John Doty
5f89f460e5
The next step though will be to replace the Configuration with an integer, and intern all Configurations, along with all other objects.
1067 lines
37 KiB
Python
1067 lines
37 KiB
Python
"""I wanted to try to use the code in `parser.py` to do real work, and as you
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might expect the code did NOT work acceptibly.
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This version has some performance work done.
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2023
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"""
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import dataclasses
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import functools
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import typing
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###############################################################################
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# LR0
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#
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# We start with LR0 parsers, because they form the basis of everything else.
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###############################################################################
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class Configuration:
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"""A rule being tracked in a state.
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(Note: technically, lookahead isn't used until we get to LR(1) parsers,
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but if left at its default it's harmless. Ignore it until you get to
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the part about LR(1).)
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"""
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__slots__ = (
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'name',
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'symbols',
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'position',
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'lookahead',
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'next',
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'at_end',
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'_vals',
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'_hash',
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)
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name: str
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symbols: typing.Tuple[str, ...]
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position: int
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lookahead: typing.Tuple[str, ...]
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next: str | None
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at_end: bool
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_vals: typing.Tuple
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_hash: int
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def __init__(self, name, symbols, position, lookahead) -> None:
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self.name = name
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self.symbols = symbols
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self.position = position
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self.lookahead = lookahead
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at_end = position == len(symbols)
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self.at_end = at_end
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self.next = symbols[position] if not at_end else None
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self._vals = (name, symbols, position, lookahead)
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self._hash = hash(self._vals)
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@classmethod
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def from_rule(cls, name: str, symbols: typing.Tuple[str, ...], lookahead=()):
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return Configuration(
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name=name,
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symbols=symbols,
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position=0,
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lookahead=lookahead,
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)
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def __hash__(self) -> int:
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return self._hash
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def __eq__(self, value: object, /) -> bool:
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if value is self:
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return True
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if not isinstance(value, Configuration):
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return NotImplemented
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return (
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value._hash == self._hash and
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value.name == self.name and
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value.position == self.position and
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value.symbols == self.symbols and
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value.lookahead == self.lookahead
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)
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def __lt__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals < value._vals
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def __gt__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals > value._vals
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def __le__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals <= value._vals
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def __ge__(self, value) -> bool:
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if not isinstance(value, Configuration):
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return NotImplemented
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return self._vals >= value._vals
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def replace_position(self, new_position):
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return Configuration(
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name=self.name,
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symbols=self.symbols,
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position=new_position,
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lookahead=self.lookahead,
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)
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def clear_lookahead(self):
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return Configuration(
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name=self.name,
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symbols=self.symbols,
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position=self.position,
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lookahead=(),
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)
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@property
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def rest(self):
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return self.symbols[(self.position+1):]
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def at_symbol(self, symbol):
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return self.next == symbol
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def __str__(self):
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la = ", " + str(self.lookahead) if self.lookahead != () else ""
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return "{name} -> {bits}{lookahead}".format(
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name=self.name,
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bits=' '.join([
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'* ' + sym if i == self.position else sym
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for i, sym in enumerate(self.symbols)
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]) + (' *' if self.at_end else ''),
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lookahead=la,
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)
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ConfigSet = typing.Tuple[Configuration, ...]
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class TableBuilder(object):
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def __init__(self):
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self.errors = []
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self.table = []
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self.row = None
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def flush(self):
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self._flush_row()
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if len(self.errors) > 0:
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raise ValueError("\n\n".join(self.errors))
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return self.table
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def new_row(self, config_set):
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self._flush_row()
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self.row = {}
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self.current_config_set = config_set
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def _flush_row(self):
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if self.row:
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actions = {k: v[0] for k,v in self.row.items()}
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self.table.append(actions)
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def set_table_reduce(self, symbol, config):
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action = ('reduce', config.name, len(config.symbols))
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self._set_table_action(symbol, action, config)
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def set_table_accept(self, config):
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action = ('accept',)
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self._set_table_action('$', action, config)
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def set_table_shift(self, index, config):
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action = ('shift', index)
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self._set_table_action(config.next, action, config)
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def set_table_goto(self, symbol, index):
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action = ('goto', index)
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self._set_table_action(symbol, action, None)
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def _set_table_action(self, symbol, action, config):
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"""Set the action for 'symbol' in the table row to 'action'.
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This is destructive; it changes the table. It raises an error if
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there is already an action for the symbol in the row.
