Outcome builders in Swift – The.Swift.Dev.

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If you wish to make a end result builder in Swift, this text will make it easier to to cope with the most typical instances when making a DSL.

Swift

Swift end result builder fundamentals


The end result builder proposal (initially it was referred to as operate builders) was carried out in Swift 5.4. This function permits us to construct up a end result worth utilizing a sequence of elements. At first sight, you would possibly suppose, hey this appears to be like like an array with a sequence of components, besides the coma in between the gadgets, however nope, that is fully totally different. However why is it good for us?


Outcome builder can be utilized to create totally new Area-Particular Languages (DSLs) inside Swift. Making a DSL has many benefits, since DSLs are often tied to a selected drawback, the syntax that you just use to explain the language may be very light-weight, but highly effective and succesful. Since Swift DSLs are kind protected, 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 primary management flows inside these embedded micro-languages. 🤔


Let me provide you with 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 protected, 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 make it easier to to detect the semantic points in any respect. Now if we change the next code with a DSL, we’ll significantly 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 write down higher code. In our case the checklist of tags goes to be a predefined set of values, so you will not be capable of present a unsuitable tag or miss the closing tag, in different phrases your DSL goes to be syntactically legitimate. After all you continue to can have logical errors, however that is all the time the case, it doesn’t matter what instrument 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 appears to be like 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 write down the characters in any respect and the compiler can kind verify the whole lot for us, so type-o accidents cannot occur. ✅


Earlier than you suppose that end result builders are simply syntactic sugar over underlying knowledge sorts, I’ve to guarantee you that they’re much more advanced than this. It’s a particularly superior and highly effective function that you need to undoubtedly learn about.


You possibly can create all types of end result builders, for instance I am utilizing them to construct validators, person interface components and format constraints. After all SGML (HTML, XML) and CSS can be an excellent use-case, however the checklist is infinite. Let me present you the best way to construct a easy end result builder.




Constructing a HTML tree construction


I’ll present you ways I created my SwiftHtml HTML DSL library, as a result of it was a enjoyable challenge to work with and I’ve realized lots about it, it is also going to interchange the Leaf/Tau template in my future initiatives. The principle concept behind SwiftHtml was that I needed to comply with the HTML specs as carefully as doable. So I’ve created a Node construction to symbolize a node contained in the doc tree.


public struct Node {

    public enum `Kind` {
        case commonplace     
        case remark      
        case empty        
        case group        
    }

    public let kind: `Kind`
    public let title: String?
    public let contents: String?

    public init(kind: `Kind` = .commonplace,
                title: String? = nil,
                contents: String? = nil) {
        self.kind = kind
        self.title = title
        self.contents = contents
    }
}


A node has 4 variants outlined by the Kind. A regular node will render as a typical HTML tag utilizing the title and the contents. A remark will solely use the contents and empty tag will not have a closing tag and use the title property as a tag title. 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 comprise the visible illustration of a tag, however I’ve determined to separate the hierarchical relationship from this degree. That is why I really 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 closing selection was to make use of reference sorts as a substitute of worth sorts. 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 appears to be like 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, akin to the potential of including frequent attributes and I am additionally capable of create subclasses for the tags.


public closing class Html: Tag {

    public init(_ youngsters: [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "html", contents: nil), youngsters: youngsters)
    }
}

public closing class Head: Tag {

    public init(_ youngsters: [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "head", contents: nil), youngsters: youngsters)
    }
}

public closing class Title: Tag {

    public init(_ contents: String) {
        tremendous.init(.init(kind: .commonplace, title: "title", contents: contents))
    }
}

public closing class Physique: Tag {

    public init(_ youngsters: [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "physique", contents: nil), youngsters: youngsters)
    }
}

public closing class H1: Tag {

    public init(_ contents: String) {
        tremendous.init(.init(kind: .commonplace, title: "h1", contents: contents))
    }
}

public closing class P: Tag {

    public init(_ contents: String) {
        tremendous.init(.init(kind: .commonplace, title: "p", contents: contents))
    }
}


All proper, now we’re capable of initialize our Tag tree, however I warn you, it’ll 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 just isn’t 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 assets are superb. 🤓


Constructing an array of components


The principle factor that I do not like about our earlier buildWebpage operate is that I’ve to consistently write brackets and comas, as a way to construct our construction. This may be simply eradicated by introducing a brand new end result builder for the Tag objects. We simply need to 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 permit 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 alter 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 wish to use and mark the complete operate argument with the @TagBuilder key phrase.


public closing class Html: Tag {
    public init(@TagBuilder _ builder: () -> [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "html", contents: nil), youngsters: builder())
    }
}

public closing class Head: Tag {
    public init(@TagBuilder _ builder: () -> [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "head", contents: nil), youngsters: builder())
    }
}

public closing class Physique: Tag {
    public init(@TagBuilder _ builder: () -> [Tag]) {
        tremendous.init(.init(kind: .commonplace, title: "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. For those who check out the introduction part contained in the proposal you will get a greater concept about what occurs beneath 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 remodel our advanced array based mostly code into one thing clear and good by profiting from the Swift compiler. I really like this strategy, however there may be extra.


