Discussion on: Getting started with fp-ts: Either vs Validation

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Replies for: Thank you so much for those articles, Fp-ts has done a lot to increase my speed everyday at work! I'm struggling to figure what this would look li...

This is what sequenceT is doing under the hood (when specialized to getValidation(getSemigroup<string>()) + three validations)

function specializedSequenceT(
  firstValidation: Either<NonEmptyArray<string>, string>,
  secondValidation: Either<NonEmptyArray<string>, string>,
  thirdValidation: Either<NonEmptyArray<string>, string>
): Either<NonEmptyArray<string>, [string, string, string]> {
  // Applicative instance for `Either<NonEmptyArray<string>, A>`
  const V = getValidation(getSemigroup<string>())

  // builds a tuple from three strings
  const tuple = (a: string) => (b: string) => (c: string): [string, string, string] => [a, b, c]

  // manual lifting, check out the "Lifting" section in "Getting started with fp-ts: Applicative"
  return V.ap(V.ap(V.map(firstValidation, tuple), secondValidation), thirdValidation)
}

function validatePassword(s: string): Either<NonEmptyArray<string>, string> {
  return pipe(
    specializedSequenceT(minLengthV(s), oneCapitalV(s), oneNumberV(s)),
    map(() => s)
  )
}

Edouard Penin • Nov 30 '19

Ohhhh, okay, the tuple of results is what you apply to. Thank you so much for your answer!
(And yes, going back to the applicative article right now 😉)