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LoganDark

Doesn't support algebraic data types, though.

FWIW I think this article mentioning them at all is sort of weird because the article doesn't propose them either, lmao

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Which Python 3.10 and static type checkers much of the benefits of algebraic data types is already available [1].

[1]: https://stackoverflow.com/questions/16258553/how-can-i-defin...

Right, the article talks about "type Point = Point1D | Point2D | Point3D" syntax at the end... which would work just as well with dataclasses as with structs!

It already does too. Just remove the `type` bit and you have an alias for a union, which has been supported since typing was added (though the `|` syntax was only added in 3.10). And AFAIK the type checkers do handle completeness on unions.

The typed equivalent of a sum type in python is a union (https://docs.python.org/3/library/typing.html#typing.Union). Works with pretty much every type you can think of, but has the usual union limitation that you can't have two variants of the same type (e.g. int | int | float).

Then again, you can just create newtype wrappers, and that's essentially the same thing as different constructors for a sum type.

You'd need to create newtype wrappers anyway in order to tell them apart, I'd assume. Otherwise it wouldn't really be an algebraic data type as much as a... union type.

Hence “equivalent”. Of your variants all have trivial and non-overlapping payloads you can just use them as-is, or erlang-style tagged tuples, in most cases it’ll do fine.

Do you mean Python's lack of a labeled union type?

Pydantic does though.

    class A(BaseModel):
      status: Literal["success"]

    class B(BaseModel):
      status: Literal["error"]

    class C(BaseModel, ABC):
      __root__: Union[A, B] = Field(discriminator="status")

    aorb = C.parse_obj(blah…).__root__

    if aorb.status == "error":
      # aorb will type check as B.

You are a friend and a scholar.

I was looking for a solution in pydantic that is wildly relevant. Thanks, I didn’t realize that was implemented/existed.

My only objection is using ABC in your MRO. Haha, just a little bit overloaded with abstract base classes and using A, B, and C.

I got it eventually! Cool.

Ya know what, on reading that that does seem confusing. My brain had long since internalized ABC as a single token I didn't even notice.

I believe this is a good example of type narrowing :)

Yeah. Been spoiled by Rust.

It doesn't seem that far off though, it does have a union type and with pattern matching you can achieve pretty much the same thing (sure you may need a few extra classes if the types aren't distinct already but that seems like a small difference).

I don't know if type checkers can figure out if a pattern match statement is exhaustive yet though, that would be interesting.

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