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Because many people start learning in one of the industry incumbent, not necessarily good by design languages. Java, C, C++, python, Javascript, go... As a se

• pjmlp 3y ago
  Java enums are also quite powerful versus what C, C++ and C# can do. They are real objects, can have associated data and methods. So if they are surprised by Rust, haven't learnt Java properly
  • ongy 3y ago
    I Haven't used java in ages. Though from a quick search, it's still just scalar/unary values. Rhough one of the better implementations of those. But the big advantage of the more sum type Style enums rust provides is the ability to nest values in them while preserving type safety guarantees. So I still see them as miles better as a language concept.
    • fifticon 3y ago
      erh no, those in java allow you to do just that. It is one of my favorite features of java. In essence, you get to embed multiple synchronized maps inside a java enum (if you want to). All handled 'invisibly' for you.
      • Ygg2 3y ago
        Still, you get to see all fields from all variants, while Rust variants are more isolated.
      • Sharlin 3y ago
        No, Java enums can contain data but do so in a manner orthogonal to Rust enums. Java enums are not sum types; each variant is simply a (`final`) instance of the enum type which, mostly, is just a regular class type with a private constructor so that no other instances can be created. All variants have the same shape and the author of the enum decides what data the variants contain. Rust enums are sum types, which means that each variant can have a totally different shape and contain different data, and it's the user of the enum that supplies the data when they create an instance of a variant. This quintessential use case for sum types not possible with Java enums (although it is now sort of possible with records and sealed traits): enum Result<T, E> { Ok(T), Err(E) }
    • papercrane 3y ago
      Java enums are a class with a fixed number of instances. The thing in Java that is closest to Rust enum's would be sealed classes, which was a preview feature in Java 15 and finalized and released with Java 17. https://openjdk.org/jeps/409 https://openjdk.org/jeps/409
    • pjmlp 3y ago
      Still, better than C, C++ and C#, which was my point. For the real deal, there are sealed classes now, modeled on how Scala does it.
  • CodesInChaos 3y ago
    Java enums store the associated data once per option, while Rust enums store it per value. So they're very different. You couldn't model Rust's `Option` or `Result` as a java enum.
    • pjmlp 3y ago
      Sure, I didn't say it was the same, only they are more powerful than C, C++ and C#.
    • andrybak 3y ago
      > You couldn't model Rust's `Option` or `Result` as a java enum. For reference, they can be modeled as a "sealed" class since Java 17. https://openjdk.org/jeps/409 https://openjdk.org/jeps/409 Prior to Java 17, sealed classes feature can be partially recreated via private constructors in the super classes. This approach forces you to put all classes in a single Java file, which can be awkward to navigate.