity of seemingly simple features.
This blog post explores a simplified variant of Generalized LR (GLR) parsing called "right-nulled" GLR. Instead of maintaining a graph-structured stack during parsing ambiguities, this technique uses a single stack and resolves conflicts by prioritizing reduce actions over shift actions. When a conflict occurs, the parser performs all possible reductions before attempting to shift. This approach sacrifices some of GLR's generality, as it cannot handle all types of grammars, but it significantly reduces the complexity and overhead associated with maintaining the graph-structured stack, leading to a faster and more memory-efficient parser. The post provides a conceptual overview, highlights the limitations compared to full GLR, and demonstrates the algorithm with a simple example.
Hacker News users discuss the practicality and efficiency of GLR parsing, particularly in comparison to other parsing techniques. Some commenters highlight its theoretical power and ability to handle ambiguous grammars, while acknowledging its potential performance overhead. Others question its suitability for real-world applications, suggesting that simpler methods like PEG or recursive descent parsers are often sufficient and more efficient. A few users mention specific use cases where GLR parsing shines, such as language servers and situations requiring robust error recovery. The overall sentiment leans towards appreciating GLR's theoretical elegance but expressing reservations about its widespread adoption due to perceived complexity and performance concerns. A recurring theme is the trade-off between parsing power and practical efficiency.
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https://news.ycombinator.com/item?id=43191667
Hacker News users discuss a mechanically verified garbage collector for OCaml, focusing on the practical implications of such verification. Several commenters express skepticism about the real-world performance impact, questioning whether the verification translates to noticeable improvements in speed or reliability for average users. Some highlight the trade-offs between provable correctness and potential performance limitations. Others note the significance of the work for critical systems where guaranteed safety and predictable behavior are paramount, even at the cost of some performance. The discussion also touches on the complexity of garbage collection and the challenges in achieving both efficiency and correctness. Some commenters raise concerns about the applicability of the specific approach to other languages or garbage collection algorithms.
The Hacker News post discussing the mechanically verified garbage collector for OCaml has several comments exploring various aspects of the work.
Several commenters express appreciation for the accomplishment of verifying a garbage collector, acknowledging the complexity and difficulty inherent in such an undertaking. They see this as a significant step towards more reliable and robust software, particularly in areas where memory safety is critical.
One commenter delves into the specifics of the Coq proof assistant, used for the verification, mentioning the challenges associated with its steep learning curve and the significant time investment required to become proficient. They further highlight the value of Coq in ensuring the correctness of complex systems like garbage collectors.
Discussion arises around the practicality and performance implications of verified software. Some commenters question whether the performance overhead introduced by the verification process is acceptable, while others express optimism about the potential for future optimizations and the long-term benefits of increased reliability.
The topic of formal verification in general is also touched upon, with commenters discussing its growing importance in various fields and the potential for broader adoption in the future. The complexities and trade-offs of formal methods are acknowledged, but the overall sentiment appears to be one of encouragement for continued research and development in this area.
One commenter specifically points out the significance of verifying a concurrent garbage collector, highlighting the added difficulty this presents due to the intricate interactions and potential race conditions inherent in concurrent systems.
The use of OCaml as the target language is also mentioned, with some commenters expressing interest in the implications for the OCaml ecosystem and the potential for wider adoption of verified components within the language.
Finally, a commenter questions the extent of the verification, asking whether the entire garbage collector or only specific properties were verified. This highlights the importance of clearly defining the scope and limitations of formal verification efforts. Another commenter mentions that the work is being done in the context of the "Verdi" framework, which is itself formally verified, adding another layer of confidence to the results.