A (Minimal) Lisp Interpreter written in Python (Tahsin's First New Programming Language)

 

import operator as op

# 1. Tokenizer: split input string into tokens

def tokenize(s):

    return s.replace('(', ' ( ').replace(')', ' ) ').split()

# 2. Parser: convert tokens into nested Python lists (AST)

def parse(tokens):

    if len(tokens) == 0:

        raise SyntaxError("Unexpected EOF")

    token = tokens.pop(0)

    if token == '(':

        L = []

        while tokens[0] != ')':

            L.append(parse(tokens))

        tokens.pop(0)  # remove ')'

        return L

    elif token == ')':

        raise SyntaxError("Unexpected )")

    else:

        return atom(token)

# 3. Atom: numbers become int/float, others are symbols (strings)

def atom(token):

    try:

        return int(token)

    except ValueError:

        try:

            return float(token)

        except ValueError:

            return str(token)

# 4. Environment: built-in functions

def standard_env():

    env = {}

    env.update({

        '+': op.add,

        '-': op.sub,

        '*': op.mul,

        '/': op.truediv,

        'eq?': op.eq,

        'lt?': op.lt,

        'gt?': op.gt,

        'print': print

    })

    return env

# 5. Evaluator

def eval(x, env):

    if isinstance(x, str):          # variable reference

        return env[x]

    elif not isinstance(x, list):   # constant literal

        return x

    elif x[0] == 'define':          # (define var expr)

        (_, var, expr) = x

        env[var] = eval(expr, env)

    else:                           # procedure call

        proc = eval(x[0], env)

        args = [eval(arg, env) for arg in x[1:]]

        return proc(*args)

# 6. REPL

def repl():

    env = standard_env()

    while True:

        try:

            expr = input('lisp> ')

            if expr.strip() == "exit":

                break

            val = eval(parse(tokenize(expr)), env)

            if val is not None:

                print(val)

        except Exception as e:

            print("Error:", e)

if __name__ == "__main__":

    repl()

🧪 Example Usage

lisp> (+ 12 7)

19

lisp> (* 4 5)

20

lisp> (define x 10)

lisp> (+ x 7)

17

lisp> (print "Hello, Syed")

Hello, Syed