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#######################################################################
# Name: calc_peg.py
# Purpose: Simple expression evaluator example using PEG language
# Author: Igor R. Dejanovic <igor DOT dejanovic AT gmail DOT com>
# Copyright: (c) 2009 Igor R. Dejanovic <igor DOT dejanovic AT gmail DOT com>
# License: MIT License
#
# This example is functionally equivalent to calc.py. The difference is that
# in this example grammar is specified using PEG language instead of python constructs.
# Semantic actions are used to calculate expression during semantic
# analysis.
# Parser model as well as parse tree exported to dot files should be
# the same as parser model and parse tree generated in calc.py example.
#######################################################################
from arpeggio import *
from arpeggio.peg import ParserPEG
from arpeggio.export import PMDOTExport, PTDOTExport
import logging
# Semantic actions
from calc import ToFloat, Factor, Term, Expr, Calc
# Grammar is defined using textual specification based on PEG language.
calc_grammar = """
number <- r'\d*\.\d*|\d+';
factor <- number / "(" expression ")";
term <- factor (( "*" / "/") factor)*;
expression <- ("+" / "-")? term (("+" / "-") term)*;
calc <- expression EndOfFile;
"""
# Rules are mapped to semantic actions
sem_actions = {
"number" : ToFloat(),
"factor" : Factor(),
"term" : Term(),
"expression" : Expr(),
"calc" : Calc()
}
try:
logging.basicConfig(level=logging.DEBUG)
# First we will make a parser - an instance of the calc parser model.
# Parser model is given in the form of PEG notation therefore we
# are using ParserPEG class. Root rule name (parsing expression) is "calc".
parser = ParserPEG(calc_grammar, "calc")
# Then we export it to a dot file.
PMDOTExport().exportFile(parser.parser_model,
"calc_peg_parser_model.dot")
# An expression we want to evaluate
input = "-(4-1)*5+(2+4.67)+5.89/(.2+7)"
# Then parse tree is created out of the input expression.
parse_tree = parser.parse(input)
# We save it to dot file in order to visualise it.
PTDOTExport().exportFile(parse_tree,
"calc_peg_parse_tree.dot")
# getASG will start semantic analysis.
# In this case semantic analysis will evaluate expression and
# returned value will be evaluated result of the input expression.
# Semantic actions are supplied to the getASG function.
print "%s = %f" % (input, parser.getASG(sem_actions))
except NoMatch, e:
print "Expected %s at position %s." % (e.value, str(e.parser.pos_to_linecol(e.position)))