Swift end result builder fundamentals
The end result builder proposal (initially it was referred to as perform builders) was carried out in Swift 5.4. This characteristic permits us to construct up a end result worth utilizing a sequence of elements. At first sight, you may assume, hey this seems like an array with a sequence of components, besides the coma in between the gadgets, however nope, that is utterly totally different. However why is it good for us?
End result builder can be utilized to create fully new Area-Particular Languages (DSLs) inside Swift. Making a DSL has many benefits, since DSLs are often tied to a selected downside, the syntax that you simply use to explain the language may be very light-weight, but highly effective and succesful. Since Swift DSLs are kind secure, it’s a lot safer to make use of one as a substitute of manually concatenate objects. Swift DSLs additionally permits us to make use of fundamental management flows inside these embedded micro-languages. 🤔
Let me offer you an instance: you possibly can write HTML in Swift, you possibly can merely write out all of the tags and glue a bunch of String values collectively, however that would not be so secure, proper?
func buildWebpage(title: String, physique: String) -> String {
"""
<html>
<head>
<title>(title)</title>
</head>
<physique>
<h1>(title)</h1>
<h1>(physique)</h1>
</physique>
</html>
"""
}
let html = buildWebpage(title: "Lorem ipsum", physique: "dolor sit amet")
print(html)We will all agree that that is ugly and the compiler will not show you how to to detect the semantic points in any respect. Now if we substitute the next code with a DSL, we are going to drastically advantage of the Swift compiler options. Swift will give us kind security, so our code can be much less error susceptible. A DSL can have many constraints and restrictions that’ll assist others to put in writing higher code. In our case the listing of tags goes to be a predefined set of values, so you will not be capable to present a flawed tag or miss the closing tag, in different phrases your DSL goes to be syntactically legitimate. In fact you continue to can have logical errors, however that is all the time the case, it doesn’t matter what software you select. 🧠
import SwiftHtml
func buildWebpage(title: String, physique: String) -> String {
let doc = Doc(.unspecified) {
Html {
Head {
Title(title)
}
Physique {
H1(title)
P(physique)
}
}
}
return DocumentRenderer().render(doc)
}As you possibly can see the snippet above seems far more Swifty and we have been additionally capable of take away the duplicate HTML closing tags from the code. We do not have to put in writing the < and > characters in any respect and the compiler can kind test every thing for us, so type-o accidents cannot occur. ✅
Earlier than you assume that end result builders are simply syntactic sugar over underlying knowledge varieties, I’ve to guarantee you that they’re much more complicated than this. It’s a particularly superior and highly effective characteristic that it’s best to undoubtedly find out about.
You’ll be able to create all types of end result builders, for instance I am utilizing them to construct validators, consumer interface components and format constraints. In fact SGML (HTML, XML) and CSS can be a terrific use-case, however the listing is limitless. Let me present you how one can construct a easy end result builder.
Constructing a HTML tree construction
I will present you ways I created my SwiftHtml HTML DSL library, as a result of it was a enjoyable mission to work with and I’ve realized lots about it, it is also going to switch the Leaf/Tau template in my future tasks. The primary thought behind SwiftHtml was that I wished to observe the HTML specs as intently as doable. So I’ve created a Node construction to symbolize a node contained in the doc tree.
public struct Node {
public enum `Sort` {
case commonplace
case remark
case empty
case group
}
public let kind: `Sort`
public let identify: String?
public let contents: String?
public init(kind: `Sort` = .commonplace,
identify: String? = nil,
contents: String? = nil) {
self.kind = kind
self.identify = identify
self.contents = contents
}
}A node has 4 variants outlined by the Sort. An ordinary node will render as an ordinary HTML tag utilizing the identify and the contents. A remark will solely use the contents and empty tag will not have a closing tag and use the identify property as a tag identify. Lastly the group node can be used to group collectively a number of nodes, it will not render something, it is only a grouping ingredient for different tags.
