ExHal

An easy to use HAL API client for elixir.

Usage

Consider a resource http://example.com/hal whose HAL representation looks like

{ "name": "Hello!",
  "_links": {
     "self"   : { "href": "http://example.com" },
      "profile": [{ "href": "http://example.com/special" },
                  { "href": "http://example.com/normal" }]
  }
}

iex> {:ok, doc, response_header} = ExHal.client
...> |> ExHal.Client.add_headers("User-Agent": "MyClient/1.0")
...> |> ExHal.Client.get("http://example.com/hal")
%ExHal.Document{...}

Navigation

Now we have an entry point to the API. From there we can follow links to navigate around.

iex> ExHal.follow_link(doc, "profile")
{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}

iex> ExHal.follow_link(doc, "self")
{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}

iex> ExHal.follow_links(doc, "profile")
[{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}, {:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}]

We can specify headers for each request in addition to the headers specified in the client.

iex> ExHal.follow_links(doc, "profile",
                        headers: ["Accept": "application/vnd.custom.json+type"])
[{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}, {:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}]

If we try to follow a non-existent or compound link with ExHal.follow_link it will return an error tuple.

iex> ExHal.follow_link(doc, "nonexistent")
{:error, %ExHal.Error{reason: "no such link"}}

iex> ExHal.follow_link(doc, "profile", strict: true)
{:error, %ExHal.Error{reason: "multiple choices"}}

If we try to follow a non-existent with ExHal.follow_links it will return a list of error tuples.

iex> ExHal.follow_links(doc, "nonexistent")
[{:error, %ExHal.Error{reason: "no such link"}}]

Actions

If you want to take an action (ie, make a PUT, POST, etc) request you can do that too.

name_change = """
  { "name": "Bye!",
    "_links": {
       "self"   : { "href": "http://example.com" },
       "profile": [{ "href": "http://example.com/special" },
       { "href": "http://example.com/normal" }]
    }
  }
  """

# make a request that returns a HAL response
iex> ExHal.put(doc, "self", name_change)
{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}

# make a request that just returns a response without a body
iex> {:ok, resp} = ExHal.post(doc, "add-child", "{\"name\": \"child\"}")
{:ok, %ExHal.NonHalResponse{status_code: 201, headers: [{"Location", "http://example.com/child"}, ...], body: ""}}
iex> ExHal.url(resp)
"http://example.com/child"

Collections

Consider a resource http://example.com/hal-collection whose HAL representation looks like

{ "_links": {
     "self"   : { "href": "http://example.com/hal-collection" },
      "item": [{ "href": "http://example.com/beginning" },
               { "href": "http://example.com/middle" }],
      "next": { "href": "http://example.com/hal-collection?p=2" }
  }
}

and a resource http://example.com/hal-collection?p=2 whose HAL representation looks like

{ "_links": {
     "self"   : { "href": "http://example.com/hal-collection?p=2" },
      "item": [{ "href": "http://example.com/end" }]
  }
}

If we get the first HAL collection resource and turn it into a stream we can use all our favorite Stream functions on it.

iex> collection = ExHal.client
...> |> ExHal.Client.add_headers("User-Agent": "MyClient/1.0")
...> |> ExHal.Client.get("http://example.com/hal-collection")
...> |> ExHal.to_stream
#Function<11.52512309/2 in Stream.resource/3>

iex> Stream.map(collection, fn follow_results ->
...>   case follow_results do
...>     {:ok, a_doc, %ResponseHeader{}} -> ExHal.url(a_doc)
...>     {:ok, a_doc} -> ExHal.url(a_doc)
...>     {:error, _}  -> :error
...>   end
...> end )
["http://example.com/beginning", "http://example.com/middle", "http://example.com/end"]

Serialization

Collections and Document can render themselves to a json-like structure that can then be serialized using your favorite json encoder (e.g. Poison):

ExHal.Collection.render!

