source-academy / py-slang / 42494967454

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Typed Python §4, phase 3: the evaluators (#636)

* Typed Python §4, phase 3: a step before validation in every engine

The type checker is a step of validating a chunk, after parsing and before the engine's own checks,
in py2js, the CSE machine and the environment stepper alike. It cannot be a FeatureValidator (those
are synchronous and per node; the checker is a worker, looks at the whole text and keeps what it saw
from chunk to chunk), so each engine takes it as an option, `beforeValidation`, called with the text
and the parsed program of each chunk (not the preludes), which may throw and may change the program.

* checkTypedChunk (src/typecheck/typed-chunk.ts), the one function that all engines call: the rules
  of Typed §4 on annotations (the first violation), the checker with the text as written (all errors,
  as StaticTypeErrors; warnings and hints do not stop a program), then the erasure of the types.
* LazyChecker / startBrowserChecker (loader.ts): fetches TypedPy4Checker.js and starts it as a
  worker from a blob: URL when first needed (preload() starts at once), and again after a failure;
  a checker that cannot be started is an error and the program is not run unchecked.
* StaticTypeError(s): line and column, and describe(source), the message with its place, because the
  CSE machine says where an error is only in its message.
* Py2JsSession, PyCseEvaluatorBase and PyEStepperEvaluatorBase take the option; the evaluators of
  Python §4 are unchanged. The helpers that the plain evaluators need to report a type error are in
  conductor/staticTypeErrors.ts, apart from the ones that have the checker (typed.ts), so that a plain
  evaluator does not contain it (the bundles of PyCseEvaluator4 etc. have no trace of it).

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com>

* Typed Python §4, phase 3: the evaluators TypedPy2Js, TypedPyCse and TypedPyEStepper

Typed Python §4 on three engines, each checking the types first: TypedPy2JsEvaluator4,... (continued)

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