Functional Interactor

Based around https://github.com/collectiveidea/interactor, reimagined to use Kase with composability operators.

Getting Started

Add Interactor to your Gemfile and bundle install.

gem "functional_interactor", "~> 0.0.1"

This implementation is meant to be used with the Kase gem. This is also an experiment -- some of the ideas such as generic interactors are somethign we (Legal.io engineering) is trying due to the sheer complexity of our code base. The examples we have in the advanced usage section can use a lot of improvement.

What is an Interactor?

An interactor is a simple, single-purpose object.

Interactors are used to encapsulate your application's business logic. Each interactor represents one thing that your application does.

Call and Return Protocol

An Interactor must respond to the method call, and takes a single object.

An Interactor following this protocol will accept a single object which encapsulates the state or context. By convention, we use a Hash-like object so that interactors can be composed into higher-order interactions.

Success

When the action succeeds, return

[:ok, context]

The return value of context can be anything, though it is suggested that you stick with a Hash-like object so that interactors can be chained together.

Failure

When the action fails, return an array where the first element is the symbol :error. Examples:

[:error, "This failed"]
[:error, :invalid, [{'field' => 'must be present'}]]
[:error, :stripe_error, StripeException.new]

You are typically going to use Kase to handle errors:

Kase.kase PushUserToElasticSearch.call(user) do
  on(:ok) do |ctx|
    # Do something
  end
  
  on(:error, :network) do |reason|
    NotifyHuman.log "failed to push user ##{user.id} to ElasticSearch"
  end
end

Context

An interactor is given a context. The context contains everything the interactor needs to do its work.

When an interactor does its single purpose, it affects its given context.

Context are assumed to be a Hash like object.

Adding to the Context

As an interactor runs it can add information to the context.

context[:user] = user

Hooks

This implementation has no hooks.

An Example Interactor

Your application could use an interactor to authenticate a user.

class AuthenticateUser
  include FunctionalInteractor

  def call(context = {})
    user = User.authenticate(context[:email], context[:password])

    return [:error, :not_authenticated] unless user

    context[:user] = user
    context[:token] = user.secret_token

    # Return a new context so we are not modifying the original
    [:ok,  { user: user, token: user.secret_token }]
  end
end

To define an interactor, simply create a class that includes the Interactor module and give it a call instance method. The interactor can access its context from within call.

Interactors in the Controller

Most of the time, your application will use its interactors from its controllers. The following controller:

class SessionsController < ApplicationController
  def create
    if user = User.authenticate(session_params[:email], session_params[:password])
      session[:user_token] = user.secret_token
      redirect_to user
    else
      flash.now[:message] = "Please try again."
      render :new
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

can be refactored to:

class SessionsController < ApplicationController
  def create
    Kase.kase AuthenticateUser.call(session_params) do
      on(:ok) do |result|
        session[:user_token] = result[:token]
        redirect_to root_path
      end
      
      on(:error, :not_authenticated) do
        flash.now[:message] = t(result.message)
        render :new
      end
    end
  end

  private

  def session_params
    params.require(:session).permit(:email, :password)
  end
end

The .call class method simply instantiates a new AuthenticatedUser interactor and passes the context to it. This allows us to create generic interactors that can be inlined and composed together. This is discussed in the following section.

Advanced Usage

Sequences

creativeideas/interactor has an Organizer class. We have a similar code called Interactors::Sequence.

Let's define a second interactor:

class NotifyLogin
  include FunctionalInteractor
  
  def call(context = {})
    NotificationsMailer.login(user: context[:user]).deliver
    [:ok, context]
  end
end

We can then chain them together like so:

interactions = Interactors::Sequence.new
interactions.compose(AuthenticatedUser)
interactions.compose(NotifyLogin)

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

Here, the Interactors::Sequence object holds a sequence of interactions. It will call them one by one, starting from the top. If at any point, it returns something with [:error, ...] then the chain will stop. We can then use Kase to handle the error.

`#compose` and `|`

We do not actually have to create an Interactors::Sequence object. The #compose method will create an Interactors::Sequence for you. You can chain them together like so:

interactions = AuthenticatedUser.compose(NotifyLogin)

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

We also aliased | so you can use that instead:

interactions = AuthenticatedUser | NotifyLogin

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

Generic Interactors

Sometimes we want to dynamically create an interactor. We can change the notification interactor to:

interactions = AuthenticatedUser \
| Interactors::Anonymous.new do
    NotificationsMailer.login(user: context[:user]).deliver
    [:ok, context]
  end

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

There is a helper, Interactors.new that can simplify that:

interactions = AuthenticatedUser \
| Interactors.new do
    NotificationsMailer.login(user: context[:user]).deliver
    [:ok, context]
  end

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

Since we don't care about handling errors, we can Interactors::Simple instead:

interactions = AuthenticatedUser \
| Interactors::Simple.new { NotificationsMailer.login(user: context[:user]).deliver }

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

This might seem like a lot for just a simple mailer. The real value comes from when there is a long chain of interactions:

interactions = AuthenticatedUser \
| FraudDetector \
| Interactors::Simple.new { NotificationsMailer.login(user: context[:user]).deliver } \
| ActivityLogger.new(:user_logs_in, controller: self) \
| Interactors::RPC.new(service: :presence, module: :'Elixir.Presence.RPC', func: :register)

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

Custom Generic Interactors

Generic interactors work because we can override the constructor. In the case of a Rails mailer, maybe we want to have a generic mailer:

class Interactors::Mailer
  include FunctionalInteractor
  
  def new(mailer:, method:)
    @mailer = mailer
    @method = method
  end
  
  def call(context = {})
    mailer.send(method, context)
    [:ok, context]
  end
end

In which case, we can then use that:

interactions = AuthenticatedUser \
| Interactors::Mailer.new(mailer: NotificationsMailer, method: :login)

Kase.kase interactions.call(session_params) do
  on(:ok)    { |context| puts "Yay! Logged in!" }
  on(:error) { |context| puts "Failed to login" }
end

Further Discussion

collectiveideas/interactor has a great section discussing on when to use interactors: https://github.com/collectiveidea/interactor#when-to-use-an-interactor