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"""
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assert self.row is not None
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existing, existing_config = self.row.get(symbol, (None, None))
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if existing is not None and existing != action:
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config_old = str(existing_config)
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config_new = str(config)
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max_len = max(len(config_old), len(config_new)) + 1
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error = (
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"Conflicting actions for token '{symbol}':\n"
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" {config_old: <{max_len}}: {old}\n"
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" {config_new: <{max_len}}: {new}\n".format(
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config_old=config_old,
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config_new=config_new,
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max_len=max_len,
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old=existing,
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new=action,
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symbol=symbol,
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)
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)
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self.errors.append(error)
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self.row[symbol] = (action, config)
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class GenerateLR0(object):
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"""Generate parser tables for an LR0 parser.
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The input grammars are of the form:
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grammar_simple = [
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('E', ['E', '+', 'T']),
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('E', ['T']),
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('T', ['(', 'E', ')']),
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('T', ['id']),
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]
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Which is to say, they are a list of productions. Each production is a
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tuple where the first element of the tuple is the name of the
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non-terminal being added, and the second elment of the tuple is the
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list of terminals and non-terminals that make up the production.
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There is currently no support for custom actions or alternation or
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anything like that. If you want alternations that you'll have to lower
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the grammar by hand into the simpler form first.
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Don't name anything with double-underscores; those are reserved for
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the generator. Don't add '$' either, as it is reserved to mean
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end-of-stream. Use an empty list to indicate nullability, that is:
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('O', []),
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means that O can be matched with nothing.
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Implementation notes:
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- This is implemented in the dumbest way possible, in order to be the
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most understandable it can be. I built this to learn, and I want to
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make sure I can keep learning with it.
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- We tend to use tuples everywhere. This is because tuples can be
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compared for equality and put into tables and all that jazz. They might
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be a little bit slower in places but like I said, this is for
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learning. (Also, if we need this to run faster we can probably go a
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long way by memoizing results, which is much easier if we have tuples
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everywhere.)
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"""
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grammar: dict[str, list[typing.Tuple[str, ...]]]
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nonterminals: set[str]
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terminals: set[str]
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alphabet: list[str]
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def __init__(self, start: str, grammar: list[typing.Tuple[str, list[str]]]):
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"""Initialize the parser generator with the specified grammar and
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start symbol.
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"""
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# Turn the incoming grammar into a dictionary, indexed by nonterminal.
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#
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# We count on python dictionaries retaining the insertion order, like
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# it or not.
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full_grammar = {}
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for name, rule in grammar:
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rules = full_grammar.get(name)
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if rules is None:
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rules = []
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full_grammar[name] = rules
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rules.append(tuple(rule))
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self.grammar = full_grammar
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self.nonterminals = set(self.grammar.keys())
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self.terminals = {
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sym
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for _, symbols in grammar
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for sym in symbols
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if sym not in self.nonterminals
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}
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self.alphabet = list(sorted(self.terminals | self.nonterminals))
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# Check to make sure they didn't use anything that will give us
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# heartburn later.
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reserved = [a for a in self.alphabet if a.startswith('__') or a == '$']
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if reserved:
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raise ValueError(
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"Can't use {symbols} in grammars, {what} reserved.".format(
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symbols=' or '.join(reserved),
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what="it's" if len(reserved) == 1 else "they're",
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)
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)
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self.grammar['__start'] = [(start,)]
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self.terminals.add('$')
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self.alphabet.append('$')
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@functools.cache
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def gen_closure_next(self, config: Configuration):
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"""Return the next set of configurations in the closure for
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config.
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If the position for config is just before a non-terminal, then the
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next set of configurations is configurations for all of the
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productions for that non-terminal, with the position at the
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beginning. (If the position for config is just before a terminal,
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or at the end of the production, then the next set is empty.)
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"""
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next = config.next
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if next is None:
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return ()
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else:
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return tuple(
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Configuration.from_rule(next, rule)
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for rule in self.grammar.get(next, ())
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)
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@functools.cache
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def gen_closure(self, seeds: typing.Iterable[Configuration]) -> ConfigSet:
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"""Compute the closure for the specified configs. The closure is all
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of the configurations we could be in. Specifically, if the position
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for a config is just before a non-terminal then we must also consider
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configurations where the rule is the rule for the non-terminal and
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the position is just before the beginning of the rule.
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(We have replaced a recursive version with an iterative one.)
