Lisp Eval: Building The Core Evaluation Infrastructure

Welcome, fellow code explorers! Today, we're diving deep into the heart of our Lisp implementation within the PTC Runner. Our mission? To forge the foundational eval infrastructure that will allow us to breathe life into our Abstract Syntax Trees (ASTs). Think of it as building the engine and control panel before we can actually drive the car. We're talking about creating the essential type definitions, establishing the main entry point for evaluation, and, crucially, enabling the evaluation of literals and collections. This is the bedrock upon which all future, more complex Lisp Eval features will be built.

The Genesis of Eval: From Code to Runtime Values

Our journey into Lisp Eval begins with a clear understanding of where we are and where we're going. The Lisp module in our project now boasts a robust Parser and an astute Analyzer. The Parser diligently transforms our source code into a Raw AST, a preliminary structure. Following this, the Analyzer takes that Raw AST, validates it, and then elegantly desugars it into a Core AST. This Core AST is a more refined representation, ready for the next crucial stage: evaluation. We've meticulously defined the types for this Core AST in lib/ptc_runner/lisp/core_ast.ex. However, the star of our current show, the eval module (lib/ptc_runner/lisp/eval.ex), and its essential companion, the env module (lib/ptc_runner/lisp/env.ex), are yet to be born. These modules will be the workhorses of our Lisp Eval process. The Eval layer is designed to receive this polished Core AST from the Analyzer and, through a series of intricate steps, produce actual runtime values that our system can understand and utilize. This transition from a static code structure to dynamic runtime values is the core promise of any programming language's evaluation mechanism, and our Lisp Eval implementation is no exception. We're laying the groundwork for a powerful and flexible system that can interpret and execute Lisp code within the PTC Runner environment. The architecture reference document, specifically sections 1 and 2, provides a detailed roadmap for this endeavor, ensuring we're building a cohesive and well-structured evaluation layer.

Defining the Building Blocks: Types and the Environment

Before we can truly evaluate anything, we need to define what we're evaluating into and how we'll manage the context of that evaluation. This brings us to the crucial first steps in establishing our Lisp Eval infrastructure: defining the types for our evaluation results and setting up the environment. We'll create a new module, PtcRunner.Lisp.Eval, which will house our core evaluation logic. This module will come equipped with precise type specifications, ensuring that our evaluation functions behave predictably and safely. Alongside this, we need a skeleton for PtcRunner.Lisp.Env. Initially, this environment module will be quite basic, serving as an empty container. However, as our Lisp Eval capabilities grow, this environment will become the central hub for managing variables, function scopes, and other contextual information essential for executing Lisp code. The eval/5 function will be our main entry point. It's designed to accept the Core AST, a context (ctx), the current memory state, the environment (env), and a tool_executor. This signature is carefully crafted to handle the complexities of evaluation, including passing state and allowing for future extensions like tool interactions. The return value is equally important: we'll adhere to the {:ok, value, memory} or {:error, reason} tuple format, ensuring a clear and consistent way to handle both successful evaluations and potential failures. This strict typing and structured return format are vital for building a reliable and maintainable Lisp Eval system. The memory state, in particular, will be threaded through the evaluation process, allowing for operations that might modify or access memory without breaking the functional purity of the evaluation itself. This careful consideration of types and environment management forms the essential scaffolding for all subsequent Lisp Eval developments.

The Art of Literal Evaluation: Bringing Primitives to Life

Now, let's get our hands dirty with the actual Lisp Eval logic. The first set of AST nodes we need to tackle are the literals – the fundamental building blocks of any data representation. These are the simplest forms of data that Lisp can understand directly, and mastering their evaluation is the first major milestone for our eval infrastructure. We're talking about nil, true, false, various numeric types, strings (represented as {:string, _} in our Core AST), and keywords (represented as {:keyword, _}). The goal here is straightforward: when the evaluator encounters these literal nodes, it should simply return their corresponding runtime values without any fuss. For example, an `{:string,