127 lines
3.2 KiB
Python
127 lines
3.2 KiB
Python
"""
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Recursive-descent parser for arithmetic expressions.
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AST node shapes:
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Num(value) — a numeric literal; value is int or float
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BinOp(op, left, right) — binary operation; op is '+', '-', '*', or '/'
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Unary(op, operand) — unary minus; op is '-'
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Grammar (precedence low → high):
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expr := term (('+' | '-') term)*
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term := unary (('*' | '/') unary)*
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unary := '-' unary | primary
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primary := NUMBER | '(' expr ')'
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"""
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from __future__ import annotations
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from dataclasses import dataclass
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from typing import Union
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class ParseError(Exception):
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pass
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@dataclass
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class Num:
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value: Union[int, float]
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def __repr__(self) -> str:
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return f"Num({self.value!r})"
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@dataclass
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class BinOp:
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op: str
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left: "Node"
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right: "Node"
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def __repr__(self) -> str:
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return f"BinOp({self.op!r}, {self.left!r}, {self.right!r})"
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@dataclass
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class Unary:
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op: str
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operand: "Node"
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def __repr__(self) -> str:
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return f"Unary({self.op!r}, {self.operand!r})"
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Node = Union[Num, BinOp, Unary]
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class _Parser:
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def __init__(self, tokens: list) -> None:
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self._tokens = tokens
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self._pos = 0
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def _peek(self):
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return self._tokens[self._pos]
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def _consume(self, kind: str = None):
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tok = self._tokens[self._pos]
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if kind is not None and tok.kind != kind:
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raise ParseError(
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f"expected {kind!r}, got {tok.kind!r} ({tok.value!r})"
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)
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self._pos += 1
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return tok
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def parse(self) -> Node:
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if self._peek().kind == "EOF":
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raise ParseError("empty input")
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node = self._expr()
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if self._peek().kind != "EOF":
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tok = self._peek()
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raise ParseError(
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f"unexpected token {tok.kind!r} ({tok.value!r}) after expression"
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)
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return node
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def _expr(self) -> Node:
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node = self._term()
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while self._peek().kind in ("PLUS", "MINUS"):
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op = self._consume().value
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right = self._term()
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node = BinOp(op, node, right)
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return node
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def _term(self) -> Node:
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node = self._unary()
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while self._peek().kind in ("STAR", "SLASH"):
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op = self._consume().value
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right = self._unary()
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node = BinOp(op, node, right)
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return node
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def _unary(self) -> Node:
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if self._peek().kind == "MINUS":
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op = self._consume().value
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operand = self._unary()
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return Unary(op, operand)
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return self._primary()
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def _primary(self) -> Node:
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tok = self._peek()
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if tok.kind == "NUMBER":
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self._consume()
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return Num(tok.value)
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if tok.kind == "LPAREN":
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self._consume()
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node = self._expr()
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if self._peek().kind != "RPAREN":
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raise ParseError(
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f"expected ')' but got {self._peek().kind!r}"
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)
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self._consume("RPAREN")
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return node
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raise ParseError(
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f"unexpected token {tok.kind!r} ({tok.value!r})"
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)
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def parse(tokens: list) -> Node:
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"""Parse a token list (from lexer.tokenize) into an AST Node."""
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return _Parser(tokens).parse()
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