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Supported types

✅ In general, Mokkery is able to mock a type that is fully overridable. That includes:

  • 🧩 Interfaces
  • ➡️ Functional types
  • 🏗️ Abstract/open classes with no-args constructor and all overridable public members
  • 🏛️ Final classes with all-open plugin

❌ Things that Mokkery is unable to mock:

  • ➡️ Functions (extensions included)
  • 🏛️ Final classes that are already compiled (it's not possible to apply all-open plugin)
  • 🔢 Primitives
  • 🔐 Sealed types
  • 🗽 Objects

Value classes (WASM)

Mokkery supports mocking methods involving value classes, whether they serve as return types or parameters. However, for WASM, this support is restricted only to value classes originating from the standard library (kotlin.Result and kotlin.time.Duration).

Any other value class requires registering AutofillProvider that returns some empty value:

AutofillProvider.forInternals.types.register { ValueClass(null) }

Calling mock & spy

Type passed to spy and mock must be directly specified. Following code is illegal:

inline fun <reified T : Any> myMock() = mock<T>()

However, it is not completely forbidden to use generic parameters. Following code is allowed:

fun <T : Any> myListMock() = mock<List<T>>()

Calling every & verify

To ensure that every and verify work as expected, compiler plugin transforms the code inside their blocks. This transformation currently restricts those blocks from extracting their parts into separate functions. It also dictates that block parameter must always be a lambda expression (not function reference nor lambda assigned to a variable). Following code is illegal:

fun test() {
// ...
verify {

private fun {

However, it is perfectly fine to extract whole verify or every call to separate function:

fun test() {
// ...
private fun foo() {
verify {

Using matchers

All matchers

The biggest limitation is that you must not assign matchers to variables. Following code is illegal:

everySuspend {
val matcher = any<Int>()
} returns Book(...)

Composite matchers

When using composite matchers (whenever matcher accepts other matcher like logical matchers), you cannot use literals. Code below is illegal:

everySuspend { repository.findById(or("1", "2")) } returns Book(...)

You must use eq matcher explicitly:

everySuspend { repository.findById(or(eq("1"), eq("2"))) } returns Book(...)

Varargs matchers

If you pass varargs as array, it might sometimes lead to ambiguity. Calls presented below are prohibited:

everySuspend { repository.findAllById(ids = arrayOf("1", *anyVarargs(), "3")) } returns emptyList()
everySuspend { repository.findAllById(ids = arrayOf("1", any())) } returns emptyList()

While passing varargs as arrays make sure that you don't mix matchers with literals. Calls presented below are allowed:

everySuspend { repository.findAllById(ids = arrayOf(eq("1"), *anyVarargs(), eq("3"))) } returns emptyList()
everySuspend { repository.findAllById(ids = arrayOf(eq("1"), any())) } returns emptyList()