• BatmanAoD@programming.dev
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    1 year ago

    It’s not possible to instantiate or assign, which is more like a never type than a unit; and it is not possible to define new types with the same properties, which is also more like bottom than unit. But you’re right that it’s not actually a true never type since it can’t represent function divergence.

    I think the truth is just that Java’s type system isn’t very mathematically disciplined.

    • Aloso@programming.dev
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      1 year ago

      It’s not possible to instantiate or assign, which is more like a never type than a unit

      Actually, this is because void is not a type, it is just a keyword, a placeholder used instead of the return type when a function doesn’t return anything.

      If it were a bottom type, that would mean that a method returning void must diverge, which is simply not true.

      Also, if it were a bottom type, it would be possible to write an “unreachable” method

      void unreachable(void bottom) {
          return bottom;
      }
      

      Even though it couldn’t be called, it should be possible to define it, if void was a bottom type. But it is not, because void isn’t a bottom type, it’s no type at all.

      • BatmanAoD@programming.dev
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        1 year ago

        The post has been edited; it looks like someone on reddit made essentially the same point. You’re right of course that void isn’t a true type in Java, but the post now also discusses Void, which I suppose just shows how void infects the type system despite not being a type.