Rescript Validation Applicative - just to avoid losing it

gistfile1.res

module Validation = {
  type t<'s, 'f> =
    | Success('s)
    | Failure('f)

  let pure = v => Success(v)

  let apply = (selector, source, combine) =>
    switch selector {
    | Success(map) =>
      switch source {
      | Success(s) => Success(map(s))
      | Failure(f) => Failure(f)
      }
    | Failure(f1) =>
      switch source {
      | Success(_) => Failure(f1)
      | Failure(f2) => combine(f1, f2)->Failure
      }
    }

  let traverse = (list, fn, combine) => {
    let cons = (head, tail) => list{head, ...tail}

    let initState = pure(list{})
    let folder = (. head, tail) => {
      cons->pure->apply(fn(head), combine)->apply(tail, combine)
    }

    Js.List.foldRight(folder, list, initState)
  }
}