Optionals and additional construct blocks


If you wish to present if help inside your DSL it’s a must to implement some further strategies inside your end result builder object. Do that code, but it surely 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 wish 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 wish 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(_ part: [Tag]?) -> [Tag] {
        part ?? []
    }
}


However wait, why is it not working? Nicely, 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? Nicely, we are able to introduce a brand new buildBlock technique that may remodel 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(_ part: [Tag]?) -> [Tag] {
        part ?? []
    }
}


func buildWebpage(title: String, physique: String) -> Html {
    Html {
        Head {
            Title(title)
        }
        Physique { 
            if title == "magic" { 
                H1("Whats up")
                P("World")
            } 

            
    }
}


I hope it is not too difficult, but it surely’s all about constructing the correct return kind for the underlying technique. We needed to have simply an array of tags, however with the if help 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. If you’d like to check out a extra easy instance, you need to learn this put up. ☺️


If and else help and both blocks


If blocks can return non-obligatory values, now what about if-else blocks? Nicely, it is fairly an analogous strategy, we simply wish 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(_ part: [Tag]?) -> [Tag] {
        part ?? []
    }

    public static func buildEither(first part: [Tag]) -> [Tag] {
        part
    }

    public static func buildEither(second part: [Tag]) -> [Tag] {
        part
    }
}

func buildWebpage(title: String, physique: String) -> Html {
    Html {
        Head {
            Title(title)
        }
        Physique {
            if title == "magic" {
                H1("Whats up")
                P("World")
            }
            else {
                P(physique)
            }
        }
    }
}

let html = buildWebpage(title: "title", physique: "physique")


As you possibly can see now we do not want further constructing blocks, since we have already lined the variadic Tag array concern with the non-obligatory help. Now it’s doable to write down if and else blocks inside our HTML DSL. Appears fairly good up to now, what’s subsequent? 🧐


Enabling for loops and maps via expressions


Think about that you’ve a bunch of paragraphs within the physique that you just’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 resolve the issue? After all 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’ll be an array of Tag arrays: [[Tag]].


The buildArray technique can remodel these array of tag arrays into Tag arrays, that is ok to supply for help, however we nonetheless want yet another technique to have the ability to use it correctly. We’ve 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 part: [Tag]) -> [Tag] {
        part
    }

    public static func buildEither(second part: [Tag]) -> [Tag] {
        part
    }

    public static func buildOptional(_ part: [Tag]?) -> [Tag] {
        part ?? []
    }

    public static func buildExpression(_ expression: Tag) -> [Tag] {
        [expression]
    }

    public static func buildArray(_ elements: [[Tag]]) -> [Tag] {
        elements.flatMap { $0 }
    }
}


This fashion our for loop will work. The construct expression technique may be very highly effective, it permits us to supply numerous enter sorts and switch them into the information kind that we really want. I’ll present you yet another construct expression instance on this case to help the map operate 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 part: [Tag]) -> [Tag] {
        part
    }

    public static func buildEither(second part: [Tag]) -> [Tag] {
        part
    }

    public static func buildOptional(_ part: [Tag]?) -> [Tag] {
        part ?? []
    }

    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 practical 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’d had some issues unsuitable, this was the very first DSL that I’ve made, however up to now so good, it serves all my wants.




A easy HTML renderer


Earlier than we shut this text I might like to indicate 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: " ", depend: 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)
        }
    }

    non-public 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
    }
    
    non-public func open(_ tag: Tag) -> String {
        return "<" + tag.node.title! + ">"
    }
    
    non-public func shut(_ tag: Tag) -> String {
        "</" + tag.node.title! + ">"
    }
}


As you possibly can see it is a fairly easy, but advanced struct. The open and shut strategies are easy, the fascinating half occurs within the render strategies. The very first render operate can render a tag utilizing the node kind. We simply change the kind and return the HTML worth based on it. if the node is a typical or a gaggle kind we additionally render the youngsters utilizing the identical technique.


After all the ultimate implementation is a little more advanced, 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 evaluate 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 actually need them, however hey issues change, and I’ve additionally modified my thoughts about this function. Now I can not dwell with out end result builders and I really like the code that I can write by utilizing them. I actually hope that this text helped you to grasp them a bit higher. 🙏



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