# Is there a way to clearly see whether an expression is being used as the "result" of a when?

**URL:** <https://discuss.kotlinlang.org/t/is-there-a-way-to-clearly-see-whether-an-expression-is-being-used-as-the-result-of-a-when/6332>\
**Category:** Support\
**Created:** [January 23, 2018, 6:49am UTC](https://discuss.kotlinlang.org/t/is-there-a-way-to-clearly-see-whether-an-expression-is-being-used-as-the-result-of-a-when/6332 "2018-01-23T06:49:01Z")\
**Posts on this page:** 2\
**Page:** 1

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**Author:** ![aaaidan](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/aaaidan/32/4685_2.png) [@aaaidan](https://discuss.kotlinlang.org/u/aaaidan)\
**Post date:** [January 23, 2018, 6:49am UTC](https://discuss.kotlinlang.org/t/is-there-a-way-to-clearly-see-whether-an-expression-is-being-used-as-the-result-of-a-when/6332/1 "2018-01-23T06:49:01Z")

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Hi,

First post, so apologies in advance. I am relatively new to Kotlin. It’s my favourite language.

I recently found a subtle bug in my code that evaded a couple of code reviewer keen eyes, so I thought it was worth posting a simplified version here as feedback/discussion on this part of the language and/or IDE.

```
class House {

    enum class State {
        DOOR_CLOSED, DOOR_OPEN
    }

    var state = State.DOOR_CLOSED

    fun processAction(action:String) {
        val newState = when (state) {
            State.DOOR_CLOSED -> {
                if (action == "openDoor") State.DOOR_OPEN
                State.DOOR_CLOSED
            }
            else -> State.DOOR_CLOSED // self-closing door :)
        }
        state = newState
    }

}

fun main(args: Array<String>) {
    var h = House() 
    h.processAction("openDoor")
    println("${h.state}, but could easily expect DOOR_OPEN")
}

```

It’s clear to me now that, of course, the line with `State.DOOR_OPEN` will not set the value of the `when` expression, because it’s not the last expression in that block. The `if` statement is missing an `else` clause.

However, since I’ve become accustomed to the rich and helpful advice of kotlin/IntelliJ, I half expected it would somehow make it obvious if it wasn’t working as I intended. Clearly this is a case of user error, but it would have been very helpful if there was some way to be able to verify whether a particular expression is considered a “return value” for a block like this.

I notice, in Android Studio at least, return expressions inside `let` blocks are annotated with a `^let` tag, and something like this would have helped me a lot in this case.

Thoughts?

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**Author:** ![pdvrieze](https://sea1.discourse-cdn.com/flex019/user_avatar/discuss.kotlinlang.org/pdvrieze/32/1882_2.png) [@pdvrieze](https://discuss.kotlinlang.org/u/pdvrieze)\
**Post date:** [January 23, 2018, 2:56pm UTC](https://discuss.kotlinlang.org/t/is-there-a-way-to-clearly-see-whether-an-expression-is-being-used-as-the-result-of-a-when/6332/2 "2018-01-23T14:56:30Z")

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This would be something the IDE could/should warn about (you may wish to file a feature request). As State.DOOR\_OPEN is a compile time constant it’s conditional referencing can be recognised as a problem reasonably easy. It has almost no side-effect (it can trigger class loading, the class initializer can have arbitrary side-effects - and yes that is very poor coding practice).