``` @PublishedApi internal object Function1Functor...

# arrow-contributors

[Eugenio]

@PublishedApi internal object Function1FunctorInstance_ : Function1FunctorInstance<Any?>

@InlineOnly
inline fun <I, A, B> Kind<Function1PartialOf<I>, A>.map(noinline f: (A) -> B): Function1<I, B> = 
    with(Function1FunctorInstance_ as Function1FunctorInstance<I>) { [e-mail](f) }

fun test(): Function1<String, Double> = 
    fun(it: String): Int { return it.length }
        .k()
        .let { it as Kind<Function1PartialOf<String>, Int> } // just to show we're using our extension!
        .map { it.toDouble() }