145 lines
3.7 KiB
Python
145 lines
3.7 KiB
Python
"""Recursive-descent parser for arithmetic expressions.
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AST nodes:
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Num(value) -- numeric literal (int or float)
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BinOp(op, left, right) -- binary op; op in {'+', '-', '*', '/'}
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Unary(op, operand) -- unary op; op == '-'
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Grammar:
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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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class ParseError(Exception):
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pass
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class Num:
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__slots__ = ("value",)
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def __init__(self, value):
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self.value = value
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def __repr__(self):
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return f"Num({self.value!r})"
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def __eq__(self, other):
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return isinstance(other, Num) and self.value == other.value
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class BinOp:
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__slots__ = ("op", "left", "right")
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def __init__(self, op, left, right):
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self.op = op
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self.left = left
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self.right = right
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def __repr__(self):
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return f"BinOp({self.op!r}, {self.left!r}, {self.right!r})"
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def __eq__(self, other):
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return (
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isinstance(other, BinOp)
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and self.op == other.op
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and self.left == other.left
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and self.right == other.right
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)
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class Unary:
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__slots__ = ("op", "operand")
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def __init__(self, op, operand):
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self.op = op
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self.operand = operand
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def __repr__(self):
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return f"Unary({self.op!r}, {self.operand!r})"
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def __eq__(self, other):
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return (
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isinstance(other, Unary)
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and self.op == other.op
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and self.operand == other.operand
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)
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class _Parser:
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def __init__(self, tokens):
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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=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}, 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):
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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):
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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):
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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):
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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):
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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("LPAREN")
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node = self._expr()
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if self._peek().kind != "RPAREN":
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raise ParseError("unclosed parenthesis")
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self._consume("RPAREN")
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return node
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if tok.kind == "EOF":
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raise ParseError("unexpected end of input")
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raise ParseError(f"unexpected token {tok.kind!r} ({tok.value!r})")
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def parse(tokens) -> object:
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"""Parse a token list (from tokenize()) into an AST node.
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Raises ParseError on malformed input.
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"""
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return _Parser(tokens).parse()
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