Ratatouille

Ratatouille is a declarative terminal UI kit for Elixir for building rich text-based terminal applications similar to how you write HTML.

It builds on top of the termbox API (using the Elixir bindings from ex_termbox).

For the API Reference, see: https://hexdocs.pm/ratatouille.

Toby, a terminal-based Erlang observer built with Ratatouille

Table of Contents

• Ratatouille
  • Getting Started
    • Building an Application
      • init/1
      • update/2
      • render/1
      • Running it
  • Views
    • The DSL
    • Adding Logic
    • Styling
    • Views are Strict
  • Example Applications
  • Under the Hood
  • Packaging and Distributing
    • Defining an OTP Application
    • Executable Releases with Distillery
  • Projects using Ratatouille
  • Installation
    • From Hex
    • From Source
  • Roadmap
  • Contributing
    • Running the Tests

Getting Started

Ratatouille implements The Elm Architecture (TEA) as a way to structure application logic. This fits quite naturally in Elixir and is part of what makes Ratatouille declarative. If you've already used TEA on the web, this should feel very familiar.

As with a GenServer definition, Ratatouille apps only implement a behaviour by defining callbacks and don't know how to start or run themselves. It's the application runtime that handles all of those (sometimes tricky) details.

Building an Application

Let's build a simple application that displays an integer counter which can be incremented when the user presses + and decremented when the user presses -.

First a quick clarification, since we're using the word "application" a lot. For our purposes, an application is a terminal application, and not necessarily an OTP application, but your terminal application could also be an OTP application. We'll cover that in Packaging and Distributing Applications below.

Back to the counter app. First we'll look at the entire example, then we'll go through it line by line to see what each line does. You can also find this example in the repo and run it with mix run.

# examples/counter.exs

defmodule Counter do
  @behaviour Ratatouille.App

  import Ratatouille.View

  def init(_context), do: 0

  def update(model, msg) do
    case msg do
      {:event, %{ch: ?+}} -> model + 1
      {:event, %{ch: ?-}} -> model - 1
      _ -> model
    end
  end

  def render(model) do
    view do
      label(content: "Counter is #{model} (+/-)")
    end
  end
end

Ratatouille.run(Counter)

At the top, we define a new module (Counter) for the app and we inform Elixir that it will implement the Ratatouille.App behaviour. This just ensures we're warned if we forget to implement a callback and serves as documentation that this is a Ratatouille app.

defmodule Counter do
  @behaviour Ratatouille.App

  # ...
end

Next, we import the View DSL from Ratatouille.View:

import Ratatouille.View

The View DSL provides element builder functions like view, row, table, label that you can use to define views. Think of them like HTML tags.

init/1

The init/1 callback defines the initial model. "Model" is the Elm architecture's term for what we often call "state" in Elixir/Erlang. As with a GenServer, the state (our model) will later be passed to callbacks when things happen in order to allow the app to update it.

The model can be any Erlang term. For larger apps, it's helpful to use maps or structs to organize different pieces of the state. Here, we just have an integer counter, so we return 0 as our initial model:

defmodule Counter do
  # ...

  def init(_context), do: 0

  # ...
end

update/2

The update/2 callback defines how to transform the model when a particular message is received. Ratatouille's runtime will automatically call update/2 when terminal events occur (pressing a key, resizing the window, clicking the mouse, etc.). We can also send ourselves messages via subscriptions and commands.

Here, we'd like to increment the counter when we get a ?+ key press and decrement it when get a ?-. Event messages are based on the underlying termbox events and characters are given as code points (e.g., ?a is 97).

defmodule Counter do
  # ...

  def update(model, msg) do
    case msg do
      {:event, %{ch: ?+}} -> model + 1
      {:event, %{ch: ?-}} -> model - 1
      _ -> model
    end
  end

  # ...
end

It's a good idea to provide a fallback clause in case we don't know how to handle a message. This way the app won't crash if the user presses a key that the app doesn't handle. But if things stop working as you expect, try removing the fallback to see if important messages are going unmatched.

render/1

The render/1 callback defines a view to display the model. The runtime will call it as needed when it needs to update the terminal window.