The trick in my answer is that these Node objects solely include the visible illustration of a tag, however I’ve determined to separate the hierarchical relationship from this degree. That is why I truly launched a Tag class that may have a number of youngsters. In my earlier article I confirmed a number of methods to construct a tree construction utilizing Swift, I’ve experimented with all of the doable options and my remaining alternative was to make use of reference varieties as a substitute of worth varieties. Do not hate me. 😅
open class Tag {
public var node: Node
public var youngsters: [Tag]
public init(_ node: Node, youngsters: [Tag] = []) {
self.node = node
self.youngsters = youngsters
}
}Now that is how a Tag object seems like, it is fairly easy. It has an underlying node and a bunch of youngsters. It’s doable to increase this tag and supply functionalities for all of the HTML tags, equivalent to the potential of including widespread attributes and I am additionally capable of create subclasses for the tags.
public remaining class Html: Tag {
public init(_ youngsters: [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "html", contents: nil), youngsters: youngsters)
}
}
public remaining class Head: Tag {
public init(_ youngsters: [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "head", contents: nil), youngsters: youngsters)
}
}
public remaining class Title: Tag {
public init(_ contents: String) {
tremendous.init(.init(kind: .commonplace, identify: "title", contents: contents))
}
}
public remaining class Physique: Tag {
public init(_ youngsters: [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "physique", contents: nil), youngsters: youngsters)
}
}
public remaining class H1: Tag {
public init(_ contents: String) {
tremendous.init(.init(kind: .commonplace, identify: "h1", contents: contents))
}
}
public remaining class P: Tag {
public init(_ contents: String) {
tremendous.init(.init(kind: .commonplace, identify: "p", contents: contents))
}
}All proper, now we’re capable of initialize our Tag tree, however I warn you, it is going to look very awkward.
func buildWebpage(title: String, physique: String) -> Html {
Html([
Head([
Title(title),
]),
Physique([
H1(title),
P(body),
]),
])
}It’s nonetheless not doable to render the tree and the syntax shouldn’t be so eye-catchy. It is time to make issues higher and we should always undoubtedly introduce some end result builders for good.
The anatomy of Swift end result builders Now that we have now our knowledge construction ready, we should always give attention to the DSL itself. Earlier than we dive in, I extremely suggest to fastidiously learn the official proposal and watch this WWDC video about end result builders, since each sources are wonderful. 🤓
Constructing an array of components
The primary factor that I do not like about our earlier buildWebpage perform is that I’ve to continually write brackets and comas, to be able to construct our construction. This may be simply eradicated by introducing a brand new end result builder for the Tag objects. We simply should mark an enum with the @resultBuilder attribute and supply a static buildBlock technique with the given kind.
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
}It will enable us to make use of a listing of elements within our DSL constructing blocks, however earlier than we might use it we even have to vary our particular HTML tag init strategies to reap the benefits of this newly created end result builder. Simply use a closure with the return kind that we need to use and mark the whole perform argument with the @TagBuilder key phrase.
public remaining class Html: Tag {
public init(@TagBuilder _ builder: () -> [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "html", contents: nil), youngsters: builder())
}
}
public remaining class Head: Tag {
public init(@TagBuilder _ builder: () -> [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "head", contents: nil), youngsters: builder())
}
}
public remaining class Physique: Tag {
public init(@TagBuilder _ builder: () -> [Tag]) {
tremendous.init(.init(kind: .commonplace, identify: "physique", contents: nil), youngsters: builder())
}
}Now we are able to refactor the construct webpage technique since it may well now use the underlying end result builder to assemble the constructing blocks based mostly on the elements. In the event you check out the introduction part contained in the proposal you will get a greater thought about what occurs underneath the hood.
func buildWebpage(title: String, physique: String) -> Html {
Html {
Head {
Title(title)
}
Physique {
H1(title)
P(physique)
}
}
}
let html = buildWebpage(title: "title", physique: "physique")Anyway, it is fairly magical how we are able to rework our complicated array based mostly code into one thing clear and good by benefiting from the Swift compiler. I like this strategy, however there’s extra.
