[uzerfcwn]

That looks more like ad hoc union types than sum types. What happens if you do:

    data FooResult = Number | Number

If you can't distinguish the two cases, then it means they're not an actual sum type, since e.g. the set size is |FooResult| = |Number| whereas a sum type should have 2*|Number|.

The reason ad hoc union types are avoided in Hindley-Milner (HM) languages is that their usability depends on subtyping, which is incompatible with HM. You could get around this by saying that ad hoc sum types require the types to be distinct, but this would greatly impede usability. For example:

    ErrorA = FileNotFoundError | ParsingError
    ErrorB = FileNotFoundError | PermissionError
    ErrorC = ErrorA | ErrorB // Oops, trying to ad hoc sum FileNotFoundError twice

The tried and true solution in HM is to use generic types like Result<A,B>, where you separate the cases with labels Ok and Err.

On the other hand, languages with subtyping aren't that averse to ad hoc union types. TS and Python already have them and C# is adding them soonish [1].

[1] https://github.com/dotnet/csharplang/blob/main/proposals/Typ...