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Consequence builders in Swift – The.Swift.Dev.


If you wish to make a end result builder in Swift, this text will show you how to to take care of the commonest 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 applied in Swift 5.4. This characteristic permits us to construct up a end result worth utilizing a sequence of elements. At first sight, you would possibly suppose, hey this seems to be like an array with a sequence of components, besides the coma in between the objects, however nope, that is fully totally different. However why is it good for us?


Consequence builder can be utilized to create solely 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 could be very light-weight, but highly effective and succesful. Since Swift DSLs are sort secure, it’s a lot safer to make use of one as an alternative of manually concatenate objects. Swift DSLs additionally permits us to make use of fundamental management flows inside these embedded micro-languages. 🤔


Let me provide you with an instance: you’ll be able to write HTML in Swift, you’ll be able to 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 change the next code with a DSL, we are going to significantly advantage of the Swift compiler options. Swift will give us sort security, so our code will likely be much less error inclined. A DSL can have many constraints and restrictions that’ll assist others to put in writing higher code. In our case the checklist 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 at all times 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’ll be able to see the snippet above seems to be far more Swifty and we had been additionally capable of take away the duplicate HTML closing tags from the code. We do not have to put in writing the characters in any respect and the compiler can sort examine every little thing 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 complicated than this. It’s an especially superior and highly effective characteristic that it’s best to positively find out about.


You possibly can create every kind 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 also be an important use-case, however the checklist is countless. Let me present you the right way to 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 discovered so much about it, it is also going to switch the Leaf/Tau template in my future initiatives. The principle thought behind SwiftHtml was that I wished to comply with the HTML specs as carefully as attainable. So I’ve created a Node construction to signify a node contained in the doc tree.


public struct Node {

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

    public let sort: `Sort`
    public let identify: String?
    public let contents: String?

    public init(sort: `Sort` = .commonplace,
                identify: String? = nil,
                contents: String? = nil) {
        self.sort = sort
        self.identify = identify
        self.contents = contents
    }
}


A node has 4 variants outlined by the Sort. A normal node will render as a regular 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 will likely be used to group collectively a number of nodes, it will not render something, it is only a grouping factor for different tags.


The trick in my resolution 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 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 attainable options and my ultimate selection was to make use of reference sorts as an alternative 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 seems to be like, it is fairly easy. It has an underlying node and a bunch of kids. It’s attainable to increase this tag and supply functionalities for all of the HTML tags, akin to the aptitude of including frequent attributes and I am additionally capable of create subclasses for the tags.


public ultimate class Html: Tag {

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

public ultimate class Head: Tag {

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

public ultimate class Title: Tag {

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

public ultimate class Physique: Tag {

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

public ultimate class H1: Tag {

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

public ultimate class P: Tag {

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


All proper, now we’re capable of initialize our Tag tree, however I warn you, it should look very awkward.


func buildWebpage(title: String, physique: String) -> Html {
    Html([
        Head([
            Title(title),
        ]),
        Physique([
            H1(title),
            P(body),
        ]),
    ])
}


It’s nonetheless not attainable to render the tree and the syntax isn’t so eye-catchy. It is time to make issues higher and we should always positively introduce some end result builders for good.




The anatomy of Swift end result builders

Now that we’ve got our knowledge construction ready, we should always deal with the DSL itself. Earlier than we dive in, I extremely advocate to rigorously learn the official proposal and watch this WWDC video about end result builders, since each assets are wonderful. 🤓


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, 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 must mark an enum with the @resultBuilder attribute and supply a static buildBlock methodology with the given sort.


@resultBuilder
public enum TagBuilder {
    public static func buildBlock(_ elements: Tag...) -> [Tag] {
        elements
    }
}


This can enable us to make use of an inventory of elements within our DSL constructing blocks, however earlier than we may use it we even have to vary our particular HTML tag init strategies to make the most of this newly created end result builder. Simply use a closure with the return sort that we wish to use and mark the whole operate argument with the @TagBuilder key phrase.


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

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

public ultimate class Physique: Tag {
    public init(@TagBuilder _ builder: () -> [Tag]) {
        tremendous.init(.init(sort: .commonplace, identify: "physique", contents: nil), youngsters: builder())
    }
}


Now we are able to refactor the construct webpage methodology since it could now use the underlying end result builder to assemble the constructing blocks primarily based on the elements. If you happen to 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 primarily based code into one thing clear and good by profiting from the Swift compiler. I 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 important to implement some further strategies inside your end result builder object. Do that code, however 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’ve got 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 may very well be expressed as a [Tag]? sort however we at all times wish to have a bunch of [Tag] components, now that is simple.


@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 factor 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 methodology 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, however it’s all about constructing the correct return sort for the underlying methodology. We wished to have simply an array of tags, however with the if help we have ended up with an inventory of tag arrays, that is why we’ve got to remodel it again to a flattened array of tags with the brand new construct block. In order for you to check out a extra easy instance, it’s best to learn this publish. ☺️


If and else help and both blocks


If blocks can return non-obligatory values, now what about if-else blocks? Properly, 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(_ 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’ll be able to see now we do not want further constructing blocks, since we have already coated the variadic Tag array situation with the non-obligatory help. Now it’s attainable to put in writing if and else blocks inside our HTML DSL. Seems fairly good up to now, what’s subsequent? 🧐


Enabling for loops and maps by expressions


Think about that you’ve got a bunch of paragraphs within the physique that you simply’d like to make use of. Fairly simple, 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 sort 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], ultimately, it should be an array of Tag arrays: [[Tag]].


The buildArray methodology can rework these array of tag arrays into Tag arrays, that is ok to offer for help, however we nonetheless want yet another methodology to have the ability to use it correctly. We’ve got 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 fashion our for loop will work. The construct expression methodology could be very highly effective, it allows us to offer numerous enter sorts and switch them into the info sort that we really need. I will 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 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 an alternative of for loops if we desire useful 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 flawed, 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.sort {
        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.identify! + ">"
    }
    
    non-public func shut(_ tag: Tag) -> String {
        "</" + tag.node.identify! + ">"
    }
}


As you’ll be able to see it is a fairly easy, but complicated struct. The open and shut strategies are simple, the fascinating half occurs within the render strategies. The very first render operate can render a tag utilizing the node sort. We simply change the sort and return the HTML worth based on it. if the node is a regular or a gaggle sort we additionally render the youngsters utilizing the identical methodology.


In fact the ultimate implementation is a little more complicated, it entails HTML attributes, it helps minification and customized indentation degree, however for academic 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 primarily based resolution we are able to say that the distinction is large. Actually talking I used to be afraid of end result builders for a really very long time, I assumed 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 am unable to dwell with out end result builders and I like the code that I will write through the use of them. I actually hope that this text helped you to know them a bit higher. 🙏






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