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Conflicting examples.

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This commit is contained in:
John Doty 2016-12-08 06:28:21 -08:00
parent 9781765043
commit 5a0b1654a9
1 changed files with 28 additions and 10 deletions
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38
parser.py
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@ -1,6 +1,7 @@
# This is doty playing with parser tables. # This is doty playing with parser tables.
from collections import namedtuple from collections import namedtuple
class Configuration( class Configuration(
namedtuple('Configuration', ['name', 'symbols', 'position']) namedtuple('Configuration', ['name', 'symbols', 'position'])
): ):
@ -188,8 +189,8 @@ class GenerateLR0(object):
def gen_table(self): def gen_table(self):
"""Generate the parse table. """Generate the parse table.
The parse table is a list of states. The first state in the list is the starting The parse table is a list of states. The first state in the list is
state. Each state is a dictionary that maps a symbol to an the starting state. Each state is a dictionary that maps a symbol to an
action. Each action is a tuple. The first element of the tuple is a action. Each action is a tuple. The first element of the tuple is a
string describing what to do: string describing what to do:
@ -259,16 +260,13 @@ class GenerateLR0(object):
there is already an action for the symbol in the row. there is already an action for the symbol in the row.
""" """
existing = row.get(symbol, None) existing = row.get(symbol, None)
if existing is not None: if existing is not None and existing != action:
raise ValueError( raise ValueError(
"Conflict: {old} vs {new}", "Conflict: {old} vs {new}".format(old=existing, new=action)
old=existing,
new=action,
) )
row[symbol] = action row[symbol] = action
def parse(table, input, trace=False): def parse(table, input, trace=False):
"""Parse the input with the generated parsing table and return the """Parse the input with the generated parsing table and return the
concrete syntax tree. concrete syntax tree.
@ -370,7 +368,7 @@ def format_table(generator, table):
return '\n'.join(lines) return '\n'.join(lines)
# OK, this is # OK, this is a very simple LR0 grammar.
grammar_simple = [ grammar_simple = [
('E', ['E', '+', 'T']), ('E', ['E', '+', 'T']),
('E', ['T']), ('E', ['T']),
@ -383,5 +381,25 @@ table = gen.gen_table()
tree = parse(table, ['id', '+', '(', 'id', ')']) tree = parse(table, ['id', '+', '(', 'id', ')'])
print(format_node(tree)) print(format_node(tree))
grammar_lr0_conflict = [ # This one doesn't work with LR0, though, it has a shift/reduce conflict.
] try:
grammar_lr0_conflict = grammar_simple + [
('T', ['id', '[', 'E', ']']),
]
gen = GenerateLR0(grammar_lr0_conflict, 'E')
table = gen.gen_table()
assert False
except ValueError as e:
print(e)
# Nor does this: it has a reduce/reduce conflict.
try:
grammar_lr0_conflict = grammar_simple + [
('E', ['V', '=', 'E']),
('V', ['id']),
]
gen = GenerateLR0(grammar_lr0_conflict, 'E')
table = gen.gen_table()
assert False
except ValueError as e:
print(e)