Money for Nothing, ...
Brian Goetz
brian.goetz at oracle.com
Wed Jun 14 17:50:53 UTC 2023
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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