I wonder if this can be done at all. Consider this...

# codingconventions

[Michal Klimczak]

I wonder if this can be done at all. Consider this structure

sealed interface GeneralFormError {
    object FieldRequired : GeneralFormError
}

sealed interface SpecificFormError : GeneralFormError {
    object PostCodeInvalid : SpecificFormError
}

fun GeneralFormError.message() : String = when(this) {
    GeneralFormError.FieldRequired -> "Field is required"
    else -> throw IllegalStateException("Subtype of GeneralFormError not handled")
}

fun SpecificFormError.message() : String = when(this) {
    SpecificFormError.PostCodeInvalid -> "Post code is invalid"
    is GeneralFormError -> (this as GeneralFormError).message()
}

Then I have a variable val postCodeError : GeneralFormError which can be both FieldRequired or PostCodeInvalid . If I call postCodeError.message() it will throw the ISE. Is there an elegant way of doing this properly (without checking the type in use site).

[ephemient]

add

is SpecificFormError -> message() // this calls the SpecificFormError overload since it has been smart casted

to GeneralFormError.message()

[Michal Klimczak]

I could, but then GeneralFormError would have tk know about all of the specific error (there will be lots of them in different places)

[ephemient]

if it's a sealed interface they must be enumerable

[ephemient]

that has to be done either within this function or at all calllers