Tiddly Lisp

#### tiddlylisp.py # # Based on Peter Norvig's lispy (http://norvig.com/lispy.html), # copyright by Peter Norvig, 2010. # # Adaptations by Michael Nielsen. See # http://michaelnielsen.org/ddi/lisp-as-the-maxwells-equations-of-software/ import sys import traceback #### Symbol, Env classes Symbol = str class Env(dict): "An environment: a dict of {'var':val} pairs, with an outer Env." def __init__(self, params=(), args=(), outer=None): self.update(zip(params, args)) self.outer = outer def find(self, var): "Find the innermost Env where var appears." return self if var in self else self.outer.find(var) def add_globals(env): "Add some built-in procedures and variables to the environment." import operator env.update( {'+': operator.add, '-': operator.sub, '*': operator.mul, '/': operator.div, '>': operator.gt, '<': operator.lt, '>=': operator.ge, '<=': operator.le, '=': operator.eq }) env.update({'True': True, 'False': False}) return env global_env = add_globals(Env()) isa = isinstance #### eval def eval(x, env=global_env): "Evaluate an expression in an environment." if isa(x, Symbol): # variable reference return env.find(x)[x] elif not isa(x, list): # constant literal return x elif x[0] == 'quote' or x[0] == 'q': # (quote exp), or (q exp) (_, exp) = x return exp elif x[0] == 'atom?': # (atom? exp)

(_, exp) = x return not isa(eval(exp, env), list) elif x[0] == 'eq?': # (eq? exp1 exp2) (_, exp1, exp2) = x v1, v2 = eval(exp1, env), eval(exp2, env) return (not isa(v1, list)) and (v1 == v2) elif x[0] == 'car': # (car exp) (_, exp) = x return eval(exp, env)[0] elif x[0] == 'cdr': # (cdr exp) (_, exp) = x return eval(exp, env)[1:] elif x[0] == 'cons': # (cons exp1 exp2) (_, exp1, exp2) = x return [eval(exp1, env)]+eval(exp2,env) elif x[0] == 'cond': # (cond (p1 e1) ... (pn en)) for (p, e) in x[1:]: if eval(p, env): return eval(e, env) elif x[0] == 'null?': # (null? exp) (_, exp) = x return eval(exp,env) == [] elif x[0] == 'if': # (if test conseq alt) (_, test, conseq, alt) = x return eval((conseq if eval(test, env) else alt), env) elif x[0] == 'set!': # (set! var exp) (_, var, exp) = x env.find(var)[var] = eval(exp, env) elif x[0] == 'define': # (define var exp) (_, var, exp) = x env[var] = eval(exp, env) elif x[0] == 'lambda': # (lambda (var*) exp) (_, vars, exp) = x return lambda *args: eval(exp, Env(vars, args, env)) elif x[0] == 'begin': # (begin exp*) for exp in x[1:]: val = eval(exp, env) return val else: # (proc exp*) exps = [eval(exp, env) for exp in x] proc = exps.pop(0) return proc(*exps) #### parsing def parse(s): "Parse a Lisp expression from a string." return read_from(tokenize(s)) def tokenize(s): "Convert a string into a list of tokens." return s.replace("(", " ( ").replace(")", " ) ").split() def read_from(tokens): "Read an expression from a sequence of tokens." if len(tokens) == 0: raise SyntaxError('unexpected EOF while reading') token = tokens.pop(0) if '(' == token:

L = [] while tokens[0] != ')': L.append(read_from(tokens)) tokens.pop(0) # pop off ')' return L elif ')' == token: raise SyntaxError('unexpected )') else: return atom(token) def atom(token): "Numbers become numbers; every other token is a symbol." try: return int(token) except ValueError: try: return float(token) except ValueError: return Symbol(token) def to_string(exp): "Convert a Python object back into a Lisp-readable string." if not isa(exp, list): return str(exp) else: return '('+' '.join(map(to_string, exp))+')' #### Load from a file and run def load(filename): """ Load the tiddlylisp program in filename, execute it, and start the repl. If an error occurs, execution stops, and we are left in the repl. Note that load copes with multi-line tiddlylisp code by merging lines until the number of opening and closing parentheses match. """ print "Loading and executing %s" % filename f = open(filename, "r") program = f.readlines() f.close() rps = running_paren_sums(program) full_line = "" for (paren_sum, program_line) in zip(rps, program): program_line = program_line.strip() full_line += program_line+" " if paren_sum == 0 and full_line.strip() != "": try: val = eval(parse(full_line)) if val is not None: print to_string(val) except: handle_error() print "\nThe line in which the error occurred:\n%s" % full_line break full_line = "" repl() def running_paren_sums(program): """ Map the lines in the list program to a list whose entries contain a running sum of the per-line difference between the number of '('

parentheses and the number of ')' parentheses. """ count_open_parens = lambda line: line.count("(")-line.count(")") paren_counts = map(count_open_parens, program) rps = [] total = 0 for paren_count in paren_counts: total += paren_count rps.append(total) return rps #### repl def repl(prompt='tiddlylisp> '): "A prompt-read-eval-print loop." while True: try: val = eval(parse(raw_input(prompt))) if val is not None: print to_string(val) except KeyboardInterrupt: print "\nExiting tiddlylisp\n" sys.exit() except: handle_error() #### error handling def handle_error(): """ Simple error handling for both the repl and load. """ print "An error occurred. Here's the Python stack trace:\n" traceback.print_exc() #### on startup from the command line if __name__ == "__main__": if len(sys.argv) > 1: load(sys.argv[1]) else: repl()