Like an HTML document, a view is defined as a tree of elements (nodes). Elements have attributes (e.g., text: bold) and children (nested content). While helper functions can return arbitrary element trees, the render/1 callback must return a view tree starting with a root view element---it's sort of like the <body> tag in HTML.

defmodule Counter do
  # ...

  def render(model) do
    view do
      label(content: "Counter is #{model} (+/-)")
    end
  end

  # ...
end

Running it

There's a final and very important line at the bottom:

Ratatouille.run(Counter)

This starts the application runtime with our app definition. Options can be passed as a second argument. This is an easy way to run simple apps. For more complicated ones, it's recommended to define an OTP application.

That's it---now you can run the program with mix run <file>. To run the bundled example:

$ mix run examples/counter.exs

You should see the counter we defined, be able to make changes to it with + and -, and be able to quit using q.

Views

Ratatouille's views are trees of elements similar to HTML in structure. For example, here's how to define a two-column layout:

view do
  row do
    column size: 6 do
      panel title: "Left Column" do
        label(content: "Text on the left")
      end
    end

    column size: 6 do
      panel title: "Right Column" do
        label(content: "Text on the right")
      end
    end
  end
end

The DSL

As you might have noticed, Ratatouille provides a small DSL on top of Elixir for defining views. These are functions and macros which accept attributes and/or child elements in different formats. For example, a column element can be defined in all of the following ways:

column()

column(size: 12)

column do
  # ... child elements ...
end

column size: 12 do
  # ... child elements ...
end

All of these evaluate to a %Ratatouille.Renderer.Element{tag: :column} struct. The macros provide syntactic sugar, but under the hood it's all structs.

Here's a list of all the elements provided by Ratatouille.View:

Adding Logic

Because it's just Elixir code, you can freely mix in Elixir syntax and abstract views using functions:

label(content: a_variable)

view do
  case current_tab do
    :one -> render_tab_one()
    :two -> render_tab_two()
  end
end

if window.width > 80 do
  row do
    column(size: 6)
    column(size: 6)
  end
else
  row do
    column(size: 12)
  end
end

Styling

Attributes are used to style text and other content:

# Labels are block-level, so this makes text within the whole block red.
label(content: "Red text", color: :red)

# Nested inline text elements can be used to style differently within a label.
label do
  text(content: "R", color: :red)
  text(content: "G", color: :green)
  text(content: "B", color: :blue)
end

# `color` sets the foreground, while `background` sets the background.
label(content: "Black on white", color: :black, background: :white)

# `attributes` accepts a list of text attributes, here `:bold` and `:underline`.
label(content: "Bold and underlined text", attributes: [:bold, :underline])

Styling is still being developed, so it's not currently possible to style every aspect of every element, but this will improve with time.

Views are Strict

Most web browsers will happily try to make sense of any HTML you give them. For example, you can put a td directly under a div and the content will likely still be rendered.

Ratatouille takes a different, more strict approach and first validates that the view tree is well-structured. If it's not valid, an error is raised explaining the problem. This is intended to provide quick feedback when something's wrong. Restricting the set of valid views also helps to simplify the rendering implementation.

It's helpful to keep the following things in mind when defining views:

• Each tag has a list of allowed child tags. For example, a row may only have elements with the column tag as direct descendants.
• Each tag has a list of attributes. Some attributes are required, and these must be set. Optional attributes have some default behavior when unset. It's not allowed to set an attribute that's not in the list.
• A view element must be the root element of any view tree you'd like to render.

See the list of elements above for documentation on each element.

Example Applications

The following examples show off different aspects of the framework:

With the repository cloned locally, run an example with mix run examples/<example>.exs. Examples can be quit with q or CTRL-c (unless indicated otherwise).

Under the Hood

The application runtime abstracts away many of the details concerning how the terminal window is updated and how events are received. If you're interested in how these things actually work, or if the runtime doesn't support your use case, see this guide:

https://hexdocs.pm/ratatouille/under-the-hood.html

Packaging and Distributing

Warning: This part is still rough around the edges.

