How to bind a Kotlin Function in Guice

Question

I have a Kotlin class similar to this:

class MyClass @Inject constructor(val work: (Int) -> Unit)) { ... }

Neither the bind nor the @Provides is working:

class FunctionModule : AbstractModule() {

    override fun configure() {
        bind(object : TypeLiteral<Function1<Int, Unit>>() {}).toInstance({})
    }

    @Provides
    fun workFunction(): (Int) -> Unit = { Unit }
    }
}

I keep getting the error:

No implementation for kotlin.jvm.functions.Function1< ? super java.lang.Integer, kotlin.Unit> was bound.

How can I inject an implementation for a Kotlin function using Guice?

Answer 1

tl;dr - Use:

bind(object : TypeLiteral<Function1<Int, @JvmSuppressWildcards Unit>>() {})
    .toInstance({})

---

In the class

class MyClass @Inject constructor(val work: (Int) -> Unit)) { ... }

the parameter work has a type (as least according to Guice) of:

kotlin.jvm.functions.Function1<? super java.lang.Integer, kotlin.Unit>

However,

bind(object : TypeLiteral<Function1<Int, Unit>>() {}).toInstance({})

registers a type of kotlin.jvm.functions.Function1<? super java.lang.Integer, **? extends** kotlin.Unit>

Changing bind to bind(object : TypeLiteral<Function1<Int, **@JvmSuppressWildcards** Unit>>() {}).toInstance({}) to remove the variance on the return type allows Guice to properly inject the function.