#zeta is a small lisp interpreter written in Python 3.

It started as a project for a class, and now I just hack on it occasionally for fun.

#Running To start the interpreter, run 'python zeta.py'. By default, the interpreter runs as a read-evaluate-print loop. However, one can pass a filename as the first argument to execute a script non-interactively:

python zeta script.lisp

The interpreter adjusts to avoid printing unnecessary prompts.

To see what names are defined in the global environment, type (help) at the prompt.

To exit interactive mode, type ctrl-D or ctrl-C.

#Features

• Built-in types: integer, float, bool, string, symbol, and list
• Scheme-like define syntax
• Tail-call optimization
• Parentheses-aware REPL

#Canonical Example

;; Applications in tail position will execute in
;; constant stack space
(define (factorial n)
    (define (loop acc x)
        (if (< x 2)
            acc
            (loop (* x acc) (- x 1))))
    (loop 1 n))

;; Nicer to use a fold
(define (factorial^ n)
    (foldl * 1 (range 1 n)))

;; Even better if the library expects such a thing
(define (factorial^^ n)
    (product (range 1 n)))

(print (factorial   10))
(print (factorial^  10))
(print (factorial^^ 10))

#Credits

• Originally a port of the Common Lisp meta-circular evaluator in Dr. Raphael Finkel's book "Advanced Programming Language Design".
• Figured out tail-call optimization by reading Peter Norvig's "(An ((Even Better) Lisp) Interpreter (in Python))".

#Miscellaneous Requires the pyparsing library.

Released under the MIT License.