monkeycpp-interpreter

name: monkeycpp-interpreter description: >- Guides implementation of the Monkey-style C++ interpreter in lang2 (lexer, parser, AST, objects, environment, evaluator, REPL). Use when adding language features, fixing parser/eval bugs, or changing the REPL; use when the user mentions monkeycpp, lang2, or this interpreter codebase.

Monkey-style C++ interpreter (lang2)

Architecture

Data flow: Lexer (token.h, lexer.*) → Parser (parser.*, ast.h) → Evaluator (evaluator.*) using Environment (environment.*) and Value (object.*). Builtins: builtins.*. Entry: main.cpp (REPL or argv[1] file).

Project conventions

• C++17, sources under src/, include path -I src (or CMake target_include_directories on src).
• Match existing style: minimal comments, straightforward dynamic_cast for AST dispatch, std::shared_ptr for env and heap objects, std::unique_ptr for AST owned by Program.
• Do not add unrelated refactors or new docs unless asked.

Critical invariants

1. AST lifetime
   FunctionObject / ClassObject methods hold BlockStatement* (and similar) into the parse tree. The REPL keeps std::vector<std::unique_ptr<Program>> program_storage in main.cpp so closures survive across lines. Any new code path that stores AST pointers must keep the owning Program alive for as long as those pointers are used.

2. this in methods
   applyFunction(..., this_binding) sets this on the function env. Member access requires Value::Kind::Instance.

3. Parser pitfalls (already fixed once; preserve behavior)

   • new Name(arg): After (, decide empty vs args using curTokenIs(RPAREN), not peekTokenIs only—otherwise a single arg is skipped when peek is already ).
   • Empty () in parseFunctionParameters: do not advance past ) twice; parent uses expectPeek(LBRACE) for method body.
   • Class body: parse methods while curTokenIs(FUNCTION), then expect closing }; stray ; after } is handled by parseProgram skipping leading ;.

How to add a language feature

Work in order; compile after each step.

1. token.h / token.cpp
   New TokenType and Token::typeName entry.

2. lexer.cpp
   Single-char tokens in nextToken; lookupIdent for new keywords.

3. ast.h
   New node types; statement vs expression; ownership (unique_ptr for child AST).

4. parser.h / parser.cpp

   • New statement: branch in parseStatement.
   • New prefix: branch in parsePrefixExpression.
   • New infix: extend parseExpression loop / peekPrecedence / tokenToPrecedence (e.g. [ for index = between CALL and MEMBER or as specified).

5. object.h / object.cpp
   Extend Value::Kind and inspect() if a new runtime type is needed (e.g. array for indexing).

6. evaluator.cpp
   New dynamic_cast branches in evalStatement / evalExpression; update callValue if new callable type.

7. main.cpp
   If REPL should print new expression forms, adjust the single-statement print rule (currently skips printing null).

Build

c++ -std=c++17 -Wall -Wextra -o monkeycpp src/main.cpp src/token.cpp src/lexer.cpp src/parser.cpp src/object.cpp src/environment.cpp src/builtins.cpp src/evaluator.cpp -I src

Or CMake: cmake -B build -S . && cmake --build build (add new .cpp files to CMakeLists.txt).

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