Validation versus disjunction

by

This is probably not the answer you’re looking, but I just noticed Validation has the following methods

/** Run a disjunction function and back to validation again. Alias for `@\/` */
def disjunctioned[EE, AA](k: (E \/ A) => (EE \/ AA)): Validation[EE, AA] =
  k(disjunction).validation

/** Run a disjunction function and back to validation again. Alias for `disjunctioned` */
def @\/[EE, AA](k: (E \/ A) => (EE \/ AA)): Validation[EE, AA] =
  disjunctioned(k)

When I saw them, I couldn’t really see their usefulness until I remembered this question. They allow you to do a proper bind by converting to disjunction.

def checkParses(p: (String, String)):
  ValidationNel[NumberFormatException, (Int, Int)] =
  p.bitraverse[
    ({ type L[x] = ValidationNel[NumberFormatException, x] })#L, Int, Int
  ](
    _.parseInt.toValidationNel,
    _.parseInt.toValidationNel
  )

def checkValues(p: (Int, Int)): InvalidSizes \/ (Int, Int) =
  (p._1 >= p._2) either InvalidSizes(p._1, p._2) or p

def parse(input: List[(String, String)]):
  ValidationNel[Throwable, List[(Int, Int)]] = input.traverseU(p =>
    checkParses(p).@\/(_.flatMap(checkValues(_).leftMap(_.wrapNel)))
  )

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