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But then you should compare a trait with no default method to a switch with no default case. And a switch with no default case would be reported as an error if

But then you should compare a trait with no default method to a switch with no default case. And a switch with no default case would be reported as an error if some cases are not covered. Also a switch with no default case is still open to extension. But the extensibility happens in a different dimension, as described in the article - the analogue of adding more implementations of an interface is adding more switches, not adding more case clauses. That's why a switch is not a replacement for interfaces.

lmm 6y ago
> And a switch with no default case would be reported as an error if some cases are not covered. In what language? It's not an error in Scala, which is what the post is talking about.
- pkolaczk 6y ago
  In Scala or OCaml you get a compile-time warning + runtime error. In Java 14, Kotlin and Rust there is a compile-time error if switch/match expression is non-exhaustive.
  - lmm 6y ago
    You don't get a compile-time warning in Scala, only a runtime error. https://scastie.scala-lang.org/6fGnDSzDSdmI4USN5k9OTA https://scastie.scala-lang.org/6fGnDSzDSdmI4USN5k9OTA
    - pkolaczk 6y ago
      If you're matching on a sealed type, you do get a warning.
      - lmm 6y ago
        Right - like I said, if it's "closed" you get a warning, but if it's "open" it's silently accepted and fails at runtime.
        pkolaczk 6y ago
        The openness of match/switch over a sealed hierarchy is in another dimension - you can add more switches without changing old code. This is something that interfaces don't provide.
        lmm 6y ago
        Visitor-style interfaces do provide that flexibility, though they're cumbersome.