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Swift String Showcase: Privacy Logs

May 2021

Here I'm (a year late to the party πŸ₯²) explaining what Swift String Interpolations is all about and showcasing a custom string interpolation type.

Here's the full implementation and bits of context to follow along.

Introduction to String Interpolation

According to Rich Hickey's vibes, we're up to look for interpolation's definition. Without further ado:

  1. the insertion of something of a different nature into something else
    • "the interpolation of songs into the piece"
  2. a remark interjected in a conversation
    • "as the evening progressed their interpolations became more ridiculous"

A remark interjected in a conversation 🧐? Voila, a thing to interpolate in the everyday dandy dictionary.

Anyway, talking about the definitions, we're going to concern ourselves with the first one. Most modern programming languages support string interpolation in some way:

  • Rust println!("The {species}'s name is {name}.");
  • JS console.log(`The ${species}'s name is {name}.`);
  • Elixir IO.puts("The #{species}'s name is #{name}.")
  • Swift print("The \(species)'s name is \(name).")
  • You have the idea...

Nothing particularly new here, right? Except here is Swift 5's custom string interpolations SE-0228 proposal. In case you've missed it completely, it gives control over the default string interpolation. Here are the proposed potential uses:

// Use printf-style format strings:
"The price is $\(cost, format: "%.2f")"

// Use UTS #35 number formats:
"The price is \(cost, format: "Β€###,##0.00")"

// Use Foundation.NumberFormatter, or a new type-safe native formatter:
"The price is \(cost, format: moneyFormatter)"

// Mimic String.init(_:radix:uppercase:)
"The checksum is 0x\(checksum, radix: 16)"

What I did a year ago - I've checked said uses, which are totally rad, thought, "yeah, that's cool," and was off with it. But I've entirely missed the way of said proposal to land - custom interpolation types.

Don't be me, read to the end, or review or revise the proposal, or some other explanation.

Extend String Interpolation

Before we talk about custom interpolation types, I want to mention that types are not the only available extension point. Most of the time, it's much quicker to drop a String.StringInterpolation extension.

Let's have a look at what it takes to implement the purposed use cases:

let πŸ‡ΊπŸ‡Έ = Locale(identifier: "en_US")

extension String.StringInterpolation {
// Use printf-style format strings:
mutating func appendInterpolation(_ value: Double, format: String) {
appendInterpolation(String(format: format, value))

// Use UTS #35 number formats:
mutating func appendInterpolation(_ value: NSNumber, format: String) {
let formatter = NumberFormatter()
formatter.locale = πŸ‡ΊπŸ‡Έ
formatter.format = format // .format @available(macOS 10.0, *)
appendInterpolation(formatter.string(from: value)!)

// Use Foundation.NumberFormatter, or a new type-safe native formatter:
mutating func appendInterpolation(_ value: Double, format: NumberFormatter) {
appendInterpolation(format.string(from: value as NSNumber)!)

// Mimic String.init(_:radix:uppercase:)
mutating func appendInterpolation<Value: BinaryInteger>(_ value: Value, radix: Int, uppercase: Bool = false) {
appendInterpolation(String(value, radix: radix, uppercase: uppercase))

let moneyFormatter = NumberFormatter()
moneyFormatter.locale = πŸ‡ΊπŸ‡Έ
moneyFormatter.numberStyle = .currency
moneyFormatter.maximumFractionDigits = 2

let cost = 12.8
"The price is $\(cost, format: "%.2f")" // The price is $12.80
"The price is \(cost as NSNumber, format: "Β€###,##0.00")" // The price is $12.80
"The price is \(cost, format: moneyFormatter)" // The price is $12.80

let checksum = 123123123
"The checksum is 0x\(checksum, radix: 16)" // The checksum is 0x756b5b3

Well, it wasn't that complicated or horrible (aside from emoji locales). Oh, you can find more examples of StringInterpolation extensions here.

If you follow along with the implementation, you may notice that things kind of become messy with every override of appendInterpolation. We were forced to juggle with types (NSNumber/Double) and compromise on APIs, far from optimal. That's where custom string interpolation types come into play.

Custom String Interpolation Type

Aside from reducing String type clutter, custom string interpolation types rein how and what is out to be interpolated. But to think about it, where can it be useful?

