source-academy / py-slang / 28845351947

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CSE machine: Spec-driven operator semantics for Python §1–§4: ==, is, ordering (#198) (#207)

* Factor the operator typing tables with \input and revise specs (#198 Part A)

The two operator typing tables (python_typing.tex for Python §1/§2,
python_typing_3.tex for §3/§4) had drifted apart by hand. Split them so
the common rows live once:

  front + middle_12 + back  ->  Python §1/§2
  front + middle_34 + back  ->  Python §3/§4

The diff between the middle fragments is now exactly the documented
chapter delta: §1/§2 restrict ==/!= to numeric x numeric and
string x string (no is operator); §3/§4 give ==, != and is
any x any -> bool, behaving as in Python.

* Make ==, != and is total (any x any) at Python §3/§4, per the revised spec

Implements the semantics of docs/specs/python_typing_middle_34.tex
(#198): at Python §3/§4, `==`/`!=` and `is`/`is not` accept any operand
types and behave as in Python.

- `==`/`!=` on lists: structural (element-wise, recursive) equality,
  replacing the previous UnsupportedOperandTypeError. Numbers compare
  across int/float/complex inside lists too ([1] == [1.0] is True).
  An identity shortcut lets comparisons of shared substructure
  terminate early; self-referential lists without shared identity
  exhaust the stack, mirroring CPython's RecursionError.
- `is`/`is not` on any operands: lists and functions compare by
  reference (sharing is what `is` exists to observe); immutable values
  compare by value, staying within Python's unspecified interning
  behavior while remaining deterministic (1 is 1 -> True,
  1 is 1.0 -> False, None is None -> True).

Two pre-existing bugs surfaced and fixed along the way:

- `x is not y` evaluated as `x is y`: the grammar kept TokenType.IS on
  the token and only changed its lexeme, which nothing reads. The
  postprocess now assigns TokenType.ISNOT (fix in python.ne and the
  generated python-grammar.ts).
- NoIsOperatorValidator (chapters 1/2) never fired: `is` parses as an
  ExprNS.Co... (continued)

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