Optionals and additional construct blocks
If you wish to present if assist inside your DSL you must implement some extra strategies inside your end result builder object. Do that code, nevertheless it will not compile:
func buildWebpage(title: String, physique: String) -> Html {
Html {
Head {
Title(title)
}
Physique {
if title == "magic" {
H1(title)
P(physique)
}
}
}
}The construct an non-obligatory end result with an if assertion we have now to consider what occurs right here. If the title is magic we want to return an array of Tags, in any other case nil. So this might be expressed as a [Tag]? kind however we all the time need to have a bunch of [Tag] components, now that is straightforward.
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
public static func buildOptional(_ element: [Tag]?) -> [Tag] {
element ?? []
}
}However wait, why is it not working? Properly, since we return an array of tags, however the outer Physique ingredient was anticipating Tag components one after one other, so a [Tag] array will not match our wants there. What can we do about this? Properly, we are able to introduce a brand new buildBlock technique that may rework our [Tag]... values right into a plain Tag array. Let me present you actual this fast.
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
public static func buildBlock(_ elements: [Tag]...) -> [Tag] {
elements.flatMap { $0 }
}
public static func buildOptional(_ element: [Tag]?) -> [Tag] {
element ?? []
}
}
func buildWebpage(title: String, physique: String) -> Html {
Html {
Head {
Title(title)
}
Physique {
if title == "magic" {
H1("Hiya")
P("World")
}
}
}I hope it is not too sophisticated, nevertheless it’s all about constructing the correct return kind for the underlying technique. We wished to have simply an array of tags, however with the if assist we have ended up with a listing of tag arrays, that is why we have now to remodel it again to a flattened array of tags with the brand new construct block. In order for you to try a extra easy instance, it’s best to learn this submit. ☺️
If and else assist and both blocks
If blocks can return non-obligatory values, now what about if-else blocks? Properly, it is fairly an identical strategy, we simply need to return both the primary or the second array of tags.
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
public static func buildBlock(_ elements: [Tag]...) -> [Tag] {
elements.flatMap { $0 }
}
public static func buildOptional(_ element: [Tag]?) -> [Tag] {
element ?? []
}
public static func buildEither(first element: [Tag]) -> [Tag] {
element
}
public static func buildEither(second element: [Tag]) -> [Tag] {
element
}
}
func buildWebpage(title: String, physique: String) -> Html {
Html {
Head {
Title(title)
}
Physique {
if title == "magic" {
H1("Hiya")
P("World")
}
else {
P(physique)
}
}
}
}
let html = buildWebpage(title: "title", physique: "physique")As you possibly can see now we do not want extra constructing blocks, since we have already lined the variadic Tag array problem with the non-obligatory assist. Now it’s doable to put in writing if and else blocks inside our HTML DSL. Seems to be fairly good to date, what’s subsequent? 🧐
Enabling for loops and maps by expressions
Think about that you’ve a bunch of paragraphs within the physique that you simply’d like to make use of. Fairly straightforward, proper? Simply change the physique into an array of strings and use a for loop to remodel them into P tags.
func buildWebpage(title: String, paragraphs: [String]) -> Html {
Html {
Head {
Title(title)
}
Physique {
H1(title)
for merchandise in paragraphs {
P(merchandise)
}
}
}
}
let html = buildWebpage(title: "title", paragraphs: ["a", "b", "c"])Not so quick, what is the precise return kind right here and the way can we remedy the issue? In fact the primary impression is that we’re returning a Tag, however in actuality we would like to have the ability to return a number of tags from a for loop, so it is a [Tag], in the long run, it is going to be an array of Tag arrays: [[Tag]].
