Hypertext Application Language (HAL)

What HTML does for web browsers, HAL does for applications.

HTML gives you pages of marked-up information for presentation to users; HAL gives you marked-up representations for application consumption. Applications can easily extract information about remote resources, including relationships between resources. What HAL calls a representation, HTML calls a web page; such pages have links to other pages. HTML was designed for presenting information to humans. HAL was designed for presenting information to applications.

This framework provides a suite of Swift classes for rendering and parsing resource representations, including their links, properties and nested representations of embedded resources.

What Can You Do With It?

The framework lets you encode and decode HAL representations. Representations internally render and parse via dictionaries, or [String: Any] types in Swift, but can code between other types via the dictionary. So for example, you can construct JSON strings from Representation instances, and vice versa.

To start with, you can easily construct a resource representation and configure it with links, embedded representations and properties. The following example constructs a ficticious customer resource.

let representation = Representation()
  .with(rel: Link.SelfRel, href: "https://example.com/api/customer/123456")
  .with(name: "ns", ref: "https://example.com/apidocs/ns/" + NamespaceManager.Rel)
  .with(name: "role", ref: "https://example.com/apidocs/role/" + NamespaceManager.Rel)
  .with(link: Link(rel: "ns:parent", href: "https://example.com/api/customer/1234")
    .with(name: "bob")
    .with(title: "The Parent")
    .with(hreflang: "en"))
  .with(rel: "ns:users", href: "https://example.com/api/customer/123456?users")
  .with(name: "id", value: 123456)
  .with(name: "age", value: 33)
  .with(name: "name", value: "Example Resource")
  .with(name: "optional", value: true)
  .with(name: "expired", value: false)

This extract shows the use of cascading with methods for configuring a representation. But the Representation and Link classes also provide more conventional methods for loading their attributes with values.

Converting a representation to JSON is easy.

try! representation.jsonString(options: .prettyPrinted)!

Notice the try. Converting to JSON throws an error if JSONSerialization fails. Notice also the 'bang' for the optional string result, which can produce nil if the JSON data fails to convert to a UTF-8 string for any reason.

The jsonString(options:) method produces a string containing JSON-encoded hypertext application language, as follows:

{
  "optional" : true,
  "age" : 33,
  "name" : "Example Resource",
  "id" : 123456,
  "expired" : false,
  "_links" : {
    "self" : {
      "href" : "https://example.com/api/customer/123456"
    },
    "curies" : [
      {
        "name" : "ns",
        "templated" : true,
        "href" : "https://example.com/apidocs/ns/{rel}"
      },
      {
        "name" : "role",
        "templated" : true,
        "href" : "https://example.com/apidocs/role/{rel}"
      }
    ],
    "ns:parent" : {
      "name" : "bob",
      "title" : "The Parent",
      "href" : "https://example.com/api/customer/1234",
      "hreflang" : "en"
    },
    "ns:users" : {
      "href" : "https://example.com/api/customer/123456?users"
    }
  }
}

Notice that the string does not escape the forward slashes.

Converting this back to a representation is also very simple. Assuming the JSON-encoded string is in a variable called string, then the following expression constructs a new representation object from the JSON string.

try! Representation.from(json: string.data(using: String.Encoding.utf8)!)

The result matches the original. Of course, you would catch the errors and guard the optionals in a real application.

Deviations

The interfaces and implementations largely echo those written in Java, but there are some deliberate deviations.

The Swift framework adds some consistency in naming. Representation is the name used to describe an object that represents some remote resource. The term “resource” describes the actual remote resource. Representations represent resources only; they are not the resource themselves.

The use of “currie” has been replaced as it partially overlaps the idea of currying and curried functions in mathematics and computer science, when in fact the term ‘curie’ only refers to a compact URI. The acronym only coincidentally resembles the word “curry,” a delicious Asian meal.