Money for Nothing, ...

[Brian Goetz]

Here's how we would define this in Haskell:

     data Result t = Succ t | Error Throwable

or maybe (Either-style)

     data Result t e = Succ t | Error e

Note that in either, `Error` is still generic in `t`, its just that it 
doesn't use `t`, so it doesn't say `t`.  Similarly, in the latter, both 
alternatives are generic in both tvars, but each uses only one of them.

The Java analogue here is:

     sealed interface Result<T> { }
     record Succ<T>(T t) implements Result<T> { }
     record Fail<T>(Throwable t) implements Result<T> { }

What you are probably reacting to here is "but why does Fail have to say 
T, it doesn't use it."  And the answer is: "get over that, and then 
you're done."

You are trying to invent a new generics feature to avoid putting a `T` 
you don't use in your Error declaration.  But that T (and maybe E) are 
needed to unify the two under Result<T,E> -- just like in the second 
Haskell example above.



On 6/13/2023 3:17 PM, Remi Forax wrote:
> Hello,
> currently, it's not possible to write a lot of generics sealed type because Java has no way to denote the bottom type.
>
> By example, if a Result can be either a Success or an Error, we want to be able to write this kind of switch
>
>    public static void main(String[] args) {
>      Result<String, IOException> result = ...
>      var val = switch(result) {
>        case Error<IOException> error -> 1;
>        case Success<String> success -> 2;
>      };
>    }
>
> But i do not see a way to do that without introducing a way to denote the bottom type (named "Nothing" here)
>
>    sealed interface Result<T,E extends Exception> {}
>    record Error<E extends Exception>(E error) implements Result<Nothing, E> {}
>    record Success<T>(T value) implements Result<T, Nothing> { }
>
> Nothing being the return type of a method that either never terminate (by example, using a for(;;)) or always throw an exception.
>
> So, should we add Nothing to Java or is there another way to model this kind of sealed types ?
>
> regards,
> Rémi
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