ExHal.Document.render!

Transcoding

ExHal also supports interpreting HAL documents. The following is a HAL document interpreter

Given a document like

{
  "name": "Jane Doe",
  "mailingAddress": "123 Main St",
  "_links": {
    "app:department": { "href": "http://example.com/dept/42" },
    "app:manager":    { "href": "http://example.com/people/84" },
    "tag": [
      {"href": "foo:1"},
      {"href": "http://2"},
      {"href": "urn:1"}
    ]
  }
}

We can define an transcoder for it.

defmodule PersonTranscoder do
  use ExHal.Transcoder

  defproperty "name"
  defproperty "mailingAddress", param: :address
  deflink     "app:department", param: :department_url
  deflink     "app:manager",    param: :manager_id, value_converter: PersonUrlConverter
  deflinks    "tags"
end

PersonUrlConverter is a module that has adopted the ExHal.ValueConverter behavior.

defmodule PersonUrlConverter do
  @behaviour ExHal.ValueConverter

  def from_hal(person_url) do
    to_string(person_url)
    |> String.split("/")
    |> List.last
  end

  def to_hal(person_id) do
    "http://example.com/people/#{person_id}"
  end
end

We can use this transcoder to to extract the pertinent parts of the document into a map.

iex> PersonTranscoder.decode!(doc)
%{name: "Jane Doe",
  address: "123 Main St",
  department_url: "http://example.com/dept/42",
  manager_id: 84}
iex> PersonTranscoder.encode!(%{name: "Jane Doe",
  address: "123 Main St",
  department_url: "http://example.com/dept/42",
  manager_id: 84})
~s(
{
  "name": "Jane Doe",
  "mailingAddress": "123 Main St",
  "_links": {
    "app:department": { "href": "http://example.com/dept/42" },
    "app:manager":    { "href": "http://example.com/people/84" }
   }
} )

This can be used to, for example, build Ecto changesets via a changeset/2 functions and to render HAL responses to HTTP requests.

Patching

ExHal.Transcoder also supports modifying objects with JSON patch.

defmodule PetTranscoder do
  use ExHal.Transcoder

  defproperty "name"
  defproperty "animalType",   param: species, protected: true
  deflink     "favoriteToy",  param: :favorite_toy
  deflinks    "friends"
end

Create an object from the transcoder, then modify it

iex> spot = PetTranscoder.decode!(doc)
%{name: "Spot",
  species: "dog",
  favorite_toy: "http://a.co/56guxwO"
  friends: ["https://petbook.com/u/fifi", "https://petbook.com/u/fido"]}

iex> json_patches = [
  %{"op" => "replace", "path" => "/name",       "value" => "Bowser"},
  %{"op" => "replace", "path" => "/animalType", "value" => "dragon"},
  %{"op" => "replace", "path" => "/_links/favoriteToy", "value" => %{"href" => "http://a.co/9cs2VQd"}},
  %{"op" => "add",     "path" => "/_links/friends/-",   "value" => %{"href" => "https://doggo.biz/12345"}}]
...

iex> spot |> PetTranscoder.patch!(json_patches)
%{name: "Bowser",
  species: "dog",
  favorite_toy: "http://a.co/9cs2VQd",
  friends: ["https://petbook.com/u/fifi", "https://petbook.com/u/fido", "https://doggo.biz/12345"]}

"replace" operations are supported for properties and links. "add" (append) is supported for link collections (deflinks) with /propertyName/- path syntax. Any properties or links marked with protected: true cannot be changed via patch!. Patch operations against properties or links not defined in the transcoder are ignored.

Composing

Transcoders are also chainable. For example, given a ManagerTranscoder the following would produce a Map that includes all person params and all the manager params: PersonTranscoder.decode!(doc) |> ManagerTranscoder.decode!(doc). Similarly PersonTranscoder.encode!(model) |> ManagerTranscoder.encode!(module) would produce an ExHal.Document that has all the properties and links defined in those transcoders.