The buildArray technique can rework these array of tag arrays into Tag arrays, that is adequate to offer for assist, however we nonetheless want another technique to have the ability to use it correctly. We now have to construct an expression from a single Tag to show it into an array of tags. 🔖
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
public static func buildBlock(_ elements: [Tag]...) -> [Tag] {
elements.flatMap { $0 }
}
public static func buildEither(first element: [Tag]) -> [Tag] {
element
}
public static func buildEither(second element: [Tag]) -> [Tag] {
element
}
public static func buildOptional(_ element: [Tag]?) -> [Tag] {
element ?? []
}
public static func buildExpression(_ expression: Tag) -> [Tag] {
[expression]
}
public static func buildArray(_ elements: [[Tag]]) -> [Tag] {
elements.flatMap { $0 }
}
}This manner our for loop will work. The construct expression technique may be very highly effective, it allows us to offer varied enter varieties and switch them into the information kind that we really want. I will present you another construct expression instance on this case to assist the map perform on an array of components. That is the ultimate end result builder:
@resultBuilder
public enum TagBuilder {
public static func buildBlock(_ elements: Tag...) -> [Tag] {
elements
}
public static func buildBlock(_ elements: [Tag]...) -> [Tag] {
elements.flatMap { $0 }
}
public static func buildEither(first element: [Tag]) -> [Tag] {
element
}
public static func buildEither(second element: [Tag]) -> [Tag] {
element
}
public static func buildOptional(_ element: [Tag]?) -> [Tag] {
element ?? []
}
public static func buildExpression(_ expression: Tag) -> [Tag] {
[expression]
}
public static func buildExpression(_ expression: [Tag]) -> [Tag] {
expression
}
public static func buildArray(_ elements: [[Tag]]) -> [Tag] {
elements.flatMap { $0 }
}
}Now we are able to use maps as a substitute of for loops if we choose purposeful strategies. 😍
func buildWebpage(title: String, paragraphs: [String]) -> Html {
Html {
Head {
Title(title)
}
Physique {
H1(title)
paragraphs.map { P($0) }
}
}
}
let html = buildWebpage(title: "title", paragraphs: ["a", "b", "c"])That is how I used to be capable of create a DSL for my Tag hierarchy. Please notice that I would had some issues flawed, this was the very first DSL that I’ve made, however to date so good, it serves all my wants.
A easy HTML renderer
Earlier than we shut this text I would like to point out you ways I created my HTML doc renderer.
struct Renderer {
func render(tag: Tag, degree: Int = 0) -> String {
let indent = 4
let areas = String(repeating: " ", rely: degree * indent)
change tag.node.kind {
case .commonplace:
return areas + open(tag) + (tag.node.contents ?? "") + renderChildren(tag, degree: degree, areas: areas) + shut(tag)
case .remark:
return areas + "<!--" + (tag.node.contents ?? "") + "-->"
case .empty:
return areas + open(tag)
case .group:
return areas + (tag.node.contents ?? "") + renderChildren(tag, degree: degree, areas: areas)
}
}
personal func renderChildren(_ tag: Tag, degree: Int, areas: String) -> String {
var youngsters = tag.youngsters.map { render(tag: $0, degree: degree + 1) }.joined(separator: "n")
if !youngsters.isEmpty {
youngsters = "n" + youngsters + "n" + areas
}
return youngsters
}
personal func open(_ tag: Tag) -> String {
return "<" + tag.node.identify! + ">"
}
personal func shut(_ tag: Tag) -> String {
"</" + tag.node.identify! + ">"
}
}As you possibly can see it is a fairly easy, but complicated struct. The open and shut strategies are easy, the fascinating half occurs within the render strategies. The very first render perform can render a tag utilizing the node kind. We simply change the kind and return the HTML worth in line with it. if the node is an ordinary or a gaggle kind we additionally render the kids utilizing the identical technique.
In fact the ultimate implementation is a little more complicated, it includes HTML attributes, it helps minification and customized indentation degree, however for instructional functions this light-weight model is greater than sufficient. Here is the ultimate code snippet to render a HTML construction:
func buildWebpage(title: String, paragraphs: [String]) -> Html {
Html {
Head {
Title(title)
}
Physique {
H1(title)
paragraphs.map { P($0) }
}
}
}
let html = buildWebpage(title: "title", paragraphs: ["a", "b", "c"])
let output = Renderer().render(tag: html)
print(output)If we examine this to our very first string based mostly answer we are able to say that the distinction is big. Truthfully talking I used to be afraid of end result builders for a really very long time, I believed it is simply pointless complexity and we do not really want them, however hey issues change, and I’ve additionally modified my thoughts about this characteristic. Now I can not dwell with out end result builders and I like the code that I can write through the use of them. I actually hope that this text helped you to know them a bit higher. 🙏
