def demo():
"""
A demonstration showing how PCFG C{Grammar}s can be created and used.
"""
from nltk_lite.corpora import treebank, extract
from nltk_lite.parse import cfg, pcfg, pchart, treetransforms
from itertools import islice
# Create some probabilistic CFG Productions
S, A, B, C = cfg.nonterminals('S A B C')
pcfg_prods = [pcfg.Production(A, [B, B], prob=0.3),
pcfg.Production(A, [C, B, C], prob=0.7),
pcfg.Production(B, [B, 'b'], prob=0.5),
pcfg.Production(B, [C], prob=0.5),
pcfg.Production(C, ['a'], prob=0.1),
pcfg.Production(C, ['b'], prob=0.9)]
pcfg_prod = pcfg_prods[2]
print 'A PCFG production:', `pcfg_prod`
print ' pcfg_prod.lhs() =>', `pcfg_prod.lhs()`
print ' pcfg_prod.rhs() =>', `pcfg_prod.rhs()`
print ' pcfg_prod.prob() =>', `pcfg_prod.prob()`
print
# Create and print a PCFG
grammar = pcfg.Grammar(S, pcfg_prods)
print 'A PCFG grammar:', `grammar`
print ' grammar.start() =>', `grammar.start()`
print ' grammar.productions() =>',
# Use string.replace(...) is to line-wrap the output.
print `grammar.productions()`.replace(',', ',\n'+' '*26)
print
# extract productions from three trees and induce the PCFG
print "Induce PCFG grammar from treebank data:"
productions = []
for tree in islice(treebank.parsed(),3):
# perform optional in-place tree transformations, e.g.:
# treetransforms.collapseUnary(tree, collapsePOS = False)
# treetransforms.chomskyNormalForm(tree, horzMarkov = 2)
productions += tree.productions()
grammar = pcfg.induce(S, productions)
print grammar
print
print "Parse sentence using induced grammar:"
parser = pchart.InsideParse(grammar)
parser.trace(3)
sent = extract(0, treebank.raw())
print sent
for parse in parser.get_parse_list(sent):
print parse