While it's easy to run apps while developing with mix run, packaging them for others to easily run is a bit more complicated. Depending on the type of app you're building, it might not be reasonable to assume that users have any Elixir or Erlang tools installed. Terminal apps are usually distributed as binary executables so that they can just be run as such without additional dependencies. Fortunately, this is possible using OTP releases that bundle ERTS.

Defining an OTP Application

In order to create an OTP release, we first need to define an OTP application that runs the terminal application. Ratatouille.Runtime.Supervisor takes care of starting all the necessary runtime components, so we start this supervisor under the OTP application supervisor and pass it a Ratatouille app definition (along with any other runtime configuration).

For example, the OTP application for toby looks like this:

defmodule Toby do
  use Application

  def start(_type, _args) do
    children = [
      {Ratatouille.Runtime.Supervisor, runtime: [app: Toby.App]},
      # other workers...
    ]

    Supervisor.start_link(
      children,
      strategy: :one_for_one,
      name: Toby.Supervisor
    )
  end
end

Executable Releases with Distillery

We'll use Distillery to create the OTP release, as it can even create distributable, self-contained executables. Releases built on a given architecture can generally be run on machines of the same architecture.

Follow the Distillery guide to generate a release configuration:

https://hexdocs.pm/distillery/introduction/installation.html

In order to make a "batteries-included" release, it's important that you have include_erts set to true:

environment :prod do
  # ...
  set(include_erts: true)
  # ...
end

Now it's possible to generate the release:

MIX_ENV=prod mix release --executable --transient

This creates a Distillery release that bundles the Erlang runtime and the application. Start it in the foreground, e.g.:

_build/prod/rel/toby/bin/toby.run foreground

You can also move this executable somewhere else (e.g., to a directory in your $PATH). A current caveat is that it must be able to unpack itself, as Distillery executables are self-extracting archives.

Projects using Ratatouille

For inspiration or ideas on how to structure your application, check out this list of projects built with Ratatouille:

• tefter/cli - the command-line client for Tefter
• toby - a terminal-based Erlang observer

If you have a project you'd like to include here, just open a PR to add it to the list.

Installation

From Hex

Add Ratatouille as a dependency in your project's mix.exs:

def deps do
  [
    {:ratatouille, "~> 0.5.0"}
  ]
end

From Source

To try out the master branch, first clone the repo:

git clone https://github.com/ndreynolds/ratatouille.git
cd ratatouille

Next, fetch the deps:

mix deps.get

Finally, try out one of the included examples/:

mix run examples/rendering.exs

If you see lots of things drawn on your terminal screen, you're good to go. Use "q" to quit in the examples (unless otherwise specified).

Roadmap

• Apps
  • Application Runtime
  • Subscriptions
  • Commands
• Views / Rendering
  • Rendering engine with basic elements
  • More configurable charts (axis label, color, multiple lines, etc.)
  • Uniform support for text styling (incl. inheritance)
  • Automatic translation to termbox styling constants
    • For example, color: :red instead of color: Constants.color(:red).
  • Rendering optimizations (view diffing, more efficient updates, etc.)
• Events
  • Translate termbox events to a cleaner format
    • Dealing with the integer constants is inconvenient. These could be converted to atoms by the event manager.
• Terminal Backend
  • ex_termbox NIFs
  • Alternative port-based termbox backend
• Customization
  • Registering custom element renderers
    • This would support using custom elements (e.g. my_table()) that are defined outside of the core library.

Contributing

Contributions are much appreciated. They don't necessarily have to come in the form of code, I'm also very thankful for bug reports, documentation improvements, questions, and suggestions.

Running the Tests

Run the unit tests as usual:

mix test

Ratatouille also includes integration tests of the bundled examples. These aren't included in the default suite because they actually run the example apps. The integration suite can be run like so:

mix test --only integration

Copyright and License

Copyright (c) 2018 Nick Reynolds

This software is released under the MIT License.