| name | lisp-in-c-2026-05-03 |
| description | Build a Lisp interpreter in C from scratch, covering S-expression parsing, manual memory management, hash-table environments, eval-apply cycle, and REPL. Two approaches: string-only atoms (LIPS) vs typed union AST nodes. Use when building interpreters in C, understanding evaluation with explicit memory management, or studying language implementation. |
Lisp in C — Minimal Interpreter Guide
Overview
Build a complete Scheme-like Lisp interpreter in C from scratch. The interpreter supports arithmetic, comparison, variables, user-defined functions with lexical scoping, conditionals, list operations, and a REPL. Two implementation approaches are covered:
- LIPS style (hal-rock/lips): All values stored as strings, S-expressions as linked lists with sentinel nodes, custom hash table for environments. Minimalist, ~300 lines of core logic across 9 source files.
- Tutorial style (ittrip.xyz): Typed union AST nodes with enum variants, strtok-based tokenizer, linked-list environment. More type-safe at the C level, closer to compiler textbook patterns.
Both approaches implement the same eval-apply cycle — the universal mechanism powering every Lisp implementation — but differ in memory representation, error handling, and data structure choices. The C-specific concerns (malloc/free discipline, pointer arithmetic, sentinel nodes, manual hash table) make this distinct from building interpreters in garbage-collected languages.
When to Use
- Building a Lisp interpreter from scratch in C
- Understanding how language evaluation works at the systems level with explicit memory management
- Learning pointer-based data structures for language implementation (linked lists, hash tables, environment chains)
- Studying the eval-apply cycle without garbage collection abstractions
- Comparing two C representation strategies: string-only atoms vs typed union AST nodes
- Implementing lexical scoping via chained environments in C
Core Concepts
