πŸ›‚ A result type that accumulates multiple errors.





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πŸ›‚ Validated


A result type that accumulates multiple errors.

Table of Contents


The problem

Swift error handling short-circuits on the first failure. Because of this, it's not the greatest option for handling things like form data, where multiple inputs may result in multiple errors.

struct User {
  let id: Int
  let email: String
  let name: String

func validate(id: Int) throws -> Int {
  guard id > 0 else {
    throw Invalid.error("id must be greater than zero")
  return id

func validate(email: String) throws -> String {
  guard email.contains("@") else {
    throw Invalid.error("email must be valid")
  return email

func validate(name: String) throws -> String {
  guard !name.isEmpty else {
    throw Invalid.error("name can't be blank")
  return name

func validateUser(id: Int, email: String, name: String) throws -> User {
  return User(
    id: try validate(id: id),
    email: try validate(id: email),
    name: try validate(id: name)

Here we've combined a few throwing functions into a single throwing function that may return a User.

let user = try validateUser(id: 1, email: "blob@pointfree.co", name: "Blob")
// User(id: 1, email: "blob@pointfree.co", name: "Blob")

If the id, email, or name are invalid, an error is thrown.

let user = try validateUser(id: 1, email: "blob@pointfree.co", name: "")
// throws Invalid.error("name can't be blank")

Unfortunately, if several or all of these inputs are invalid, the first error wins.

let user = try validateUser(id: -1, email: "blobpointfree.co", name: "")
// throws Invalid.error("id must be greater than zero")

Handling multiple errors with Validated

Validated is a Result-like type that can accumulate multiple errors. Instead of using throwing functions, we can define functions that work with Validated.

func validate(id: Int) -> Validated<Int, String> {
  return id > 0
    ? .valid(id)
    : .error("id must be greater than zero")

func validate(email: String) -> Validated<String, String> {
  return email.contains("@")
    ? .valid(email)
    : .error("email must be valid")

func validate(name: String) -> Validated<String, String> {
  return !name.isEmpty
    ? .valid(name)
    : .error("name can't be blank")

To accumulate errors, we use a function that we may already be familiar with: zip.

let validInputs = zip(
  validate(id: 1),
  validate(email: "blob@pointfree.co"),
  validate(name: "Blob")
// Validated<(Int, String, String), String>

The zip function on Validated works much the same way it works on sequences, but rather than zipping a pair of sequences into a sequence of pairs, it zips up a group of single Validated values into single Validated value of a group.

From here, we can use another function that we may already be familiar with, map, which takes a transform function and produces a new Validated value with its valid case transformed.

let validUser = validInputs.map(User.init)
// valid(User(id: 1, email: "blob@pointfree.co", name: "Blob"))

Out group of valid inputs has transformed into a valid user.

For ergonomics and composition, a curried zip(with:) function is provided that takes both a transform function and Validated inputs.

zip(with: User.init)(
  validate(id: 1),
  validate(email: "blob@pointfree.co"),
  validate(name: "Blob")
// valid(User(id: 1, email: "blob@pointfree.co", name: "Blob"))

An invalid input yields an error in the invalid case.

zip(with: User.init)(
  validate(id: 1),
  validate(email: "blob@pointfree.co"),
  validate(name: "")
// invalid(["name can't be blank"])

More importantly, multiple invalid inputs yield an invalid case with multiple errors.

zip(with: User.init)(
  validate(id: -1),
  validate(email: "blobpointfree.co"),
  validate(name: "")
// invalid([
//   "id must be greater than zero",
//   "email must be valid",
//   "name can't be blank"
// ])

Invalid errors are held in a non-empty array to provide a compile-time guarantee that you will never encounter an empty invalid case.


You can add Validated to an Xcode project by adding it as a package dependency.


If you want to use Validated in a SwiftPM project, it's as simple as adding it to a dependencies clause in your Package.swift:

dependencies: [
  .package(url: "https://github.com/pointfreeco/swift-validated", from: "0.2.1")

Interested in learning more?

These concepts (and more) are explored thoroughly in Point-Free, a video series exploring functional programming and Swift hosted by Brandon Williams and Stephen Celis.

Validated was explored in The Many Faces of Zip: Part 2:

video poster image


All modules are released under the MIT license. See LICENSE for details.

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