Elementary: HTML Templating in Pure Swift

A modern and efficient HTML rendering library - inspired by SwiftUI, built for the web.

struct MainPage: HTMLDocument {
    var title: String = "Elementary"

    var head: some HTML {
        meta(.name(.description), .content("Typesafe HTML in modern Swift"))
    }

    var body: some HTML {
        main {
            h1 { "Features" }

            FeatureList(features: [
                "HTML in pure Swift",
                "SwiftUI-inspired composition",
                "Lightweight and fast",
                "Framework agnostic and unopinionated",
            ])

            a(.href("https://github.com/sliemeobn/elementary"), .class("fancy-style")) {
                "Learn more"
            }
        }
    }
}

struct FeatureList: HTML {
    var features: [String]

    var content: some HTML {
        ul {
            for feature in features {
                li { feature }
            }
        }
    }
}

Play with it

Check out the Hummingbird + Tailwind example app.

For a demo of ElementaryHTMX, see this Hummingbird + HTMX Demo.

For a Vapor example, see the Vapor + HTMX Demo.

Lightweight and fast

Elementary renders straight to text, optimized for serving generated HTML from a Hummingbird or Vapor server app.

Any type conforming to HTML can be rendered individually, ideal for testing or for sending fragments with htmx.

The default rendering mechanism produces chunks of HTML for efficient response streaming, so the browser can start loading a page while the server is still producing the rest of it. Swift concurrency is used to handle back pressure, so you your memory footprint stays low even for large pages.

// Stream HTML, optimized for responsiveness and back pressure-aware
try await MainPage().render(into: responseStreamWriter)

Alternatively, you can simply collect the rendered HTML in a string.

let html: String = div(.class("pretty")) { "Hello" }.render()
// <div class="pretty">Hello</div>

let fragment: String = FeatureList(features: ["Anything conforming to HTML can be rendered"]).render()
// <ul><li>Anything conforming to HTML can be rendered</li></ul>

// For testing purposes, there is also a formatted version
print(
    div {
        p(.class("greeting")) { "Hi mom!" }
        p { "Look how pretty." }
    }.renderFormatted()
)

// <div>
//   <p class="greeting">Hi mom!</p>
//   <p>Look how pretty.</p>
// </div>

Elementary has zero dependencies (not even Foundation) and does not use runtime reflection or existential containers (there is not a single any in the code base).

By design, it does not come with a layout engine, reactive state tracking, or built-in CSS styling: it just renders HTML.

Clean and composable

Structure your HTML with a SwiftUI-inspired composition API.

struct List: HTML {
    var items: [String]
    var importantIndex: Int

    var content: some HTML {
        // conditional rendering
        if items.isEmpty {
            p { "No items" }
        } else {
            ul {
                // list rendering
                for (index, item) in items.enumerated() {
                    // seamless composition of elements
                    ListItem(text: item, isImportant: index == importantIndex)
                }
            }
        }
    }
}

struct ListItem: HTML {
    var text: String
    var isImportant: Bool = false

    var content: some HTML {
        // conditional attributes
        li { text }
            .attributes(.class("important"), when: isImportant)
    }
}

First class attribute handling

Elementary utilizes Swift's powerful generics to provide an attribute system that knows what goes where. Every element knows which Tag it is for.

As in HTML, attributes go right after the "opening tag".

// staying close to HTML syntax really helps
div(.data("hello", value: "there")) {
    a(.href("/swift"), .target(.blank)) {
        img(.src("/swift.png"))
        span(.class("fancy")) { "Click Me" }
    }
}

Attributes can also be altered by using the modifier syntax, this allows for easy handling of conditional attributes.

div {
    p { "Hello" }
        .attributes(.id("maybe-fancy"))
        .attributes(.class("fancy"), when: isFancy)
}

By exposing the tag type of content, attributes will fall through and be applied correctly.

struct Button: HTML {
    var text: String

    // by exposing the HTMLTag type information...
    var content: some HTML<HTMLTag.input> {
        input(.type(.button), .value(text))
    }
}

div {
    // ... Button will know it really is an <input> element ...
    Button(text: "Hello")
        .attributes(.autofocus) // ... and pass on any attributes
}

As a sensible default, class and style attributes are merged (with a blank space or semicolon respectively). All other attributes are overwritten by default.

Seamless async support

Elementary supports Swift Concurrency in HTML content. Simply await something inside your HTML, while the first bytes are already flying towards the browser.

div {
    let text = await getMyData()
    p { "This totally works: \(text)" }
    MyComponent()
}

struct MyComponent: HTML {
    var content: some HTML {
        AsyncContent {
            "So does this: \(await getMoreData())"
        }
    }
}

By using the AsyncForEach element, any AsyncSequence can be efficiently rendered straight to HTML.

ul {
    // the full result never needs to be stored in memory...
    let users = try await db.users.findAll()
    // ...as each async sequence element...
    AsyncForEach(users) { user in
        // ...is immediately streamed out as HTML
        li { "\(user.name) \(user.favoriteProgrammingLanguage)" }
    }
}

Enviroment values

Elementary utilizes TaskLocals to provide a light-weight environment system.

enum MyValues {
    // task-locals act as keys, ...
    @TaskLocal static var userName = "Anonymous"
}

struct MyComponent: HTML {
    // ... their values can be accessed ...
    @Environment(MyValues.$userName) var userName

    var content: some HTML {
        p { "Hello, \(userName)!" }
    }
}

div {
    // ... and provided in a familiar way
    MyComponent()
        .environment(Values.$userName, "Drax the Destroyer")
}

🚧 Work in progress 🚧

The list of built-in attributes is far from complete, but adding them is really simple (and can be done in external packages as well).

Feel free to open a PR with additional attributes that are missing from the model.

Motivation and other packages

Plot, HTMLKit, and Swim are all excellent packages for doing a similar thing.

My main motivation for Elementary was to create an experience like these (Swift Forums post for more context), but

stay true to HTML tag names and conventions (including the choice of lowercase types)
avoid allocating an intermedate structure and go straight to streaming HTML
using generics to stay away from allocating a ton of lists of existential anys
have a list of attributes go before the content block
provide attribute fallthrough and merging
zero dependencies on other packages

Tokamak is an awesome project and very inspiring. It can produce HTML, but it's main focus is on a very different beast. Check it out!

swift-html and swift-dom will produce HTML nicely, but they use a different syntax for composing HTML elements.

Future directions

Experiment with embedded swift for wasm and bolt a lean state tracking/reconciler for reactive DOM manipulation on top

sliemeobn / elementary

readme