Define Result as a union of Success and Failure to express a closed sum type

[renan-r-santos]

serialite.Result aliases the returns abstract base class Result. A type checker cannot prove a match on it is exhaustive, even when both Success and Failure are handled.

MRE:

from serialite import Failure, Result, Success, raise_errors

def unwrap(r: Result[int]) -> int:
    match r:
        case Success(value):
            return value
        case Failure(errors):
            raise_errors(errors)

Both arms return or raise (raise_errors is NoReturn), so this is exhaustive. pyrefly rejects it:

ERROR Function declared to return `int`, but one or more paths are missing an explicit `return` [bad-return]

A wildcard case _ as other: assert_never(other) shows the subject is never narrowed past the two arms:

ERROR Argument `Result[int, Errors]` is not assignable to parameter `arg` with type `Never` in function `typing.assert_never` [bad-argument-type]

Root cause:

returns.result.Result is an abstract base class. Nominal subclassing is open, so a checker must assume a third subclass could exist. Making Result a union of exactly Success and Failure fixes this. Each case eliminates one arm, and the match narrows to Never. A union is needed because Python has no sealed to close the base class.

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Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.45%. Comparing base (fd45f6e) to head (1a5ef06).
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Additional details and impacted files

@@           Coverage Diff           @@
##           master      #54   +/-   ##
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  Coverage   92.45%   92.45%           
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  Files          34       34           
  Lines        1524     1524           
  Branches      232      232           
=======================================
  Hits         1409     1409           
  Misses         50       50           
  Partials       65       65           

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[drhagen]

I guess until we get the @sealed decorator, this is the best can do.