Well... How about logs? Logging is a pain point on iOS (and we're clearly not there yet). The post's idea emerged while I was lurking around Peter's post and stumbled upon the next lines:

let logger = Logger(subsystem: "com.steipete.LoggingTest", category: "main")"Logging \(obj.description, privacy: .public)")

Aha (obj.description, privacy: .public)! Clearly, a custom string interpolation is in play. I immediately went off to check its implementation at Apples' Swift Log repo. Alas, this is not a complete os.Logger implementation, and it misses interpolation details.

I guess we have no choice but to try and implement our own logs privacy policy :) Let's define an API surface:

func log(_ message: Message) { print(message) }

log("Hello, \("World!", privacy: .public)") // Hello, world!
log("Hello, \("Secret!", privacy: .private)") // Hello, ******!
log("Static strings say whatever they want") // Static strings say whatever they want

By the way, Logger's 'privacy: is set to .auto. All of that without saying that OSLogPrivacy is way more intricate than our example implementation:

struct Message {
enum Privacy {
case `public`
case `private`

var value: String

So, we already can log Message instances:

log(Message(value: "Hello, World!")) // Message(value: "Hello, World!")

But they aren't looking pretty, are they? Thankfully we are one protocol conformance away from fixing the situation:

extension Message: CustomStringConvertible {
var description: String { value }

log(Message(value: "Hello, World!")) // Hello, World!

How about dealing with static strings or string literals before tackling interpolations? Plus ExpressibleByStringLiteral conformance is a requirement for ExpressibleByStringInterpolation anyway.

extension Message: ExpressibleByStringLiteral {
init(stringLiteral value: StringLiteralType) {
self.init(value: value)

log("Hello, World!") // Hello, World!

Now we're ready to deal with the whole interpolation thing:

extension Message: ExpressibleByStringInterpolation {
init(stringInterpolation: StringInterpolation) {
self.init(value: stringInterpolation.string)

struct StringInterpolation: StringInterpolationProtocol {
var string = ""

init(literalCapacity: Int, interpolationCount: Int) {

mutating func appendLiteral(_ literal: String) {

mutating func appendInterpolation<T>(_ value: T, privacy: Privacy = .private) where T: CustomStringConvertible {
switch privacy {
case .public:
case .private:
Array(repeating: "*", count: value.description.count).joined()

The main workhorse of ExpressibleByStringInterpolation is Message.StringInterpolation type and its appendInterpolation implementation. You can check out full implementation here.

Logger Nuances

As I've mentioned before, an actual Logger implementation is way more intricate. We can get some insights into how things work by checking the os interface:

public struct Logger {
public func log(_ message: OSLogMessage)

public struct OSLogMessage : ExpressibleByStringInterpolation, ExpressibleByStringLiteral {
public init(stringInterpolation: OSLogInterpolation)

So, OSLogInterpolation is an analogue to our Message.StringInterpolation type. By digging further, we can see tons of overrides of appendInterpolation:

public mutating func appendInterpolation(
_ argumentString: @autoclosure @escaping () -> String,
align: OSLogStringAlignment = .none,
privacy: OSLogPrivacy = .auto
"Hello, \("World!",
align: OSLogStringAlignment.right(columns: 20))

// > Hello, World!

public func appendInterpolation(
_ number: @autoclosure @escaping () -> Double,
format: OSLogFloatFormatting = .fixed,
align: OSLogStringAlignment = .none,
privacy: OSLogPrivacy = .auto
)"\(123.123, format: .exponential)")
// > 1.231230e+02

First of all, arguments are following @autoclosure @escaping () -> T signature. In that way, some arguments won't be computed when logs are switched off (either overall or reduced by a log level).

Secondly, formatting parameters like .format and .align to tune output meessages. The official OSLogInterpolation's documentation covers more of them.

Thirdly, there are many overrides for concrete types like UInt8/UInt16/UInt32/UInt64/etc.. I presume it speeds up either compilation or method resolution for specified types πŸ€”


As you can see, Swift String Interpolation is a hilarious feature that might be easily overlooked, as it feels so at home with the current implementation.

You may find a much more detailed explanation of ExpressibleByStringInterpolation at NSHipster, which covers yet another custom interpolation type example.

Know more cases of ExpressibleByStringInterpolation? Don't hesitate to hit me up on Twitter!

You can make anything, till next time :)

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Β© 2021 Egor Mihnevich, All Rights Reserved is released under a Creative Commons BY-NC License