An experimental Lisp interpreter in Python 2 & 3

This is an experimental Lisp interpreter I wrote 10 years ago (2008) in Python. It had been presented under the MIT License at http://www.oki-osk.jp/esc/llsp/ until last spring (2017), which has been shut down now.

How to use

It runs in any Python from 2.3 to 3.6.

$ python L2Lisp.py
> "hello, world"
"hello, world"
> (+ 5 6)
11
> (exit 0)
$

You can give it a file name of your Lisp script.

$ python L2Lisp.py fibs.l
5702887
$

If you put a "-" after the file name, the interpreter will begin an interactive session after running the file.

$ python L2Lisp.py fibs.l -
5702887
> (take 20 fibs)
(1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987 1597 2584 4181 6765)
> (exit 0)
$ 

Features

• A sort of subset of Emacs Lisp, but being Lisp-1 with lexical scoping

• Tail call optimization

• Automatic avoidance of free symbol capture in macro expansion, which makes traditional macros fairly hygienic

• Very few built-ins: even defun is defined in the prelude as follows:

  (defmacro defun (name args &rest body)
    `(progn (setq ,name (lambda ,args ,@body))
            ',name))

• Implicit forcing for lazy evaluation: the Fibonacci sequence can be defined as follows:

  (defun zipWith (f x y)
    (if (or (null x)
            (null y)) nil
      (cons (f (car x) (car y))
            ~(zipWith f (cdr x) (cdr y)))))
  
  (setq fibs
        (cons 1 (cons 1 ~(zipWith + fibs (cdr fibs)))))

License

This is under the MIT License. See the L2Lisp.py file or evaluate (copyright) in the Lisp.