Similarly, patch! operations can be chained:

iex> employee = PersonTranscoder.decode!(doc) |> ManagerTranscoder.decode!(doc)
...
iex> json_patches = [%{"op" => "replace", "path" => "/mailingAddress", "value" => "..."}, ...]
...
iex> employee |> PersonTranscoder.patch!(json_patches) |> ManagerTranscoder.patch!(json_patches)

Forms

ExHal supports Forms (Dwolla style)* to allow further decoupling of server and client implementations.

Given a document

{ "_links": {
    "item": [],
    "self": { "href": "http://example.com/comments" }
  },
  "_forms": {
    "create-form": {
      "_links": { "target": { "href": "http://example.com/comments" } },
      "method": "POST",
      "contentType": "application/hal+json",
      "fields": [
        { "name": "comment",
          "path": "/comment",
          "type": "string",
          "displayText": "Comment",
          "validations": {
            "required": true
          }
        }
      ]
    }
  }
}

Creating a new item in the collection is simple:

iex> ExHal.get_form(doc, "create-form")
...> |> ExHal.Form.set_field("comment", "Very good!")
...> |> ExHal.Form.submit(ExHal.client)
{:ok, %ExHal.Document{...}, %ExHal.ResponseHeader{...}}

The list of fields is also available. This makes it possible to dynamically build user interfaces.

iex> ExHal.get_form(doc, "create-form")
...> |> ExHal.Form.get_fields()
...> |> Enum.map(fn (field) -> {field.display_name, field.type} end)
[{"Comment", :string}]

* At this time there are major limitations to the forms support. Namely

only PUT and POST forms
only JSON content types
only native JSON field types

Assertions about HAL documents

Several assertion and helper functions are available to support testing. These functions accept a ExHal.Document or a string.

iex> import ExUnit.Assertions
nil
iex> import ExHal.Assertions
nil
iex> assert_property ~s({"name": "foo"}), "name"
true
iex> assert_property ~s({"name": "foo"}), "address"
** (ExUnit.AssertionError) address is absent
iex> assert_property ~s({"name": "foo"}), "name", eq "foo"
true
iex> assert_property ~s({"name": "foo"}), "name", matches ~r/fo/
true
iex> assert_property ~s({"name": "foo"}), "name", eq "bar"
** (ExUnit.AssertionError) expected property `name` to eq("bar")
iex> assert_link_target ~s({"_links": { "profile": {"href": "http://example.com" }}}),
...>   "profile"
true
iex> assert_link_target ~s({"_links": { "profile": {"href": "http://example.com" }}}),
...>   "item"
** (ExUnit.AssertionError) link `item` is absent
iex> assert_link_target ~s({"_links": { "profile": {"href": "http://example.com" }}}),
...>   "profile", eq "http://example.com"
true
iex> assert_link_target ~s({"_links": { "profile": {"href": "http://example.com" }}}),
...>   "profile", matches ~r/example.com/
true
iex> assert_link_target ~s({"_links": { "profile": {"href": "http://example.com" }}}),
...>   "profile", eq "http://bad.com"
** (ExUnit.AssertionError) expected (at least one) `item` link to eq("http://bad.com") but found only http://example.com
iex> assert collection("{}") |> Enum.empty?
true
iex> assert 1 == collection("{}") |> Enum.count
** (ExUnit.AssertionError) Assertion with == failed

Installation

Add the following to your project :deps list:

{:exhal, "~> 7.1"}

Upgrading

from 7.x to 8.x

Upgrade elixir to ~> 1.6
Upgrade httpoison to ~> 1.0
Upgrade odgn_json_pointer to ~> 2.0

From 6.0 to 7.0

All HTTP requesting functions return a three-tuple in 7.0 (rather the two-tuple in versions before that). Code that invokes those will need to handle the change in cardinatlity.

b3k/exhal