from dataclasses import dataclass
from typing import Union


class ParseError(Exception):
    pass


@dataclass
class Num:
    value: Union[int, float]

    def __repr__(self):
        return f"Num({self.value})"


@dataclass
class BinOp:
    op: str
    left: object
    right: object

    def __repr__(self):
        return f"BinOp({self.op}, {self.left!r}, {self.right!r})"


@dataclass
class Unary:
    op: str
    operand: object

    def __repr__(self):
        return f"Unary({self.op}, {self.operand!r})"


class _Parser:
    def __init__(self, tokens):
        self.tokens = tokens
        self.pos = 0

    def _peek(self):
        return self.tokens[self.pos]

    def _consume(self, kind=None):
        tok = self.tokens[self.pos]
        if kind is not None and tok.kind != kind:
            raise ParseError(f"expected {kind!r}, got {tok.kind!r}")
        self.pos += 1
        return tok

    def _expr(self):
        left = self._term()
        while self._peek().kind in ('PLUS', 'MINUS'):
            op = self._consume().kind
            right = self._term()
            left = BinOp(op, left, right)
        return left

    def _term(self):
        left = self._unary()
        while self._peek().kind in ('STAR', 'SLASH'):
            op = self._consume().kind
            right = self._unary()
            left = BinOp(op, left, right)
        return left

    def _unary(self):
        if self._peek().kind == 'MINUS':
            self._consume()
            return Unary('MINUS', self._unary())
        return self._primary()

    def _primary(self):
        tok = self._peek()
        if tok.kind == 'NUMBER':
            self._consume()
            return Num(tok.value)
        if tok.kind == 'LPAREN':
            self._consume()
            node = self._expr()
            if self._peek().kind != 'RPAREN':
                raise ParseError(f"expected ')', got {self._peek().kind!r}")
            self._consume()
            return node
        raise ParseError(f"unexpected token {tok.kind!r}")


def parse(tokens) -> object:
    """Parse a token list into an AST.

    Nodes:
      Num(value)              — numeric literal (int or float)
      BinOp(op, left, right)  — binary op; op in {'PLUS','MINUS','STAR','SLASH'}
      Unary(op, operand)      — unary minus; op == 'MINUS'

    Raises ParseError on malformed input.
    """
    p = _Parser(tokens)
    if p._peek().kind == 'EOF':
        raise ParseError("empty input")
    node = p._expr()
    if p._peek().kind != 'EOF':
        raise ParseError(f"unexpected token {p._peek().kind!r} after expression")
    return node