-- | One can easily implement a monadic 'selectM' that satisfies the laws,
-- hence any 'Monad' is 'Selective'.
selectM :: Monad f => f (Either a b) -> f (a -> b) -> f b
selectM x y = x >>= \e -> case e of Left  a -> ($a) <$> y -- execute y
                                    Right b -> pure b     -- skip y

flatMap { it.mapLeft { a -> fs.map { ff -> ff(a) } } }

flatMap { it.fold({ a -> f.map { ff -> ff(a) } }, { b -> just(b) }) }

@extension 
interface MonadSelective<F>: Selective<F> {
  
  fun M(): Monad<F>
  
  override fun <A, B> Kind<F, A>.ap(ff: Kind<F, (A) -> B>): Kind<F, B> =
    M().run { this@ap.ap(ff) }

  override fun <A> just(a: A): Kind<F, A> = M().just(a)

  override fun <A, B> Kind<F, Either<A, B>>.select(f: Kind<F, (A) -> B>): Kind<F, B> =
    M().run { this@select.selectM(f) }
}

So not the other way around, not all selective are monads?

Ideally if all selectives are also Monad

createTestCase

UnitSpec

test: suspend io.kotlintest.TestContext.() -> kotlin.Unit