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"""
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closure = set()
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pending = list(seeds)
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while len(pending) > 0:
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config = pending.pop()
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if config in closure:
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continue
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closure.add(config)
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for next_config in self.gen_closure_next(config):
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pending.append(next_config)
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return tuple(sorted(closure)) # TODO: Why tuple?
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@functools.cache
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def gen_successor(self, config_set: typing.Iterable[Configuration], symbol: str) -> ConfigSet:
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"""Compute the successor state for the given config set and the
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given symbol.
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The successor represents the next state of the parser after seeing
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the symbol.
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"""
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seeds = tuple(
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config.replace_position(config.position + 1)
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for config in config_set
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if config.at_symbol(symbol)
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)
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closure = self.gen_closure(seeds)
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return closure
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def gen_all_successors(self, config_set: typing.Iterable[Configuration]) -> list[ConfigSet]:
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"""Return all of the non-empty successors for the given config set."""
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next = []
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for symbol in self.alphabet:
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successor = self.gen_successor(config_set, symbol)
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if len(successor) > 0:
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next.append(successor)
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return next
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def gen_sets(self, config_set: typing.Tuple[Configuration,...]) -> typing.Tuple[ConfigSet, ...]:
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"""Generate all configuration sets starting from the provided set."""
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# NOTE: Not a set because we need to maintain insertion order!
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# The first element in the dictionary needs to be the intial
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# set.
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F = {}
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pending = [config_set]
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while len(pending) > 0:
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config_set = pending.pop()
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if config_set in F:
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continue
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# print(f"pending: {len(pending)} F: {len(F)}")
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F[config_set] = len(F)
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for successor in self.gen_all_successors(config_set):
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pending.append(successor)
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return tuple(F.keys())
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def gen_all_sets(self) -> typing.Tuple[ConfigSet, ...]:
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"""Generate all of the configuration sets for the grammar."""
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seeds = tuple(
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Configuration.from_rule('__start', rule)
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for rule in self.grammar['__start']
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)
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initial_set = self.gen_closure(seeds)
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return self.gen_sets(initial_set)
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def build_set_index(self, sets: typing.Tuple[ConfigSet, ...]) -> dict[ConfigSet, int]:
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return { s: index for index, s in enumerate(sets) }
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def find_set_index(self, sets: dict[ConfigSet, int], s: ConfigSet) -> int | None:
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"""Find the specified set in the set of sets, and return the
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index, or None if it is not found.
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"""
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return sets.get(s)
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def gen_reduce_set(self, config: Configuration) -> typing.Iterable[str]:
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"""Return the set of symbols that indicate we should reduce the given
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configuration.
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In an LR0 parser, this is just the set of all terminals."""
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del(config)
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return self.terminals
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def gen_table(self):
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"""Generate the parse table.
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The parse table is a list of states. The first state in the list is
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the starting state. Each state is a dictionary that maps a symbol to an
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action. Each action is a tuple. The first element of the tuple is a
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string describing what to do:
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- 'shift': The second element of the tuple is the state
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number. Consume the input and push that state onto the stack.
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- 'reduce': The second element is the name of the non-terminal being
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reduced, and the third element is the number of states to remove
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from the stack. Don't consume the input; just remove the specified
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number of things from the stack, and then consult the table again,
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this time using the new top-of-stack as the current state and the
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name of the non-terminal to find out what to do.
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- 'goto': The second element is the state number to push onto the
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stack. In the literature, these entries are treated distinctly from
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the actions, but we mix them here because they never overlap with the
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other actions. (These are always associated with non-terminals, and
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the other actions are always associated with terminals.)
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- 'accept': Accept the result of the parse, it worked.
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Anything missing from the row indicates an error.
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"""
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builder = TableBuilder()
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config_sets = self.gen_all_sets()
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set_index = self.build_set_index(config_sets)
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for config_set in config_sets:
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builder.new_row(config_set)
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# Actions
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for config in config_set:
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if config.at_end:
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if config.name != '__start':
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for a in self.gen_reduce_set(config):
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builder.set_table_reduce(a, config)
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else:
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builder.set_table_accept(config)
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else:
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if config.next in self.terminals:
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successor = self.gen_successor(config_set, config.next)
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index = self.find_set_index(set_index, successor)
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assert index is not None
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builder.set_table_shift(index, config)
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# Gotos
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for symbol in self.nonterminals:
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successor = self.gen_successor(config_set, symbol)
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index = self.find_set_index(set_index, successor)
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if index is not None:
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builder.set_table_goto(symbol, index)
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return builder.flush()
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def set_table_action(self, errors, row, symbol, action, config):
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"""Set the action for 'symbol' in the table row to 'action'.
