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You shouldn't have to specify the error states in each method, this is why what I'm advocating is different than a simple enum:

- You don't need to define a constructor to take the error string

- You don't need to implement method dispatch (all methods of an error state instance automatically throw, like null does)

Enums are close, but not quite right.

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Hm, I think what you might want is an enum / sum type, but with the ability to state that a particular variable is statically a particular variant of that enum. So, for instance, I can write fn serve(conn: &Connection::ValidConnection), and it's a type error to pass a generic unchecked Connection to it. Then serve() can go and call other functions that take a ValidConnection without doing any further error handling.

You might, for convenience, have a single method that turns a Connection to a ValidConnection, or else throws, and you can encode your error messages in one place in this method.

I think this proposal permits such a thing, although I haven't read it closely yet: http://smallcultfollowing.com/babysteps/blog/2015/08/20/virt...

Rust's error handling (try! and error-interoperability) do this well.

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