I have a `sealed class MyError`. in my unit tests, I have code that checks a list of `MyError`s prod...

# getting-started

[y]

I have a sealed class MyError. in my unit tests, I have code that checks a list of `MyError`s produced by some other source.
for some error, I'd like to assert that the error is an instance is of a specific subclass, say MyError.Foo

data class AcceptsSpecificError {
  fun check(e: MyError) {
    // assert that `e` has type `MyError.Foo`
  }
}

how can I do this?
I can't make this AcceptsSpecificError<T: MyError> because of type erasure.
also it's not feasible for AcceptsSpecificError to take a val e: MyError and then compare to that, because some of the subclasses of MyError are too complex to easily instantiate in a unit test environment.

EDIT: would taking a property of type KClass<T: MyError> work here? what I really want here is a "phantom data" kind of value I can take so that T is not type erased.

[Jack Boswell]

Does this not work? assertTrue(e is MyError.Foo)

[y]

it does, but the subclass is meant to be a generic here. intended usage is something like AcceptsSpecificError<Foo.Bar>().check(e).
I got this to work by adding a property of type KClass. which, after searching around, seems to be what I'm supposed to do here.

[Daniel Pitts]

import kotlin.reflect.KClass

sealed class MyError {
    data object Foo : MyError()
    data object Bar : MyError()
}

class AcceptsSpecificError<T : MyError>(val type: KClass<T>) {
    fun check(e: MyError) {
        check(type.isInstance(e)) { "Expected $type, got ${e::class}" }
    }
}

inline fun <reified T : MyError> AcceptsSpecificError() = AcceptsSpecificError(T::class)

[y]

@Daniel Pitts woah, that was so simple. thank you

[y]

too bad you can't completely hide away the implementation detail here, because inline fun is not allowed to access a non-public constructor

[Daniel Pitts]

You can, but there really isn't a benefit to do so.

[Daniel Pitts]

You can change the signature here:

class AcceptsSpecificError<T : MyError> @PublishedApi internal constructor(val type: KClass<T>) {

[Daniel Pitts]

This makes the constructor "internal" as far as Kotlin is concerned, but effectively "public" to the JVM. This allows it to be called from an inline function, while maintaining a little bit of implementation hiding.

[Daniel Pitts]

I don't think I'd go that far though. Users might legitimately want to pass in a KClass<T> themselves, even if they aren't in a context where they have a reified type T.

[y]

thanks again, I appreciate the explanation.

[Daniel Pitts]

You're welcome!