Swift Builders

A collection of useful result builders for Swift and Foundation value types.

Motivation

Arrays, dictionaries, and other collection-based types in Swift are relatively simple to construct and mutate.

However, things get tricky when the contained elements depend on certain conditions or awkward logic. A prime example of this is constructing a payload to send to an analytics service. The resulting code might look like this:

func checkoutAnalyticsEvent(didSucceed: Bool, purchaseAmount: Decimal, userId: String?) -> [String: String] {
    var event: [String: String] = [:]
    event["success"] = didSucceed ? "true" : "false"
    if purchaseAmount > 0 {
        event["amount"] = purchaseAmount.formatted(.number.precision(.fractionLength(2)))
    } else {
        event["isFree"] = "true"
    }
    if let userId = userId {
        event["userId"] = userId
    } else {
        event["isGuest"] = "true"
    }
    return event
}

It’s not bad, but it’s definitely not as Swifty as one would expect.

We’re sprinkling imperative code on what should just be a description of our payload. Not only does this make it harder to reason about the code at a glance, but it also leaves too much leeway for unintended mutations.

Thankfully, there’s a better way…

Getting started

Swift Builders enables result builder syntax for most Collection types in Swift and Foundation.

For example, by leveraging Dictionary.build, our use case above becomes:

import Builders

func checkoutAnalyticsEvent(didSucceed: Bool, purchaseAmount: Decimal, userId: String?) -> [String: String] {
    return [String: String].build {
        ["success": didSucceed ? "true" : "false"]
        if purchaseAmount > 0 {
            ["amount": purchaseAmount.formatted(.number.precision(.fractionLength(2)))]
        } else {
            ["isFree": "true"]
        }
        if let userId = userId {
            ["userId": userId]
        } else {
            ["isGuest": "true"]
        }
    }
}

We can even annotate our function with the @DictionaryBuilder attribute to make the function body behave like the builder body itself (think @ViewBuilder):

import Builders

@DictionaryBuilder<String, String>
func checkoutAnalyticsEvent(didSucceed: Bool, purchaseAmount: Decimal, userId: String?) -> [String: String] {
    ["success": didSucceed ? "true" : "false"]
    if purchaseAmount > 0 {
        ["amount": purchaseAmount.formatted(.number.precision(.fractionLength(2)))]
    } else {
        ["isFree": "true"]
    }
    if let userId = userId {
        ["userId": userId]
    } else {
        ["isGuest": "true"]
    }
}

This is only a small demonstration of the power of result builders applied to Swift’s native types.

The library offers a variety of builders out of the box:

• ArrayBuilder
• ArraySliceBuilder
• ContiguousArrayBuilder
• DataBuilder
• DictionaryBuilder
• SetBuilder
• SliceBuilder
• StringBuilder
• StringUTF8ViewBuilder
• StringUnicodeScalarViewBuilder
• SubstringBuilder
• SubstringUTF8ViewBuilder
• SubstringUnicodeScalarViewBuilder

Benchmarks

MacBook Pro (14-inch, 2021)
Apple M1 Pro (10 cores, 8 performance and 2 efficiency)
32 GB Memory

$ swift run -c release Benchmarks

name                              time        std        iterations
-------------------------------------------------------------------
Array<Any>.build                  1833.000 ns ±   7.60 %     757726
Array<Int>.build                   542.000 ns ±  15.49 %    1000000
Array<Int?>.build                  709.000 ns ±   9.51 %    1000000
ArraySlice<Any>.build             2750.000 ns ±   5.28 %     511759
ArraySlice<Int>.build              875.000 ns ±   8.40 %    1000000
ArraySlice<Int?>.build            1167.000 ns ±  13.55 %    1000000
ContiguousArray<Any>.build        1917.000 ns ±  12.37 %     729365
ContiguousArray<Int>.build         542.000 ns ±  23.24 %    1000000
ContiguousArray<Int?>.build        750.000 ns ±  13.97 %    1000000
Data.build                         875.000 ns ±  13.55 %    1000000
Dictionary<String, Any>.build     4209.000 ns ±   6.26 %     328025
Dictionary<String, Double>.build  2459.000 ns ±  11.92 %     562007
Dictionary<String, Double?>.build 2583.000 ns ±   5.51 %     526636
Set<Any>.build                    6333.000 ns ±  10.30 %     228224
Set<Int>.build                     750.000 ns ±  11.22 %    1000000
Set<Int?>.build                   1292.000 ns ±  11.42 %    1000000
Slice<Array<Any>>.build           2209.000 ns ±   4.95 %     629537
Slice<Array<Int>>.build            584.000 ns ±  17.10 %    1000000
Slice<Array<Int?>>.build           917.000 ns ±   8.49 %    1000000
String.build                       500.000 ns ±   8.91 %    1000000
String.UnicodeScalarView.build    3958.000 ns ±   3.04 %     351918
String.UTF8View.build              542.000 ns ±  10.33 %    1000000
Substring.build                   1709.000 ns ±   4.41 %     810685
Substring.UnicodeScalarView.build 5084.000 ns ±   3.19 %     274560
Substring.UTF8View.build          1333.000 ns ±   5.89 %    1000000