Small programming language in Python, supports subexpressions

minilang.py

#!/usr/bin/env python
from __future__ import print_function
import traceback
import sys
import json
import re

LEX_RX = re.compile(r'("[^"]+"|[^a-zA-Z0-9\$\s]|\$?[a-zA-Z0-9]+)')

CMDS = dict(
	echo=lambda *x: print(*x),
	loop=lambda *x: builtin_loop(*x),
	line=lambda *x: sys.stdin.readline().rstrip('\n'),
	eval=lambda *x: map(rs_run, x)[-1],
)

def builtin_loop(*args):
	while True:
		for arg in args:
			rs_run(arg)

def rs_arg(arg):
	if arg[0] == '"':
		return json.loads(arg)
	return arg

def rs_lex(script):
	return map(rs_arg, LEX_RX.findall(script))

def rs_exec(*args):
	cmd = args[0]
	if cmd in CMDS:
		return CMDS[cmd](*args[1:])
	raise RuntimeError(
		' '.join(["Unknown", ' '.join(args)]))

def rs_eval(script):
	N = 0
	args = []
	result = None
	while script:
		N += 1
		cmd = script[0]
		script = script[1:]
		do_exec = not len(script)
		if cmd == '[':
			skip_N, result = rs_eval(script)
			script = script[skip_N:]
			N += skip_N
			args.append(result)
		elif cmd == ']':
			do_exec = True
			script = None
		elif cmd == ';':
			do_exec = True
		elif cmd[0] == '$':
			args.append( rs_exec(cmd[1:]) ) 
		else:
			args.append(cmd)
		if do_exec:
			result = rs_exec(*args)
			args = []
	return N, result

def rs_run(line):
	return rs_eval(rs_lex(line))

def rs_repl(prelude_files=None, readin=sys.stdin, writeout=sys.stdout):
	if prelude_files:
		for filename in prelude_files:
			with open(filename, 'rb') as handle:
				for line in handle:
					rs_run(line)
	# This can be replaced with:
	#	loop "echo > ; echo [ eval $line ]"
	while True:
		writeout.write('> ')
		writeout.flush()
		try:
			command = readin.readline()
		except KeyboardInterrupt:
			continue
		except EOFError:
			break
		if not command:
			break
		try:
			rs_run(command)
		except Exception as ex:
			traceback.print_exc()

if __name__ == "__main__":
	rs_repl(sys.argv[1:])