150 lines
4.1 KiB
Python
150 lines
4.1 KiB
Python
"""
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Recursive-descent parser for the calc expression grammar.
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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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AST node shapes (stable contract for the evaluator):
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Num(value) — 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 prefix; .op is '-'
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"""
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class ParseError(Exception):
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pass
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# ---------------------------------------------------------------------------
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# AST nodes
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# ---------------------------------------------------------------------------
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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: str, 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 (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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class Unary:
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__slots__ = ('op', 'operand')
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def __init__(self, op: str, 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 (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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# Parser
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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 _advance(self):
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tok = self._tokens[self._pos]
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self._pos += 1
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return tok
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def _expect(self, kind):
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tok = self._peek()
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if 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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return self._advance()
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# expr = term ( ('+' | '-') term )*
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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._advance().value
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node = BinOp(op, node, self._term())
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return node
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# term = unary ( ('*' | '/') unary )*
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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._advance().value
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node = BinOp(op, node, self._unary())
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return node
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# unary = '-' unary | primary
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def _unary(self):
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if self._peek().kind == 'MINUS':
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self._advance()
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return Unary('-', self._unary())
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return self._primary()
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# primary = NUMBER | '(' expr ')'
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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._advance()
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return Num(tok.value)
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if tok.kind == 'LPAREN':
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self._advance()
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node = self._expr()
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self._expect('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(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 after expression: {tok.kind!r} ({tok.value!r})"
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)
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return node
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def parse(tokens) -> object:
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"""Parse a token list produced by `calc.lexer.tokenize` into an AST."""
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return _Parser(tokens).parse()
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