Interpreter for a lisp-like language in C

GitHub (link to source code)

What is it?

My own implementation of a Lisp-like language, written in C.
This is still ongoing, but currently it supports basic arithmetic/relational operators, control flow (if, switch, while), lexical scoping (in the form of let), variable assignment, anonymous functions (lambda), and named functions.
So far, I have been mixing and matching elements from Scheme, and Common Lisp.

What is Lisp?

A family of programming languages dating back to the 1950s which put a heavy emphasis on homoiconicity (code is data), giving them powerful metaprogramming capabilities. They also have very little syntax, making them generally easier to parse.
Dialects of lisp include Common Lisp, Scheme, Clojure, Emacs Lisp, etc
They have been a major source of innovation that has seeped into many other languages, ranging from functional programming features, to macros, to garbage collection.