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Yopuy - Type-safe glue for REST APIs

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An experiment with a simple wrapper for accessing JSON/REST web APIs. Inspired by an episode of Swift Talk.

The main aims for this library are:

  • Type-safety where appropriate
  • Encoding/decoding agnostic
  • HTTP client agnostic

Here is an example of it in use.

let service = Service(adapter: YourFancyAdapter(), URL: url)
let comment = / { result in


Yopuy handles the interaction with an API via types which conform to the Resource protocol. The Resource protocol, various associatedtype declarations and other related protocols — e.g. ChildResource, RootResource — are used to grant capabilities to these types. Generally these are static functions and properties which return values representing full or partial paths to a resource.

For example, a RootResource is one that has a path that exists at the root of an API. This adds a number of constraints.

  • It cannot belong to a parent resource
  • Any path it generates will be at the root of the API

A corresponding protocol is ChildResource. It has an associatedtype Parent requirement. This associates a child resource with it's parent. It then guides the type-safe construction of paths e.g. /

Fine-Grained Capabilities

By default a Resource lacks any of the path static functions or properties. These are provided by conforming to protocols. Most of the protocols correspond to a HTTP method.

  • IsListable; can be retrieved as a collection e.g. GET /posts
  • IsShowable; can be retrieved as a single record e.g. GET /posts/1
  • IsCreatable; new resources can be created e.g. POST /posts
  • IsReplaceable; a resource can be updated via PUT e.g. PUT /posts/1
  • IsPatchable; a resource can be updated via PATCH e.g. PATCH /posts/1
  • IsDeletable; a resource can be deleted via DELETE e.g. DELETE /posts/1

Here is an example of a record that can be updated, but not created or deleted.

struct Foo: RootResource, IsPatchable, IsShowable {
  // Details elided...

HTTP Client Agnostic

All requests in Yopuy are handled by an instance of the Service. It is HTTP client agnostic by delegating all requests to an adapter, which is defined via the HTTPAdapter protocol. Here is it's full definition.

public protocol HTTPAdapter {
    func perform(_ method: HTTPMethod, request: AdapterRequest, callback: @escaping (AdapterResponse) -> Void)

Only one function is required and it's signature is pretty simple at that. The adapter is free to do what ever it needs in the implementation; the only hard-requirement is that it calls the callback function.

Aside from allowing support of arbitrary HTTP clients, this approach makes testing trivial. It's easy to define a test adapter that returns the required responses.

Bring Your Own Serialisation

Yopuy uses a similar approach to integrating the serialisation library of your choice. Depending on how you define your resources, you will need to conform to one or both of the parsing functions; parse(collection:) and parse(singular:). Yopuy will choose the parsing function based on the type of request.

For example, if you conform your resource to the IsShowable protocol, it must implement parse(singular:). Another example is the IsListable protocol. This requires the parse(collection:) function.

Within those functions, you are free to implement your parsing however you choose.

A Longer Example

This is a simple example of how we can encode a type-safe hierarchy of resources. They don't have any properties defined, since they are only demonstrating how parent and child resources are related. The IsRESTFul protocol is used to extend the structs with all the HTTP verbs used in a REST API. Also note how the Commenter struct is extended by a subset of the protocols available.

struct Post: RootResource, IsRESTFul {
    typealias ID = Int
    typealias Collection = [Post]
    typealias Singular = Post
    static let path = "posts"
    let id: Int

    static func parse(collection data: Data) throws -> Collection {
      return []

    static func parse(singular data: Data) throws -> Singular {
      return Post(id: 20)

struct Comment: ChildResource, IsRESTFul {
    typealias ID = Int
    typealias Parent = Post
    typealias Collection = [Comment]
    typealias Singular = Comment
    static let path = "comments"
    let id: Int

    static func parse(collection data: Data) throws -> Collection {
      return []

    static func parse(singular data: Data) throws -> Singular {
      return Comment(id: 20)

struct Commenter: ChildResource, IsShowable, IsListable, IsDeletable {
    typealias ID = Int
    typealias Parent = Comment
    typealias Collection = [Commenter]
    typealias Singular = Commenter
    static let path = "commenter"
    let id: Int

    static func parse(collection data: Data) throws -> Collection {
      return []

    static func parse(singular data: Data) throws -> Singular {
      return Commenter(id: 20)

let e = / / Commenter.delete(1)

There are a few interesting things about the example above:

  • Resources are either RootResource or ChildResource which dictates how they can be used
  • Constructing a path like / is a type-error, since Commenter has Comment as it's parent
  • Making Post a child is a type error, since it is a RootResource
  • Constructing a path with a child resource at the root is also a type-error
  • It's configured so that a create path can't be constructed for the Commenter resource
  • Paths also encode the HTTP verb
  • Parsing is entirely delegated to your code; use whatever parsing library you prefer
  • The operations on collections and resources are encoded in separate protocols — e.g. IsDeletable, IsListable — meaning you can pick and choose what you include
  • IsRESTful is just a shortcut for including all the Is* protocols on a resource
  • Declaring a resource without the IsRESTful or Is* protocols means they're useless, all methods come from optional protocols
  • Using typealias means the results of your requests can be arbitrary types e.g. CustomCollection<Post>


Type-safe glue for building REST APIs








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