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Tagged – A wrapper type for safer, expressive code

We often work with types that are far too general or hold far too many values than what is necessary for our domain. Sometimes we just want to differentiate between two seemingly equivalent values at the type level.

An email address is nothing but a String, but it should be restricted in the ways in which it can be used. And while a User id may be represented with an Int, it should be distinguishable from an Int-based Subscription id.

Tagged can help solve serious runtime bugs at compile time by wrapping basic types in more specific contexts with ease.

The problem

Swift has an incredibly powerful type system, yet it’s still common to model most data like this:

struct User {
  let id: Int
  let email: String
  let address: String
  let subscriptionId: Int?
}

struct Subscription {
  let id: Int
}

We’re modeling user and subscription ids using the same type, but our app logic shouldn’t treat these values interchangeably! We might write a function to fetch a subscription:

func fetchSubscription(byId id: Int) -> Subscription? {
  return subscriptions.first(where: { $0.id == id })
}

Code like this is super common, but it allows for serious runtime bugs and security issues! The following compiles, runs, and even reads reasonably at a glance:

let subscription = fetchSubscription(byId: user.id)

This code will fail to find a user’s subscription. Worse yet, if a user id and subscription id overlap, it will display the wrong subscription to the wrong user! It may even surface sensitive data like billing details!

The solution

We can use Tagged to succinctly differentiate types.

import Tagged

struct User {
  let id: Id
  let email: String
  let address: String
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
}

struct Subscription {
  let id: Id

  typealias Id = Tagged<Subscription, Int>
}

Tagged depends on a generic “tag” parameter to make each type unique. Here we’ve used the container type to uniquely tag each id.

We can now update fetchSubscription to take a Subscription.Id where it previously took any Int.

func fetchSubscription(byId id: Subscription.Id) -> Subscription? {
  return subscriptions.first(where: { $0.id == id })
}

And there’s no chance we’ll accidentally pass a user id where we expect a subscription id.

let subscription = fetchSubscription(byId: user.id)

🛑 Cannot convert value of type ‘User.Id’ (aka ‘Tagged<User, Int>’) to expected argument type ‘Subscription.Id’ (aka ‘Tagged<Subscription, Int>’)

We’ve prevented a couple serious bugs at compile time!

There’s another bug lurking in these types. We’ve written a function with the following signature:

sendWelcomeEmail(toAddress address: String)

It contains logic that sends an email to an email address. Unfortunately, it takes any string as input.

sendWelcomeEmail(toAddress: user.address)

This compiles and runs, but user.address refers to our user’s billing address, not their email! None of our users are getting welcome emails! Worse yet, calling this function with invalid data may cause server churn and crashes.

Tagged again can save the day.

struct User {
  let id: Id
  let email: Email
  let address: String
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
  typealias Email = Tagged<User, String>
}

We can now update sendWelcomeEmail and have another compile time guarantee.

sendWelcomeEmail(toAddress address: Email)
sendWelcomeEmail(toAddress: user.address)

🛑 Cannot convert value of type ‘String’ to expected argument type ‘Email’ (aka ‘Tagged<EmailTag, String>’)

Handling Tag Collisions

What if we want to tag two string values within the same type?

struct User {
  let id: Id
  let email: Email
  let address: Address
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
  typealias Email = Tagged<User, String>
  typealias Address = Tagged</* What goes here? */, String>
}

We shouldn’t reuse Tagged<User, String> because the compiler would treat Email and Address as the same type! We need a new tag, which means we need a new type. We can use any type, but an uninhabited enum is nestable and uninstantiable, which is perfect here.

struct User {
  let id: Id
  let email: Email
  let address: Address
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
  enum EmailTag {}
  typealias Email = Tagged<EmailTag, String>
  enum AddressTag {}
  typealias Address = Tagged<AddressTag, String>
}

We’ve now distinguished User.Email and User.Address at the cost of an extra line per type, but things are documented very explicitly.

If we want to save this extra line, we could instead take advantage of the fact that tuple labels are encoded in the type system and can be used to differentiate two seemingly equivalent tuple types.

struct User {
  let id: Id
  let email: Email
  let address: Address
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
  typealias Email = Tagged<(User, email: ()), String>
  typealias Address = Tagged<(User, address: ()), String>
}

This may look a bit strange with the dangling (), but it’s otherwise nice and succinct, and the type safety we get is more than worth it.

Accessing Raw Values

Tagged uses the same interface as RawRepresentable to expose its raw values, via a rawValue property:

user.id.rawValue // Int

You can also manually instantiate tagged types using init(rawValue:), though you can often avoid this using the Decodable and ExpressibleByLiteral family of protocols.

