Force compiler to emit an error when not all implementations are covered in "when" statement

Question

Maybe this is an absurd question. I have a method that receives a Command (sealed class) and returns Unit and I want the compiler to crash whether all the when branches have not been implemented:

sealed class Command
class FirstCommand : Command()
class SecondCommand: Command()

fun handle(cmd: Command) {
  when(cmd) {
    is FirstCommand -> //whatever     
  }
}

The code above is ok, but I would like it not to compile.

When method returns anything different from Unit, it does not compile:

fun handle(cmd: Command) : Any {
  when(cmd) { //error, when must be exhaustive and requires all branches
    is FirstCommand -> //whatever     
  }
}

I want that behavior but returning nothing (Unit). I understand why that happens, but I'm wondering if there is any way I can change my code to achieve what I want. I need cover all the Command implementations without forget anyone that may be added later.

A point about terminology: you don't want the compiler to crash! You just want it to emit an error message. The compiler would only crash if it itself had a bug. — DodgyCodeException, Jun 19 '18 at 20:42
@DodgyCodeException Absolutely! ;D thanks for your clarification — Héctor, Jun 19 '18 at 20:43

score 5 · Accepted Answer · answered Jun 19 '18 at 18:52

5

Solved. I didn't know you can use return statement even when method returns Unit:

fun handle(cmd: Command) {
  return when(cmd) {
    is FirstCommand -> //whatever     
  }
}

Now, code above does not compile because when needs all the branches. Just what I wanted.

answered Jun 19 '18 at 18:52

Héctor

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You lost the return type in the process though (probably a typo) – Zoe Jun 19 '18 at 20:49
1

@Zoe Actually I'm returning "void" (Unit) – Héctor Jun 19 '18 at 20:51
1

You had `Any` as the return type in the question though, just wanted to make sure it wasn't a typo :) – Zoe Jun 19 '18 at 20:52
1

@Zoe It was just for clarify that, if there is a return type, compiler cmplains. But my "real" code returns Unit. Thanks anyway – Héctor Jun 19 '18 at 20:54

Force compiler to emit an error when not all implementations are covered in "when" statement

1 Answers1