Generate HTML, XML, and other web content using Swift string literal interpolation


CI Documentation

HypertextLiteral is a Swift package for generating HTML, XML, and other SGML dialects.

It uses custom string interpolation to append and escape values based on context, with built-in affordances for common patterns and an extensible architecture for defining your own behavior.

import HypertextLiteral

let attributes = [
    "style": [
        "background": "yellow",
        "font-weight": "bold"

let html: HTML = "<span \(attributes)>whoa</span>"
// <span style="background: yellow; font-weight: bold;">whoa</span>

HypertextLiteral is small and self-contained with no external dependencies. You can get up to speed in just a few minutes, without needing to learn any new APIs or domain-specific languages (DSLs). Less time fighting your tools means more time spent generating web content.

This project is inspired by and borrows implementation details from Hypertext Literal by Mike Bostock (@mbostock). You can read more about it here.


  • Swift 5.1+


Swift Package Manager

Add the HypertextLiteral package to your target dependencies in Package.swift:

import PackageDescription

let package = Package(
  name: "YourProject",
  dependencies: [
        url: "https://github.com/NSHipster/HypertextLiteral",
        from: "0.0.3"

Then run the swift build command to build your project.


Hypertext literals automatically escape interpolated values based on the context in which they appear.

  • By default, interpolated content escapes the XML entities <, >, &, ", and ' as named character references (for example, < becomes &lt;)
  • In the context of an attribute value, quotation marks are escaped with a backslash (\")
  • In a context of comment, any start and end delimiters (<!-- and -->) are removed

Interpolating Content

To get a better sense of how this works in practice, consider the following examples:

let level: Int = 1
"<h\(level)>Hello, world!</h\(level)>" as HTML
// <h1>Hello, world!</h1>

let elementName: String = "h1"
"<\(elementName)>Hello, world!</\(elementName)>" as HTML
// <h1>Hello, world!</h1>

let startTag: String = "<h1>", endTag: String = "</h1>"
"\(startTag)Hello, world!\(endTag)" as HTML
// &lt;h1&gt;Hello, world!&lt;/h1&gt;

Interpolation for an element's name in part or whole work as intended, but interpolation of the tag itself causes the string to have its angle bracket (< and >) escaped.

When you don't want this to happen, such as when you're embedding HTML content in a template, you can either pass that content as an HTML value or interpolate using the unsafeUnescaped argument label.

let startTag: HTML = "<h1>", endTag: HTML = "</h1>"
"\(startTag)Hello, world!\(endTag)" as HTML
// <h1>Hello, world!</h1>

"\(unsafeUnescaped: "<h1>")Hello, world!\(unsafeUnescaped: "</h1>")" as HTML
// <h1>Hello, world!</h1>

Note: Interpolation with the unsafeUnescaped argument label appends the provided literal directly, which may lead to invalid results. For this reason, use of HTML values for composition is preferred.

Interpolating Attribute Values

Attributes in hypertext literals may be interchangeably specified with or without quotation marks, either single (') or double (").

let id: String = "logo
let title: String = #"Swift.org | "Welcome to Swift.org""#
let url = URL(string: "https://swift.org/")!

#"<a id='\#(logo)' title="\#(title)" href=\#(url)>Swift.org</a>"# as HTML
/* <a id='logo'
      title="Swift.org | \"Welcome to Swift.org\""

Some attributes have special, built-in rules for value interpolation.

When you interpolate an array of strings for an element's class attribute, the resulting value is a space-delimited list.

let classNames: [String] = ["alpha", "bravo", "charlie"]

"<div class=\(classNames)>…</div>" as HTML
// <div class="alpha bravo charlie">…</div>

If you interpolate a dictionary for the value of an element's style attribute, it's automatically converted to CSS.

let style: [String: Any] = [
    "background": "orangered",
    "font-weight": 700

"<span style=\(style)>Swift</span>" as HTML
// <span style="background: orangered; font-weight: 700;">Swift</span>

The Boolean value true interpolates to different values depending the attribute.

<input type="text" aria-enabled=\(true)
                   translate=\(true) />
""" as HTML
/* <input type="text" aria-enabled="true"
                      translate="yes"/> */

Interpolating Attributes with Dictionaries

You can specify multiple attributes at once by interpolating dictionaries keyed by strings.

let attributes: [String: Any] = [
    "id": "primary",
    "class": ["alpha", "bravo", "charlie"],
    "style": [
        "font-size": "larger"

"<div \(attributes)>…</div>" as HTML
/* <div id="primary"
        class="alpha bravo charlie"
        style="font-size: larger;">…</div> */

Attributes with a common aria- or data- prefix can be specified with a nested dictionary.

let attributes: [String: Any] = [
    "id": "article",
    "aria": [
        "role": "article",
    "data": [
        "index": 1,
        "count": 3,

"<section \(attributes)>…</section>" as HTML
/* <section id="article"
            data-count="3">…</section> */

Support for Other Formats

In addition to HTML, you can use hypertext literals for XML and other SGML formats. Below is an example of how HypertextLiteral can be used to generate an SVG document.

import HypertextLiteral

typealias SVG = HTML

let groupAttributes: [String: Any] = [
    "stroke-width": 3,
    "stroke": "#FFFFEE"

func box(_ rect: CGRect, radius: CGVector = CGVector(dx: 10, dy: 10), attributes: [String: Any] = [:]) -> SVG {
    <rect x=\#(rect.origin.x) y=\#(rect.origin.y)
          width=\#(rect.size.width) height=\#(rect.size.height)
          rx=\#(radius.dx) ry=\#(radius.dy)

let svg: SVG = #"""
<?xml version="1.0" encoding="utf-8"?>
<svg xmlns='http://www.w3.org/2000/svg' viewBox='0 0 100 100'>
  <g \#(groupAttributes)>
    \#(box(CGRect(x: 12, y: 28, width: 60, height: 60), attributes: ["fill": "#F06507"]))
    \#(box(CGRect(x: 27, y: 18, width: 55, height: 55), attributes: ["fill": "#F2A02D"]))
    \#(box(CGRect(x: 47, y: 11, width: 40, height: 40), attributes: ["fill": "#FEC352"]))




Mattt (@mattt)


  • Swift Tools 5.1.0
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Last updated: Wed Mar 29 2023 23:09:04 GMT-0500 (GMT-05:00)