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Comment by _old_dude_ | Hacker News Reader

Pattern matching does not work if you use an enum

  List<String> list = cons("a", nil());
  switch(list) {
    case Cons<String>(var car, var cdr) -> ...
    case Nil -> ...
  }

Nil is typed List<Void> which is not compatible with List<String>.

You may be able to use extractors as the inverse of the static methods cons() and nil() but that adds more boilerplate.

Disjunctive enums like in Rust or Scala 3 is a simpler way to define ADT.

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Pattern matching will work fine with an additional explicit nil() pattern. I agree that in this particular case it's not quite as succinct as in more functional languages, but virtually all cases in ordinary business applications are not like that, and the additional code, if any, is O(1).

Of course, Java could add rules for the special case of zero-argument records, making them more enum-like, but I really think the value in doing that is negative. Other than being able to directly translate ML examples, it would add complexity that isn't worth it.

In fact, in this particular case I'd do it like so:

    record List<T>(T car, List<? extends T> cdr) {
      public List { if (car == null) throw new IllegalArgumentException("null element"); } 
    }

Which would give you:

    switch(list) {
      case List<String>(var car, var cdr) -> ...
      case null -> ...
    }

It might not be exactly what people who like Scala might prefer, but it is very much a good representation of ADTs for those who prefer Java.

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