AST Tree-Walking Interpreter & REPL

The AdeshLang AST Tree-Walking Interpreter evaluates the abstract syntax tree directly in memory without emitting intermediate bytecode or native machine code. It offers instantaneous execution startup, transparent stack introspection, and powers the interactive AdeshLang REPL.

┌──────────────────────────────────────────────────────────┐
│                   Interpreter Pipeline                   │
├──────────────────────────────────────────────────────────┤
│  Source Code ──► Lexer & Parser ──► AST Representation   │
│                                              │           │
│                                              ▼           │
│                                      Tree-Walk Evaluator │
│                                      [Scope Environment] │
└──────────────────────────────────────────────────────────┘

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1. How the Tree-Walking Interpreter Works

1. Lexical Analysis: Source text is converted into a stream of typed tokens.
2. Parsing: Tokens are organized into an Abstract Syntax Tree (AST) node hierarchy representing statements, expressions, and declarations.
3. Environment Frames: Variable bindings and closures are stored in scoped symbol tables (Environment), chaining child scopes to parent scopes.
4. Recursive Evaluation: The evaluator traverses AST nodes, evaluating sub-expressions and returning boxed Value objects.

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2. Interactive REPL (Read-Eval-Print Loop)

Launch the interactive REPL shell by running adesh repl or simply adesh:

$ adesh repl
Welcome to AdeshLang REPL v0.2.0
Type :help for special commands, or press Ctrl+D / exit to quit.

adesh> let x = 42;
42
adesh> let double = fn(n: i64): i64 { return n * 2; };
<function: double>
adesh> double(x);
84
adesh> let numbers = [1, 2, 3, 4, 5];
[1, 2, 3, 4, 5]
adesh> numbers.map(fn(x) => x * 10);
[10, 20, 30, 40, 50]

REPL Commands

Command	Description
:help	Displays help message and available REPL commands.
:type <expr>	Inspects the inferred compile-time type of an expression.
:ast <expr>	Dumps the parsed Abstract Syntax Tree for an expression.
:env	Lists all active variables and functions in the current REPL session.
:clear	Clears current session environment bindings.
:load <file>	Loads and executes an external .adesh script into the session.
:quit / :exit	Exits the REPL session.

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3. Running Scripts with the Interpreter

To execute a script directly via the interpreter:

# Execute using AST interpreter backend
adesh run --backend=interpreter script.adesh

# Or pass the -i flag
adesh -i script.adesh

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4. Key Strengths & Trade-offs

Feature	Interpreter	JIT / AOT
Cold Start Time	< 1ms (Instant)	5ms - 50ms
Memory Overhead	Minimal	Medium (CodeGen)
Debug Introspection	Complete AST inspection	Stripped symbols
Execution Throughput	Baseline	$30\times - 150\times$ faster
Use Case	Scripts, CLI tools, REPL	Heavy compute, servers