How can I allow this to accept a String in the `fi...

# codereview

[Andrew K]

How can I allow this to accept a String in the find extension in addition to an Int? Is there anything else I can do to optimize as well?

infix fun <T> T?.ifNull(block: () -> T?) = this ?: block()

inline fun <reified T> EnumCompanion<T>.find(value: Int): T? where T : Enum<T>, T : EnumValue<T> =
    enumValues<T>().run {
        firstOrNull { (it.value as Int) == value }
            .ifNull { firstOrNull { it.name == "Unknown" } }
            .ifNull { throw Exception("Enum value does not exist") }
    }

interface EnumCompanion<T : Enum<T>>

interface EnumValue<T> {
    val value: T
}

enum class MyEnum(override val value: Int) : EnumValue<Int> {
    Unknown(0),
    First(1),
    Second(2),
    Third(3);

    companion object : EnumCompanion<MyEnum>
}

MyEnum.find(1)

[Mranders]

You need two type parameters, one for the Enum type and one for the EnumValue type. Furthermore, a couple of things:

• Your ifNull could just be the inbuilt ?: operator
• The return type is currently T?, but could be T, since it will never return null
So, the function could look like this:

inline fun <reified T, V> EnumCompanion<T>.find(value: V): T where T : Enum<T>, T : EnumValue<V> =
    enumValues<T>().run {
        firstOrNull { it.value == value }
            ?: firstOrNull { it.name == "Unknown" }
            ?: throw Exception("Enum value does not exist")
    }

From a personal perspective, I would not use the matching on name == "Unknown" to attempt to find an "Unknown". I would probably have three functions:

inline fun <reified T, V> EnumCompanion<T>.find(value: V): T? where T : Enum<T>, T : EnumValue<V> =
    enumValues<T>().firstOrNull { it.value == value }

inline fun <reified T, V> EnumCompanion<T>.findOrError(value: V): T where T : Enum<T>, T : EnumValue<V> =
    find(value) ?: throw Exception("Enum value $value does not exist")

inline fun <reified T, V> EnumCompanion<T>.findOrDefault(value: V, default: T): T where T : Enum<T>, T : EnumValue<V> =
    find(value) ?: default

And I'm not sure about the third, since the ?: can already express that at the call site.

If your domain has well defined "defaults" for enumerations, then you could expand the EnumCompanion to contain it, and then use it - something like this:

interface EnumCompanion<T : Enum<T>> {
    val default: T
}

enum class MyEnum(override val value: Int) : EnumValue<Int> {
    Unknown(0),
    First(1),
    Second(2),
    Third(3);

    companion object : EnumCompanion<MyEnum> {
        override val default = Unknown
    }
}

inline fun <reified T, V> EnumCompanion<T>.find(value: V): T where T : Enum<T>, T : EnumValue<V> =
    enumValues<T>().firstOrNull { it.value == value } ?: default