Extending Your Interpreter

Exercises to grow lisp.py toward a real Lisp:

Exercises to grow lisp.py toward a real Lisp:

Strings and comments: extend the tokenizer to handle "string literals" and ; comments.

Multi-expression bodies and let: desugar (let ((x 1) (y 2)) body...) into ((lambda (x y) body...) 1 2).

cond, and, or: add special forms (they must short-circuit, so they can't be plain functions).

Tail-call optimization: turn evaluate into a loop: for if, begin, and procedure calls in tail position, update x and env and continue instead of recursing. Then (define loop (lambda (n) (if (= n 0) 0 (loop (- n 1))))) works for a million iterations.

Quasiquote and macros: add quasiquote/unquote/unquote-splicing, then define-macro: store a macro as a procedure that receives unevaluated arguments; at evaluation time, call it and evaluate the returned expression.

Proper booleans and the empty list: add #t/#f literals and decide on truthiness rules.

Error handling: add a try/raise form using Python exceptions, or implement call/cc with exceptions for escape-only continuations.

A standard prelude: write map, filter, reduce, append, and reverse in your Lisp itself and load them at startup.

Read files: (load "file.lisp"), and support multiple top-level expressions per file.

Performance: compile expressions to Python closures once ("closure compilation") instead of re-walking the tree on every call.

Further projects: Make a Lisp (mal) (a step-by-step guide to implementing Lisp in 80+ languages), *Build Your Own Lisp* (in C), and *Lisp in Small Pieces* (Christian Queinnec) for implementation theory.

# Sketch: tail-call-optimized evaluate (replace the recursive version)
def evaluate(x, env=GLOBAL_ENV):
    while True:
        if isinstance(x, str):
            return env.find(x)[x]
        if not isinstance(x, list):
            return x
        head = x[0]
        if head == 'quote':
            return x[1]
        if head == 'if':
            _, test, conseq, alt = x
            x = conseq if evaluate(test, env) else alt      # loop instead of recursing
            continue
        if head == 'define':
            _, name, expr = x
            env[name] = evaluate(expr, env)
            return None
        if head == 'lambda':
            _, params, body = x
            return Procedure(params, body, env)
        if head == 'begin':
            for expr in x[1:-1]:
                evaluate(expr, env)
            x = x[-1]
            continue
        proc = evaluate(head, env)
        args = [evaluate(a, env) for a in x[1:]]
        if isinstance(proc, Procedure):
            x, env = proc.body, Env(proc.params, args, proc.env)   # tail call: reuse the loop
            continue
        return proc(*args)