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This is destructive; it changes the table. It raises an error if
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there is already an action for the symbol in the row.
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"""
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existing, existing_config = row.get(symbol, (None, None))
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if existing is not None and existing != action:
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config_old = str(existing_config)
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config_new = str(config)
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max_len = max(len(config_old), len(config_new)) + 1
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error = (
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"Conflicting actions for token '{symbol}':\n"
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" {config_old: <{max_len}}: {old}\n"
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" {config_new: <{max_len}}: {new}\n".format(
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config_old=config_old,
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config_new=config_new,
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max_len=max_len,
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old=existing,
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new=action,
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symbol=symbol,
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)
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)
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errors.append(error)
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row[symbol] = (action, config)
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|
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def parse(table, input, trace=False):
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"""Parse the input with the generated parsing table and return the
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concrete syntax tree.
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The parsing table can be generated by GenerateLR0.gen_table() or by any
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of the other generators below. The parsing mechanism never changes, only
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the table generation mechanism.
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input is a list of tokens. Don't stick an end-of-stream marker, I'll stick
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one on for you.
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"""
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assert '$' not in input
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input = input + ['$']
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input_index = 0
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# Our stack is a stack of tuples, where the first entry is the state number
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# and the second entry is the 'value' that was generated when the state was
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# pushed.
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stack : list[typing.Tuple[int, typing.Any]] = [(0, None)]
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while True:
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current_state = stack[-1][0]
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current_token = input[input_index]
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action = table[current_state].get(current_token, ('error',))
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if trace:
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print("{stack: <20} {input: <50} {action: <5}".format(
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stack=repr([s[0] for s in stack]),
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input=repr(input[input_index:]),
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action=repr(action)
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))
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if action[0] == 'accept':
|
|
return stack[-1][1]
|
|
|
|
elif action[0] == 'reduce':
|
|
name = action[1]
|
|
size = action[2]
|
|
|
|
value = (name, tuple(s[1] for s in stack[-size:]))
|
|
stack = stack[:-size]
|
|
|
|
goto = table[stack[-1][0]].get(name, ('error',))
|
|
assert goto[0] == 'goto' # Corrupt table?
|
|
stack.append((goto[1], value))
|
|
|
|
elif action[0] == 'shift':
|
|
stack.append((action[1], (current_token, ())))
|
|
input_index += 1
|
|
|
|
elif action[0] == 'error':
|
|
raise ValueError(
|
|
'Syntax error: unexpected symbol {sym}'.format(
|
|
sym=current_token,
|
|
),
|
|
)
|
|
|
|
|
|
###############################################################################
|
|
# SLR(1)
|
|
###############################################################################
|
|
def add_changed(items: set, item)->bool:
|
|
old_len = len(items)
|
|
items.add(item)
|
|
return old_len != len(items)
|
|
|
|
def update_changed(items: set, other: set) -> bool:
|
|
old_len = len(items)
|
|
items.update(other)
|
|
return old_len != len(items)
|
|
|
|
@dataclasses.dataclass(frozen=True)
|
|
class FirstInfo:
|
|
firsts: dict[str, set[str]]
|
|
is_epsilon: set[str]
|
|
|
|
@classmethod
|
|
def from_grammar(
|
|
cls,
|
|
grammar: dict[str, list[typing.Tuple[str,...]]],
|
|
terminals: set[str],
|
|
):
|
|
firsts = {name: set() for name in grammar.keys()}
|
|
for t in terminals:
|
|
firsts[t] = {t}
|
|
|
|
epsilons = set()
|
|
changed = True
|
|
while changed:
|
|
changed = False
|
|
for name, rules in grammar.items():
|
|
f = firsts[name]
|
|
for rule in rules:
|
|
if len(rule) == 0:
|
|
changed = add_changed(epsilons, name) or changed
|
|
continue
|
|
|
|
for index, symbol in enumerate(rule):
|
|
if symbol in terminals:
|
|
changed = add_changed(f, symbol) or changed
|
|
else:
|
|
other_firsts = firsts[symbol]
|
|
changed = update_changed(f, other_firsts) or changed
|
|
|
|
is_last = index == len(rule) - 1
|
|
if is_last and symbol in epsilons:
|
|
# If this is the last symbol and the last
|
|
# symbol can be empty then I can be empty
|
|
# too! :P
|
|
changed = add_changed(epsilons, name) or changed
|
|
|
|
if symbol not in epsilons:
|
|
# If we believe that there is at least one
|
|
# terminal in the first set of this
|
|
# nonterminal then I don't have to keep
|
|
# looping through the symbols in this rule.
|
|
break
|
|
|
|
return FirstInfo(firsts=firsts, is_epsilon=epsilons)
|
|
|
|
@dataclasses.dataclass(frozen=True)
|
|
class FollowInfo:
|
|
follows: dict[str, set[str]]
|
|
|
|
@classmethod
|
|
def from_grammar(
|
|
cls,
|
|
grammar: dict[str, list[typing.Tuple[str,...]]],
|
|
firsts: FirstInfo,
|
|
):
|
|
follows = {name: set() for name in grammar.keys()}
|
|
follows["__start"].add('$')
|
|
|
|
changed = True
|
|
while changed:
|
|
changed = False
|
|
for name, rules in grammar.items():
|
|
for rule in rules:
|
|
epsilon = True
|
|
prev_symbol = None
|
|
for symbol in reversed(rule):
|
|
f = follows.get(symbol)
|
|
if f is None:
|
|
# This particular rule can't produce epsilon.
|
|
epsilon = False
|
|
prev_symbol = symbol
|
|
continue
|
|
|
|
# While epsilon is still set, update the follow of
|
|
# this nonterminal with the follow of the production
|
|
# we're processing. (This also means that the follow
|
|
# of the last symbol in the production is the follow
|
|
# of the entire production, as it should be.)
|
|
if epsilon:
|
|
changed = update_changed(f, follows[name]) or changed
|
|
|
|
# If we're not at the end of the list then the follow
|
|
# of the current symbol contains the first of the
|
|
# next symbol.
|
|
if prev_symbol is not None:
|
|
changed = update_changed(f, firsts.firsts[prev_symbol]) or changed
|
|
|
|
# Now if there's no epsilon in this symbol there's no
|
|
# more epsilon in the rest of the sequence.
|
|
if symbol not in firsts.is_epsilon:
|
|
epsilon = False
|
|
|
|
prev_symbol = symbol
|
|
|
|
return FollowInfo(follows=follows)
|
|
|
|
|
|
|
|
class GenerateSLR1(GenerateLR0):
|
|
"""Generate parse tables for SLR1 grammars.
|
|
|
|
SLR1 parsers can recognize more than LR0 parsers, because they have a
|
|
little bit more information: instead of generating reduce actions for a
|
|
production on all possible inputs, as LR0 parsers do, they generate
|
|
reduce actions only for inputs that are in the 'follow' set of the
|
|
non-terminal.
|
|
|
|
That means SLR1 parsers need to know how to generate 'follow(A)', which
|
|
means they need to know how to generate 'first(A)', which is most of the
|
|
code in this class.
|
|
"""
|
|
_firsts: FirstInfo
|
|
|
|
def __init__(self, *args, **kwargs):
|
|
super().__init__(*args, **kwargs)
|
|
self._firsts = FirstInfo.from_grammar(self.grammar, self.terminals)
|
|
self._follows = FollowInfo.from_grammar(self.grammar, self._firsts)
|
|
|
|
def gen_first(self, symbols: typing.Iterable[str]) -> typing.Tuple[set[str], bool]:
|
|
"""Return the first set for a sequence of symbols.
|
|
|
|
Build the set by combining the first sets of the symbols from left to
|
|
right as long as epsilon remains in the first set. If we reach the end
|
|
and every symbol has had epsilon, then this set also has epsilon.
|
|
|
|
Otherwise we can stop as soon as we get to a non-epsilon first(), and
|
|
our result does not have epsilon.
|
|
"""
|
|
result = set()
|
|
for s in symbols:
|
|
result.update(self._firsts.firsts[s])
|
|
if s not in self._firsts.is_epsilon:
|
|
return (result, False)
|
|
|
|
return (result, True)
|
|
|
|
def gen_follow(self, symbol: str) -> set[str]:
|
|
"""Generate the follow set for the given nonterminal.
|
|
|
|
The follow set for a nonterminal is the set of terminals that can
|
|
follow the nonterminal in a valid sentence. The resulting set never
|
|
contains epsilon and is never empty, since we should always at least
|
|
ground out at '$', which is the end-of-stream marker.
