Function rustz::validation::apply2

pub fn apply2<A, B, R, F, E>(
    a: Validation<E, A>, 
    b: Validation<E, B>, 
    f: F
) -> Validation<E, R> where
    A: Clone,
    B: Clone,
    R: Clone,
    E: Clone + Semigroup,
    F: FnOnce(A, B) -> R,