StringInterpolation in Swift 5 — Introduction

In Swift 4, the StringInterpolation protocol got deprecated, because its original design was inefficient and inflexible, with the goal of redisigning it entirely after Swift 4. Since then, SE-0228 introduced a new design for StringInterpolation, which is going to be part of Swift 5, and opens a whole lot of powerful possibilities.

Since the implementation has already landed in the master branch of Swift, it’s already possible to play with that new StringInterpolation, by downloading a snapshot to install the latest Swift 5 toolchain and use it in Xcode… so let’s have fun!

The new StringInterpolation design

I really encourage you to read the SE-0228 proposal to get an idea of the design and motivations behind the new API.

Basically, to make a type conform to ExpressibleByStringInterpolation, you have to:

  • Make this type have a subtype StringInterpolation, that must itself conform to StringInterpolationProtocol and will be responsible to handle the interpretation of the interpolation
  • That subtype just have to implement appendLiteral(_ literal: String) and one or more appendInterpolation(…) method, with the signature of your choosing depending on what you want to support
  • This StringInterpolation subtype will serve as a “Builder Pattern” for your main type, and the compiler will call those append… methods to build the object step by step
  • Then your main type needs to implement init(stringInterpolation: StringInterpolation) to instantiate itself with the result of those incremental steps.

The fact that you can implement whatever appendInterpolation(…) method you like means that you can choose what interpolation to support. This is a super powerful feature that opens a large range of possibilities!

For example, if you implement func appendInterpolation(_ string: String, pad: Int) that means that you’ll be able to build your type using an interpolation like: "Hello \(name, pad: 10), how are you?". The interpolation just has to match one of the appendInterpolation signatures that your StringInterpolation subtype support.

Simple example

Let’s start with a simple type to demonstrate how it goes. Let’s build a GitHubComment type that would allow you to reference issue numbers and users.

The goal of that example is to be then able to write something like this:

let comment: GitHubComment = """
  See \(issue: 123) where \(user: "alisoftware") explains the steps to reproduce.

So, how do we implement this?

First let’s declare the basic struct GitHubComment and make it ExpressibleByStringLiteral (because ExpressibleByStringInterpolation inherits that protocol so let’s get that implementation out of the way) and CustomStringConvertible (for nice debugging when printing in the console)

struct GitHubComment {
  let markdown: String

extension GitHubComment: ExpressibleByStringLiteral {
  init(stringLiteral value: String) {
    self.markdown = value

extension GitHubComment: CustomStringConvertible {
  var description: String {
    return self.markdown

Then, we’ll make GitHubComment conform to ExpressibleByStringInterpolation, which means having a StringInterpolation subtype that will handle what to do when:

  • initializing itself: init(literalCapacity: Int, interpolationCount: Int) lets you the possibility to reserve some capacity to the buffer you’ll use while building the type step by step. In our case, we could simply have used a String and append the segments to it while building the instance, but I instead chose to use a parts: [String], that we’ll assemble together later
  • implement appendLiteral(_ string: String) to append the literal text to the parts
  • implement appendInterpolation(user: String) to append the markdown representation of a link to that user’s profile when encountering \(user: xxx)
  • implement appendInterpolation(issue: Int) to append the markdown representation of a link to that issue
  • then implement init(stringInterpolation: StringInterpolation) on GitHubComment to build a comment from those parts
extension GitHubComment: ExpressibleByStringInterpolation {
  struct StringInterpolation: StringInterpolationProtocol {
    var parts: [String]

    init(literalCapacity: Int, interpolationCount: Int) { = []
      // - literalCapacity is the number of characters in literal segments (L)
      // - interpolationCount is the number of interpolation segments (I)
      // We estimate that we generally have a structure like "LILILIL"
      // — e.g. "Hello \(world, .color(.blue))!" — hence the 2n+1*interpolationCount+1)

    mutating func appendLiteral(_ literal: String) {

    mutating func appendInterpolation(user name: String) {"[\(name)](\(name))")

    mutating func appendInterpolation(issue number: Int) {"[#\(number)](issues/\(number))")

  init(stringInterpolation: StringInterpolation) {
    self.markdown =

And that’s it! we’re done!

Notice how, because of the signatures of the appendInterpolation methods we’ve implemented, we’re allowed to use Hello \(user: "alisoftware") but not Hello \(user: 123), as appendInterpolation(user:) expects a String as parameter. Similarly, \(issue: 123) in your string will only allow an Int because appendInterpolation(issue:) takes an Int as parameter.

In fact, if you try to use interpolations that are not supported by your StringInterpolation subtype, the compiler will give you nice errors:

let comment: GitHubComment = """
  See \(issue: "bob") where \(username: "alisoftware") explains the steps to reproduce.
//             ^~~~~         ^~~~~~~~~
// error: cannot convert value of type 'String' to expected argument type 'Int'
// error: incorrect argument label in call (have 'username:', expected 'user:')

It’s only the beginning

This new design opens a large range of possibilities for making your own types ExpressibleByStringInterpolation. Some ideas include:

  • Making a HTML type conform so that you can write HTML using interpolations
  • Making a SQLStatement type conform so you can write SQL statements easily
  • Using string interpolation to support more custom formatting, like formatting Double or Date values inside your interpolated string
  • Making a RegEx type conform so that you can write regular expressions with a fancy syntax
  • Making a AttributedString type conform to build an NSAttributedString using string interpolation

Brent Royal-Gordon, who came up with that awesome design for the new String Interpolation alongside Michael Ilseman, provided some more example in this gist

I for one gave a try at implementing support for NSAttributedString, and I wanted to share that implementation draft with you in a dedicated post, as I find it really beautiful. See you on the part 2 of this post then!