Features

Tagged uses conditional conformance, so you don’t have to sacrifice expressiveness for safety. If the raw values are encodable or decodable, equatable, hashable, comparable, or expressible by literals, the tagged values follow suit. This means we can often avoid unnecessary (and potentially dangerous) wrapping and unwrapping.

Equatable

A tagged type is automatically equatable if its raw value is equatable. We took advantage of this in our example, above.

subscriptions.first(where: { $0.id == user.subscriptionId })

Hashable

We can use underlying hashability to create a set or lookup dictionary.

var userIds: Set<User.Id> = []
var users: [User.Id: User] = [:]

Comparable

We can sort directly on a comparable tagged type.

userIds.sorted(by: <)
users.values.sorted(by: { $0.email < $1.email })

Codable

Tagged types are as encodable and decodable as the types they wrap.

struct User: Decodable {
  let id: Id
  let email: Email
  let address: Address
  let subscriptionId: Subscription.Id?

  typealias Id = Tagged<User, Int>
  typealias Email = Tagged<(User, email: ()), String>
  typealias Address = Tagged<(User, address: ()), String>
}

JSONDecoder().decode(User.self, from: Data("""
{
  "id": 1,
  "email": "blob@pointfree.co",
  "address": "1 Blob Ln",
  "subscriptionId": null
}
""".utf8))

ExpressiblyBy-Literal

Tagged types inherit literal expressibility. This is helpful for working with constants, like instantiating test data.

User(
  id: 1,
  email: "blob@pointfree.co",
  address: "1 Blob Ln",
  subscriptionId: 1
)

// vs.

User(
  id: User.Id(rawValue: 1),
  email: User.Email(rawValue: "blob@pointfree.co"),
  address: User.Address(rawValue: "1 Blob Ln"),
  subscriptionId: Subscription.Id(rawValue: 1)
)

Numeric

Numeric tagged types get mathematical operations for free!

struct Product {
  let amount: Cents

  typealias Cents = Tagged<Product, Int>
}
let totalCents = products.reduce(0) { $0 + $1.amount }

Nanolibraries

The Tagged library also comes with a few nanolibraries for handling common types in a type safe way.

TaggedTime

The API’s we interact with often return timestamps in seconds or milliseconds measured from an epoch time. Keeping track of the units can be messy, either being done via documentation or by naming fields in a particular way, e.g. publishedAtMs. Mixing up the units on accident can lead to wildly inaccurate logic.

By importing TaggedTime you will get access to two generic types, Milliseconds<A> and Seconds<A>, that allow the compiler to sort out the differences for you. You can use them in your models:

struct BlogPost: Decodable {
  typealias Id = Tagged<BlogPost, Int>

  let id: Id
  let publishedAt: Seconds<Int>
  let title: String
}

Now you have documentation of the unit in the type automatically, and you can never accidentally compare seconds to milliseconds:

let futureTime: Milliseconds<Int> = 1528378451000

breakingBlogPost.publishedAt < futureTime
// 🛑 Binary operator '<' cannot be applied to operands of type
// 'Tagged<SecondsTag, Double>' and 'Tagged<MillisecondsTag, Double>'

breakingBlogPost.publishedAt.milliseconds < futureTime
// ✅ true

Read more on our blog post: Tagged Seconds and Milliseconds.

TaggedMoney

API’s can also send back money amounts in two standard units: whole dollar amounts or cents (1/100 of a dollar). Keeping track of this distinction can also be messy and error prone.

Importing the TaggedMoney libary gives you access to two generic types, Dollars<A> and Cents<A>, that give you compile-time guarantees in keeping the two units separate.

struct Prize {
  let amount: Dollars<Int> 
  let name: String
}

let moneyRaised: Cents<Int> = 50_000

theBigPrize.amount < moneyRaised
// 🛑 Binary operator '<' cannot be applied to operands of type
// 'Tagged<DollarsTag, Int>' and 'Tagged<CentsTag, Int>'

theBigPrize.amount.cents < moneyRaised
// ✅ true

It is important to note that these types do not encapsulate currency, but rather just the abstract notion of the whole and fractional unit of money. You will still need to track particular currencies, like USD, EUR, MXN, alongside these values.

FAQ

Installation

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

https://github.com/pointfreeco/swift-tagged

If you want to use Tagged 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-tagged", from: "0.6.0")
]
Tagged on GitHub: https://github.com/pointfreeco/swift-tagged
Platform: iOS
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