type Monoid[A any] interface {
    Empty() A
    Combine(A, A) A
}

type foldExample[A any] struct {
    monoid Monoid[A]
}

func (example foldExample[A]) foldl(xs []A, acc func([]A, A) []A) []A {
    m := example.monoid
    ans := m.Empty()
    for _, x := range xs {
        ans = m.Combine(ans, x)
    }
    return ans
}

type Functor[F_, A, B any] interface {
    Fmap(types.HKT[F_, A], func(A) B) types.HKT[F_, B]
}