Lisp Parser: AST, Helpers, And NimbleParsec Setup
Embarking on the journey to create a robust Lisp parser is an exciting endeavor, and at its core lies the establishment of solid infrastructure. This article will guide you through the essential steps of building the foundational components for a PTC-Lisp parser, focusing on defining the Abstract Syntax Tree (AST) nodes, crafting helpful parser utilities, and setting up the NimbleParsec skeleton. By laying this groundwork, we create a clear path for future development, ensuring that every subsequent parsing step is built upon a stable and well-structured base. Our initial milestone will be the successful implementation of parsing for various literals, a critical first step in understanding and processing Lisp code.
The Pillars of a PTC-Lisp Parser: AST, Helpers, and the NimbleParsec Skeleton
To effectively build a Lisp parser, we need to start with the very blueprint of how Lisp code will be represented in our system. This blueprint is the Abstract Syntax Tree, or AST. The AST acts as a hierarchical data structure that mirrors the syntactic structure of the source code, abstracting away the raw text into a more manageable and logical form. For our PTC-Lisp parser, defining the AST node types and their constructors is paramount. This means specifying exactly how different Lisp constructs, like numbers, strings, booleans, and eventually symbols and collections, will be represented. Think of it as defining the vocabulary and grammar for our internal Lisp representation. Once we have our AST defined, we need a set of reliable helper functions. These parser helpers will encapsulate common parsing logic, such as recognizing and building representations for numbers, strings (including handling escape sequences), and other literal values. These helpers reduce code duplication and make our main parser definition cleaner and more readable. Finally, the skeleton of our parser will be built using NimbleParsec, a powerful parsing library that allows us to define parsers using a declarative, combinator-based approach. Setting up this NimbleParsec skeleton involves defining the main parsing entry point and structuring how different parsing rules will interact. By diligently following the architecture reference, particularly sections 1 through 3, we ensure that our AST, helper functions, and NimbleParsec structure are aligned with the overall design goals. This structured approach, coupled with the successful completion of the API Refactor phase (issues #100, #102, #104), ensures that we are building upon a stable and well-defined codebase. This issue marks the very beginning of our Lisp parser development, with no prior PTC-Lisp parsing capabilities or existing NimbleParsec dependency. The parser plan meticulously outlines the complete AST structure and the proposed implementation strategy, serving as our trusted guide throughout this foundational phase.
Laying the Groundwork: AST Definitions and Parser Helpers
Before we can even think about parsing complex Lisp expressions, we must establish the AST node type definitions that will represent the parsed code. This involves creating a dedicated module, lib/ptc_runner/lisp/ast.ex, where we'll define Elixir structs for each type of AST node. For instance, we'll need distinct representations for integers, floats, strings, booleans (true, false), and the nil value. Each of these structs will serve as a blueprint, holding the actual parsed value and any other relevant metadata. This meticulous definition ensures that our parser doesn't just recognize code but also understands its semantic components. Following the detailed specifications in Section 1 of our parser plan is crucial here, as it ensures consistency and adherence to the overall architecture. Once the AST structure is solidified, our attention turns to parser helper functions. These functions, to be housed in lib/ptc_runner/lisp/parser_helpers.ex, are the workhorses that simplify the process of building these AST nodes from the parsed input. For example, we'll create helpers to construct integer nodes from parsed number strings, float nodes, and string nodes, paying close attention to handling escape sequences like \n, \t, and \" correctly. These helpers act as intermediaries, translating the raw output of lower-level parsing rules into the structured AST nodes we've defined. This separation of concerns makes the main parser definition much cleaner. The plan details these reduction functions in Section 4, guiding us on how to effectively use NimbleParsec's reduction capabilities to build our AST. By investing time in these foundational elements – the AST and the parser helpers – we are building a robust and maintainable parsing infrastructure. This is essential for handling the nuances of Lisp syntax and preparing for the more complex parsing tasks that lie ahead. The goal is to make the parsing process as declarative and readable as possible, ensuring that our code reflects the structure of Lisp itself.
The NimbleParsec Skeleton: Structuring the Parser
With the Abstract Syntax Tree (AST) defined and our helper functions ready, the next critical step is to construct the NimbleParsec parser skeleton. This involves setting up the main parser module, lib/ptc_runner/lisp/parser.ex, which will orchestrate the parsing process using NimbleParsec combinators. NimbleParsec is an excellent choice for this task, offering a powerful and expressive way to define complex parsing logic. Our initial focus will be on defining the basic structure, including the entry point for parsing, typically a parse/1 public API function. This function will take a string of Lisp code as input and return either {:ok, ast} upon successful parsing or {:error, {:parse_error, message}} if any issues are encountered, mirroring the error handling patterns seen in lib/ptc_runner/json/parser.ex. The skeleton will incorporate NimbleParsec's core features, such as defcombinatorp for defining reusable parsing components and defparsec for defining specific parsing rules. We'll also leverage parsec(:name) for recursive parsing, which is essential for handling nested structures in Lisp. Section 3 of our parser plan provides a detailed roadmap for this implementation. A key aspect of this skeleton is the initial implementation of parsing for all Lisp literals: nil, true, false, integers, floats, strings (including handling escape characters), and keywords. This includes carefully defining rules to distinguish between these types and to handle edge cases. For example, ensuring that nil doesn't accidentally match as a prefix for a symbol like nilly requires precise rule definition. Similarly, distinguishing between a negative integer (-5) and a symbol followed by a number (- 5) is crucial. Floats must adhere to a strict format, requiring both an integer and a decimal part (e.g., 1.0, not .5 or 5.). Strings will be limited to single lines, with newline characters represented by the \n escape sequence, and empty strings "" must be valid. Setting up this NimbleParsec skeleton and implementing literal parsing serves as our first tangible milestone. It validates our AST definitions and helper functions, and it provides a working parser for the most basic Lisp elements, paving the way for parsing more complex structures in subsequent steps. The acceptance criteria clearly outline the required deliverables, including adding NimbleParsec as a dependency in mix.exs and ensuring that all unit tests pass, including those covering literal types and boundary conditions.
