Created
September 8, 2021 02:53
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Python Implementation of LL(2) Recursive Descent Parser based loosely on Language Implementation Patterns
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list : '[' elements ']'; | |
elements : element (',' element)*; | |
element : NAME | assign | list; | |
assign : NAME '=' NAME; | |
NAME : ('a'..'z' | 'A'..'Z')+; |
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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 |
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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 Peekable | |
def parse(char_stream): | |
tokens = Peekable(xl.lex(char_stream), 2) | |
_list(tokens) | |
_match(tokens, t.Eof) | |
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 _element(tokens): | |
match (tokens[0], tokens[1]): | |
case (t.Ident(s), t.Equal()): | |
_assign(tokens) | |
case (t.Ident(s), _): | |
next(tokens) | |
case (t.Lbrack(), _): | |
_list(tokens) | |
case (token, _): | |
raise ValueError(f"Invalid token {token}") | |
def _assign(tokens): | |
_match(tokens, t.Ident) | |
_match(tokens, t.Equal) | |
_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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