|
|
"""
|
|
assert symbol in self.grammar
|
|
return self._follows.follows[symbol]
|
|
|
|
def gen_reduce_set(self, config: Configuration) -> typing.Iterable[str]:
|
|
"""Return the set of symbols that indicate we should reduce the given
|
|
config.
|
|
|
|
In an SLR1 parser, this is the follow set of the config nonterminal."""
|
|
return self.gen_follow(config.name)
|
|
|
|
|
|
class GenerateLR1(GenerateSLR1):
|
|
"""Generate parse tables for LR1, or "canonical LR" grammars.
|
|
|
|
LR1 parsers can recognize more than SLR parsers. Like SLR parsers, they
|
|
are choosier about when they reduce. But unlike SLR parsers, they specify
|
|
the terminals on which they reduce by carrying a 'lookahead' terminal in
|
|
the configuration. The lookahead of a configuration is computed as the
|
|
closure of a configuration set is computed, so see gen_closure_next for
|
|
details. (Except for the start configuration, which has '$' as its
|
|
lookahead.)
|
|
"""
|
|
def gen_reduce_set(self, config: Configuration) -> typing.Iterable[str]:
|
|
"""Return the set of symbols that indicate we should reduce the given
|
|
config.
|
|
|
|
In an LR1 parser, this is the lookahead of the configuration."""
|
|
return config.lookahead
|
|
|
|
@functools.cache
|
|
def gen_closure_next(self, config: Configuration):
|
|
"""Return the next set of configurations in the closure for
|
|
config.
|
|
|
|
In LR1 parsers, we must compute the lookahead for the configurations
|
|
we're adding to the closure. The lookahead for the new configurations
|
|
is the first() of the rest of this config's production. If that
|
|
contains epsilon, then the lookahead *also* contains the lookahead we
|
|
already have. (This lookahead was presumably generated by the same
|
|
process, so in some sense it is a 'parent' lookahead, or a lookahead
|
|
from an upstream production in the grammar.)
|
|
|
|
(See the documentation in GenerateLR0 for more information on how
|
|
this function fits into the whole process.)
|
|
"""
|
|
config_next = config.next
|
|
if config_next is None:
|
|
return ()
|
|
else:
|
|
next = []
|
|
for rule in self.grammar.get(config_next, ()):
|
|
lookahead, epsilon = self.gen_first(config.rest)
|
|
if epsilon:
|
|
lookahead.update(config.lookahead)
|
|
lookahead = tuple(sorted(lookahead))
|
|
next.append(Configuration.from_rule(config_next, rule, lookahead=lookahead))
|
|
|
|
return tuple(next)
|
|
|
|
def gen_all_sets(self):
|
|
"""Generate all of the configuration sets for the grammar.
|
|
|
|
In LR1 parsers, we must remember to set the lookahead of the start
|
|
symbol to '$'.
|
|
"""
|
|
seeds = tuple(
|
|
Configuration.from_rule('__start', rule, lookahead=('$',))
|
|
for rule in self.grammar['__start']
|
|
)
|
|
initial_set = self.gen_closure(seeds)
|
|
return self.gen_sets(initial_set)
|
|
|
|
|
|
class GenerateLALR(GenerateLR1):
|
|
"""Generate tables for LALR.
|
|
|
|
LALR is smaller than LR(1) but bigger than SLR(1). It works by generating
|
|
the LR(1) configuration sets, but merging configuration sets which are
|
|
equal in everything but their lookaheads. This works in that it doesn't
|
|
generate any shift/reduce conflicts that weren't already in the LR(1)
|
|
grammar. It can, however, introduce new reduce/reduce conflicts, because
|
|
it does lose information. The advantage is that the number of parser
|
|
states is much much smaller in LALR than in LR(1).
|
|
|
|
(Note that because we use immutable state everywhere this generator does
|
|
a lot of copying and allocation.)
|
|
"""
|
|
def merge_sets(self, config_set_a, config_set_b):
|
|
"""Merge the two config sets, by keeping the item cores but merging
|
|
the lookahead sets for each item.