Implementing Literal Parsing: The First Milestone
The immediate goal and the first major milestone for our PTC-Lisp parser infrastructure is the successful implementation of parsing for all literal values. This means ensuring our parser can correctly interpret and represent nil, true, false, integers, floats, strings (complete with escape sequences), and keywords directly from the input code. This rigorous testing of literals is fundamental because they form the building blocks of any Lisp program. For booleans and nil, we need to ensure that our parser recognizes these distinct values and doesn't confuse them with parts of other identifiers. For instance, the string true should be parsed as the boolean true, not as the start of a word like truthy. This requires defining precise boundaries in our parsing rules. Integers and floats also present unique challenges. We must correctly parse positive and negative whole numbers, as well as floating-point numbers. A critical aspect here is the definition of what constitutes a valid float: it needs both an integer and a fractional part, meaning inputs like .5 or 5. alone are invalid and should result in a parse error. Handling scientific notation, such as 2.5e10, also falls under the scope of robust float parsing. Strings are another area requiring careful attention. Our parser must correctly handle quoted strings, including empty strings "", and importantly, all valid escape sequences like \n for newline, \t for tab, \\ for a backslash, and \" for a double quote. A key constraint for this phase is that strings will be single-line only; literal newline characters within strings are not permitted and must be represented by the \n escape. Keywords, often starting with a colon like :name or :user_id, also need to be parsed accurately, including those with kebab-case naming conventions. By successfully implementing and testing the parsing of these literals, we achieve a crucial benchmark. The acceptance criteria emphasize this by requiring the parser to handle all these literal types, edge cases, and boundary conditions. Furthermore, the parse/1 public API must be functional, returning the expected {:ok, ast} or {:error, {:parse_error, message}} tuple. Unit tests are non-negotiable, covering every literal type and potential error scenario, such as unclosed strings or invalid float formats. Passing mix compile --warnings-as-errors and ensuring all existing tests remain green are the final checks that confirm the stability and correctness of our foundational literal parsing capabilities.
Testing and Validation: Ensuring Parser Correctness
Rigorous testing is the cornerstone of building a reliable Lisp parser. Once the AST, helper functions, and NimbleParsec skeleton are in place, and the literal parsing logic is implemented, a comprehensive test plan is essential to validate its correctness. The unit tests will cover every facet of literal parsing, ensuring that nil, true, and false are recognized accurately and distinctly. For numerical literals, we'll test positive, negative, and zero integers, as well as floats with various forms, including those with exponents. String parsing tests will include simple strings, empty strings, and strings with all supported escape sequences (\n, \t, \\, \"). Keywords, from simple symbols to kebab-cased ones, will also be thoroughly tested. A critical part of the unit tests involves symbol boundary tests. These are designed to confirm that our parser correctly identifies the boundaries of literals and doesn't incorrectly match them as prefixes of other tokens. For example, we need to ensure nil is parsed as nil and not as part of nilly or nil?. Beyond successful parsing, the error tests are equally vital. These tests will probe the parser's robustness by providing intentionally malformed input and verifying that appropriate parse errors are generated. Examples include testing invalid float formats like leading or trailing decimals (.5, 5.), unclosed strings ("hello), and disallowed multiline string literals. This ensures that the parser fails gracefully and provides informative error messages, akin to the error handling patterns in lib/ptc_runner/json/parser.ex. The acceptance criteria underscore the importance of these tests by explicitly requiring them for all literal types and edge cases. Additionally, we must ensure that the build process is clean, passing mix compile --warnings-as-errors, and that no existing tests are broken by these new additions. This meticulous validation process guarantees that our foundational Lisp parser is not only functional but also robust and reliable, ready to handle the complexities of Lisp syntax.
Conclusion: A Solid Foundation for PTC-Lisp
In summary, by diligently creating the AST node type definitions, crafting essential parser helper functions, and establishing the NimbleParsec parser skeleton, we have successfully laid the critical groundwork for our PTC-Lisp parser. The initial focus on parsing all literals – nil, true, false, integers, floats, strings with escapes, and keywords – represents a significant first milestone. This foundational work, guided by a clear architecture plan and validated through comprehensive unit and error testing, ensures a robust and maintainable parsing infrastructure. With this solid base, we are well-equipped to tackle the subsequent challenges of parsing symbols, collections, and handling whitespace and comments in the upcoming phases. The journey of building a powerful Lisp parser is underway, piece by carefully constructed piece.
For further insights into parsing technologies and best practices, you can explore resources from the Elixir community and documentation on NimbleParsec:
