A collection of useful result builders for Swift and Foundation value types.
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...
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:
ArrayBuilderArraySliceBuilderContiguousArrayBuilderDataBuilderDictionaryBuilderSetBuilderSliceBuilderStringBuilderStringUTF8ViewBuilderStringUnicodeScalarViewBuilderSubstringBuilderSubstringUTF8ViewBuilderSubstringUnicodeScalarViewBuilder
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