|
|
"""
|
|
assert len(config_set_a) == len(config_set_b)
|
|
merged = []
|
|
for index, a in enumerate(config_set_a):
|
|
b = config_set_b[index]
|
|
assert a.clear_lookahead() == b.clear_lookahead()
|
|
|
|
new_lookahead = a.lookahead + b.lookahead
|
|
new_lookahead = tuple(sorted(set(new_lookahead)))
|
|
merged.append(a.clear_lookahead())
|
|
|
|
return tuple(merged)
|
|
|
|
def sets_equal(self, a, b):
|
|
a_no_la = tuple(s.clear_lookahead() for s in a)
|
|
b_no_la = tuple(s.clear_lookahead() for s in b)
|
|
return a_no_la == b_no_la
|
|
|
|
def gen_sets(self, config_set):
|
|
"""Recursively generate all configuration sets starting from the
|
|
provided set, and merge them with the provided set 'F'.
|
|
|
|
The difference between this method and the one in GenerateLR0, where
|
|
this comes from, is in the part that stops recursion. In LALR we
|
|
compare for set equality *ignoring lookahead*. If we find a match,
|
|
then instead of returning F unchanged, we merge the two equal sets
|
|
and replace the set in F, returning the modified set.
|
|
"""
|
|
F = {}
|
|
pending = [config_set]
|
|
while len(pending) > 0:
|
|
config_set = pending.pop()
|
|
config_set_no_la = tuple(s.clear_lookahead() for s in config_set)
|
|
|
|
existing = F.get(config_set_no_la)
|
|
if existing is not None:
|
|
F[config_set_no_la] = self.merge_sets(config_set, existing)
|
|
else:
|
|
F[config_set_no_la] = config_set
|
|
for successor in self.gen_all_successors(config_set):
|
|
pending.append(successor)
|
|
|
|
# NOTE: We count on insertion order here! The first element must be the
|
|
# starting state!
|
|
return tuple(F.values())
|
|
|
|
def set_without_lookahead(self, config_set: ConfigSet) -> ConfigSet:
|
|
return tuple(sorted(set(c.clear_lookahead() for c in config_set)))
|
|
|
|
def build_set_index(self, sets: typing.Tuple[ConfigSet, ...]) -> dict[ConfigSet, int]:
|
|
index = {}
|
|
for s in sets:
|
|
s_no_la = self.set_without_lookahead(s)
|
|
if s_no_la not in index:
|
|
index[s_no_la] = len(index)
|
|
return index
|
|
|
|
def find_set_index(self, sets: dict[ConfigSet, int], s: ConfigSet) -> int | None:
|
|
"""Find the specified set in the set of sets, and return the
|
|
index, or None if it is not found.
|
|
"""
|
|
s_no_la = self.set_without_lookahead(s)
|
|
return sets.get(s_no_la)
|
|
|
|
|
|
###############################################################################
|
|
# Formatting
|
|
###############################################################################
|
|
def format_node(node):
|
|
"""Print out an indented concrete syntax tree, from parse()."""
|
|
lines = [
|
|
'{name}'.format(name=node[0])
|
|
] + [
|
|
' ' + line
|
|
for child in node[1]
|
|
for line in format_node(child).split('\n')
|
|
]
|
|
return '\n'.join(lines)
|
|
|
|
|
|
def format_table(generator, table):
|
|
"""Format a parser table so pretty."""
|
|
def format_action(state, terminal):
|
|
action = state.get(terminal, ('error',))
|
|
if action[0] == 'accept':
|
|
return 'accept'
|
|
elif action[0] == 'shift':
|
|
return 's' + str(action[1])
|
|
elif action[0] == 'error':
|
|
return ''
|
|
elif action[0] == 'reduce':
|
|
return 'r' + str(action[1])
|
|
|
|
header = " | {terms} | {nts}".format(
|
|
terms=' '.join(
|
|
'{0: <6}'.format(terminal)
|
|
for terminal in sorted(generator.terminals)
|
|
),
|
|
nts=' '.join(
|
|
'{0: <5}'.format(nt)
|
|
for nt in sorted(generator.nonterminals)
|
|
),
|
|
)
|
|
|
|
lines = [
|
|
header,
|
|
'-' * len(header),
|
|
] + [
|
|
"{index: <3} | {actions} | {gotos}".format(
|
|
index=i,
|
|
actions=' '.join(
|
|
'{0: <6}'.format(format_action(row, terminal))
|
|
for terminal in sorted(generator.terminals)
|
|
),
|
|
gotos=' '.join(
|
|
'{0: <5}'.format(row.get(nt, ('error', ''))[1])
|
|
for nt in sorted(generator.nonterminals)
|
|
),
|
|
)
|
|
for i, row in enumerate(table)
|
|
]
|
|
return '\n'.join(lines)
|
|
|
|
|
|
###############################################################################
|
|
# Examples
|
|
###############################################################################
|
|
def examples():
