Putting It All Together

Layers built:

What we glossed over (some are hard):

• Tail-call optimization: real Schemes require this; doable as a trampoline.
• Continuations (call/cc): Scheme's superpower. Captures and reifies the rest of the computation. Enables generators, coroutines, undo.
• Hygienic macros: avoid variable capture; standard in Scheme.
• Reader macros: customize parsing itself (custom (...) forms).
• Quasiquote: `(...) for templates with , for splice.
• Tail recursion + iteration: efficient loops via let and tail calls.

Modern Lisp dialects:

• Clojure: Lisp on JVM, immutable data, concurrency primitives. Used at Netflix, NuBank.
• Racket: Scheme + many extensions, used in research and education.
• Common Lisp (SBCL, Clozure): performance-tuned commercial Lisp. Used in Google's ITA flight search (now defunct) and other niches.
• Emacs Lisp: powers Emacs.
• Hy: Lisp syntax compiled to Python AST.

You've now built one in ~200 lines. SICP (the textbook) does this. PG's "On Lisp" goes deeper into macros.

Lisp is the language that influences ALL OTHER languages. Closures, GC, interactive REPL, dynamic dispatch — all from Lisp originally.