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# -*- coding: utf-8 -*-
import re
def unparse(ast):
"""Convert an AST back into the corresponding lisp expression"""
if isinstance(ast, str): return ast
elif len(ast) > 0 and ast[0] == "quote":
return "'%s" % unparse(ast[1])
return "(%s)" % " ".join([unparse(x) for x in ast])
def parse(source):
"""Parse string representation of one single expression
into the corresponding Abstract Syntax Tree"""
exp, rest = partition_exp(source)
if rest:
raise SyntaxError('Expected EOF')
elif exp[0] == "'":
return ["quote", parse(exp[1:])]
elif exp[0] == "(":
end = find_matching_paren(exp)
return [parse(e) for e in split_exps(exp[1:end])]
return exp
def parse_multiple(source):
"""Creates a list of ASTs from program source
constituting multiple expressions"""
return [parse(exp) for exp in split_exps(source)]
def split_exps(source):
"""Splits a source string into subexpressions
that can be parsed individually"""
rest = source.strip()
exps = []
while rest:
exp, rest = partition_exp(rest)
return exps
def partition_exp(source):
"""Split string into (exp, rest) where exp is the
first expression in the string and rest is the
rest of the string after this expression."""
source = source.strip()
if source[0] == "'":
exp, rest = partition_exp(source[1:])
return "'" + exp, rest
elif source[0] == "(":
last = find_matching_paren(source)
return source[:last + 1], source[last + 1:]
match = re.match(r"^[^\s)']+", source)
end = match.end()
atom = source[:end]
return atom, source[end:]
def find_matching_paren(source, start=0):
"""Given a string and the index of an opening parenthesis, determine
the index of the matching closing paren"""
assert source[start] == '('
pos = start
open_brackets = 1
while open_brackets > 0:
pos += 1
if len(source) == pos:
raise SyntaxError("Unbalanced expression: %s" % source[start:])
if source[pos] == '(':
open_brackets += 1
if source[pos] == ')':
open_brackets -= 1
return pos