|
|
# OK, this is a very simple LR0 grammar.
|
|
print("grammar_simple:")
|
|
grammar_simple = [
|
|
('E', ['E', '+', 'T']),
|
|
('E', ['T']),
|
|
('T', ['(', 'E', ')']),
|
|
('T', ['id']),
|
|
]
|
|
|
|
gen = GenerateLR0('E', grammar_simple)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
tree = parse(table, ['id', '+', '(', 'id', ')'])
|
|
print(format_node(tree) + "\n")
|
|
print()
|
|
|
|
# This one doesn't work with LR0, though, it has a shift/reduce conflict.
|
|
print("grammar_lr0_shift_reduce (LR0):")
|
|
grammar_lr0_shift_reduce = grammar_simple + [
|
|
('T', ['id', '[', 'E', ']']),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_lr0_shift_reduce)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Nor does this: it has a reduce/reduce conflict.
|
|
print("grammar_lr0_reduce_reduce (LR0):")
|
|
grammar_lr0_reduce_reduce = grammar_simple + [
|
|
('E', ['V', '=', 'E']),
|
|
('V', ['id']),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_lr0_reduce_reduce)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Nullable symbols just don't work with constructs like this, because you can't
|
|
# look ahead to figure out if you should reduce an empty 'F' or not.
|
|
print("grammar_nullable (LR0):")
|
|
grammar_nullable = [
|
|
('E', ['F', 'boop']),
|
|
('F', ['beep']),
|
|
('F', []),
|
|
]
|
|
try:
|
|
gen = GenerateLR0('E', grammar_nullable)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
|
|
print("grammar_lr0_shift_reduce (SLR1):")
|
|
gen = GenerateSLR1('E', grammar_lr0_shift_reduce)
|
|
first, epsilon=gen.gen_first(('E',))
|
|
print(f"First: {str(first)} (epsilon={epsilon})")
|
|
print(f"Follow: {str(gen.gen_follow('E'))}")
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
tree = parse(table, ['id', '+', '(', 'id', '[', 'id', ']', ')'], trace=True)
|
|
print(format_node(tree) + "\n")
|
|
print()
|
|
|
|
# SLR1 can't handle this.
|
|
print("grammar_aho_ullman_1 (SLR1):")
|
|
grammar_aho_ullman_1 = [
|
|
('S', ['L', '=', 'R']),
|
|
('S', ['R']),
|
|
('L', ['*', 'R']),
|
|
('L', ['id']),
|
|
('R', ['L']),
|
|
]
|
|
try:
|
|
gen = GenerateSLR1('S', grammar_aho_ullman_1)
|
|
table = gen.gen_table()
|
|
assert False
|
|
except ValueError as e:
|
|
print(e)
|
|
print()
|
|
|
|
# Here's an example with a full LR1 grammar, though.
|
|
print("grammar_aho_ullman_2 (LR1):")
|
|
grammar_aho_ullman_2 = [
|
|
('S', ['X', 'X']),
|
|
('X', ['a', 'X']),
|
|
('X', ['b']),
|
|
]
|
|
gen = GenerateLR1('S', grammar_aho_ullman_2)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
parse(table, ['b', 'a', 'a', 'b'], trace=True)
|
|
print()
|
|
|
|
# What happens if we do LALR to it?
|
|
print("grammar_aho_ullman_2 (LALR):")
|
|
gen = GenerateLALR('S', grammar_aho_ullman_2)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
print()
|
|
|
|
# A fun LALAR grammar.
|
|
print("grammar_lalr:")
|
|
grammar_lalr = [
|
|
('S', ['V', 'E']),
|
|
|
|
('E', ['F']),
|
|
('E', ['E', '+', 'F']),
|
|
|
|
('F', ['V']),
|
|
('F', ['int']),
|
|
('F', ['(', 'E', ')']),
|
|
|
|
('V', ['id']),
|
|
]
|
|
gen = GenerateLALR('S', grammar_lalr)
|
|
table = gen.gen_table()
|
|
print(format_table(gen, table))
|
|
print()
|
|
|
|
if __name__=="__main__":
|
|
examples()
|