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Python Implementation of Backtracking Recursive Descent Parser based loosely on Language Implementation Patterns
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| list : '[' elements ']'; | |
| elements : element (',' element)*; | |
| element : NAME | assign | list | list_assign; | |
| assign : NAME '=' NAME; | |
| list_assign : list '=' list; | |
| NAME : ('a'..'z' | 'A'..'Z')+; |
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| from contextlib import contextmanager | |
| class Peekable: | |
| def __init__(self, input_, k, sentinel=None): | |
| self.sentinel = sentinel | |
| self._k = k | |
| self._stream = iter(input_) | |
| self._peek = [next(self._stream, sentinel) for _ in range(k)] | |
| def __getitem__(self, n): | |
| if isinstance(n, int) and n >= self._k: | |
| raise IndexError(f"Invalid lookahead index {n} on Peekable with k={self._k}") | |
| return self._peek[n] | |
| def __iter__(self): | |
| return self | |
| def __next__(self): | |
| if self._peek[0] == self.sentinel: | |
| raise StopIteration | |
| res = self._peek[0] | |
| self._peek = self._peek[1:] | |
| self._peek.append(next(self._stream, self.sentinel)) | |
| return res | |
| class Backtrackable: | |
| def __init__(self, input_, sentinel=None): | |
| self._stream = iter(input_) | |
| self.sentinel = sentinel | |
| self.i = 0 | |
| self.buffer = [] | |
| self.backtrack_points = [] | |
| def __getitem__(self, n): | |
| if isinstance(n, int): | |
| self._fill(n+1) | |
| return self.buffer[self.i + n] | |
| else: | |
| raise IndexError(f"Unsupported operation: Indexing into Backtrackable with {n}") | |
| def __iter__(self): | |
| return self | |
| def __next__(self): | |
| self._fill(1) | |
| res = self.buffer[self.i] | |
| self.i += 1 | |
| return res | |
| def _fill(self, n): | |
| for _ in range(self.i + n - len(self.buffer)): | |
| self.buffer.append(next(self._stream, self.sentinel)) | |
| @contextmanager | |
| def backtrack_always(self): | |
| self.backtrack_points.append(self.i) | |
| try: | |
| yield | |
| finally: | |
| self.i = self.backtrack_points.pop() |
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| import string | |
| from utils import Peekable | |
| import xtokens as t | |
| def lex(char_iterable): | |
| stream = Peekable(char_iterable, 1) | |
| return _lex(stream) | |
| def _lex(stream): | |
| while True: | |
| match stream[0]: | |
| case stream.sentinel: | |
| yield t.Eof() | |
| break | |
| case '[': | |
| next(stream) | |
| yield t.Lbrack() | |
| case ']': | |
| next(stream) | |
| yield t.Rbrack() | |
| case '=': | |
| next(stream) | |
| yield t.Equal() | |
| case ',': | |
| next(stream) | |
| yield t.Comma() | |
| case c if _is_letter(c): | |
| yield _lex_ident(stream) | |
| case c if c in string.whitespace: | |
| next(stream) | |
| case c: | |
| raise ValueError(f"Invalid character {c}") | |
| def _lex_ident(stream): | |
| cs = [] | |
| while _is_letter(stream[0]): | |
| cs.append(next(stream)) | |
| return t.Ident(''.join(cs)) | |
| def _is_letter(c): | |
| return c in string.ascii_letters |
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| import xtokens as t | |
| import xlexer as xl | |
| from utils import Backtrackable | |
| def parse(char_stream): | |
| tokens = Backtrackable(xl.lex(char_stream)) | |
| _list(tokens) | |
| _match(tokens, t.Eof) | |
| def _test(tokens, f): | |
| with tokens.backtrack_always(): | |
| try: | |
| f(tokens) | |
| except ValueError: | |
| return False | |
| return True | |
| def _element(tokens): | |
| if _test(tokens, _assign): | |
| _assign(tokens) | |
| elif _test(tokens, _name): | |
| _name(tokens) | |
| elif _test(tokens, _assign_list): | |
| _assign_list(tokens) | |
| elif _test(tokens, _list): | |
| _list(tokens) | |
| else: | |
| raise ValueError(f"Invalid token {tokens[0]}") | |
| def _list(tokens): | |
| _match(tokens, t.Lbrack) | |
| _elements(tokens) | |
| _match(tokens, t.Rbrack) | |
| def _elements(tokens): | |
| _element(tokens) | |
| while tokens[0] == t.Comma(): | |
| next(tokens) | |
| _element(tokens) | |
| def _assign(tokens): | |
| _match(tokens, t.Ident) | |
| _match(tokens, t.Equal) | |
| _match(tokens, t.Ident) | |
| def _assign_list(tokens): | |
| _list(tokens) | |
| _match(tokens, t.Equal) | |
| _list(tokens) | |
| def _name(tokens): | |
| _match(tokens, t.Ident) | |
| def _match(tokens, token_type): | |
| if isinstance(tokens[0], token_type): | |
| next(tokens) | |
| else: | |
| raise ValueError(f"Failed to match {tokens[0]} to type {token_type}") |
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| from dataclasses import dataclass | |
| @dataclass | |
| class Lbrack: | |
| pass | |
| @dataclass | |
| class Rbrack: | |
| pass | |
| @dataclass | |
| class Equal: | |
| pass | |
| @dataclass | |
| class Comma: | |
| pass | |
| @dataclass | |
| class Eof: | |
| pass | |
| @dataclass | |
| class Ident: | |
| s : str |
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