Theo

Theo is a small and elegant HTML-like template language for Ruby on Rails, featuring natural partials and computed attributes.

Warning

Please note that this software is still experimental - use at your own risk.

Introduction

Thanks to Hotwire, it's now possible to build sophisticated server-rendered user interfaces in Ruby on Rails. However, ERB, Rails' most popular template language, has unintuitive partial syntax, especially for those used to working with Vue.js or React components.

With Theo, you can render a partial using HTML-like syntax:

<_button size="large" label%="label" />

Getting started

Install

Run

bundle add theo-rails

Configure

If you are using TailwindCSS, add .theo extension to the content key in your tailwind.config.js:

'./app/views/**/*.{erb,haml,html,slim,theo}'

Try

Create a new view named 'hello.html.theo(note the.theo` suffix), with the following content:

<span style%="'background-color: ' + 'yellow'">Hello from Theo!</span>

Visit the URL corresponding to this view and you should see a highlighed text.

Syntax

Computed attributes

Computing attribute value in ERB feels awkward because angle brackets <> clash with the surrounding HTML tag.

In Theo, an attribute with computed value can be expressed using %=. For example:

<a href%="root_path">Home</a>

is equivalent to:

<a href="<%= root_path %>">Home</a>

Tip

Computed attributes work with partials as well as standard HTML tags.

Short form

If value of a dynamic attribute is the same as its name, you can omit the value.

For example

<div style%>Text</div>

is equivalent to:

<div style%="style">Text</div>

which in turn is equivalent to:

<div style="<%= style %>">Text</div>

Since class is a Ruby keyword, it's treated specially:

<div class%>Text</div>

is equivalent to:

<div class="<%= binding.local_variable_get('class') %>">Text</div>

Tip

Short form is especially useful when you want to apply a class and style attribute to a partial root.

Partials

Rendering a partial in ERB requires context-switching between HTML markup and Ruby code, and the render verb makes it difficult to imagine a page as a component structure.

In Theo, you render a partial by writing a tag with '_' prefix, followed by kebab-cased partial name, for example:

<_special-button size="large" />

is equivalent to:

<%= render 'special_button', size: 'large' %>

Alternatively, you can also use PascalCase, for example:

<SpecialButton size="large" />

The benefit is that this form is recognized as valid HTML by most parsers.

Naturally, partials can also include content, e.g.:

<_button size="large">
  Create
</_button>

Tip

Rendered partials can be implemented in ERB, Theo or any other template language.

Collections

You can render a collection of partials as follows:

<_widget collection="widgets" />

which is equivalent to:

<%= render partial: 'widget', collection: widgets %>

You can also customize the local variable name via the as attribute, e.g.:

<_widget collection="widgets" as="item" />

Boolean attributes

If an attribute has no value, you can omit it, for example:

<_button disabled />

is equivalent to:

<_button disabled="" />

Path

To render a partial from another folder, use the 'path' attribute, e.g.:

<_widget path="widgets" />

is equivalent to:

<%= render 'widgets/widget' %>

yields attribute

Partials can yield a value, such as a builder object that can be used by child partials. For example:

<_widget-builder yields="widget">
  <_widget-element widget%="widget" />
</_widget-builder>

is equivalent to:

<%= render 'widget_builder' do |widget| %>
  <%= render 'widget_element', widget: %>
<% end %>

provide and inject helpers

Instead of using yields attribute, a parent partial can indirectly pass a variable to its children using the provide and inject helpers. The example above can be modified as follows:

<_widget-builder>
  <_widget-element />
</_widget-builder>

_widget_builder.html.theo:

<% provide(widget:) do %>
  <%= yield %>
<% end %>

_widget_element.html.theo:

<% widget = inject(:widget) %>

Note

This technique is used by form partials. Use it sparingly, as implicit variables can reduce code readability.

ERB backwards compatibility

You can freely mix ERB and Theo syntax, e.g.:

<% if total_amount > 100 %>
  <_free-shipping amount%="total_amount" />
<% end %>

Forms

You can build a <form> element in ERB using ActionView form helpers. Theo provides corresponding partials. For example:

<_form-with model%="widget" data-turbo-confirm="Are you sure?">
  <div>
    <_label name="name" />
    <_text-field name="name" />
  </div>

  <div>
    <_label name="size" />
    <_select name="size" options%="['Big', 'Small']" />
  </div>

  <_submit value="Create" />
</_form-with>

is equivalent to:

<%= form_with model: widget, data: { turbo_confirm: 'Are you sure?' } do |form| %>
  <div>
    <%= form.label :name %>
    <%= form.text_field :name %>
  </div>

  <div>
    <%= form.label :size %>
    <%= form.select :size, ['Big', 'Small'] %>
  </div>

  <%= form.submit "Create" %>
<% end %>

ViewComponents

Theo is compatible with ViewComponent framework.

Here's a component using Theo template syntax:

class ButtonComponent < ViewComponent::Base
  theo_template <<-THEO
    <span class%="@size"><%= content %></span>
  THEO

  def initialize(size:)
    @size = size
  end
end

If a components exists, and you use PascalCase syntax, Theo automatically renders it instead of a partial. Therefore:

<Button size="large" />

is equivalent to:

<%= render(ButtonComponent.new(size: "large")) %>

Components can yield a value:

<Button size="large" yields="component">
  <% component.with_header do %>Icon<% end %>
  Create
</Button>

You can also render a component collection as follows:

<Widget collection="widgets" />

which is equivalent to:

<%= render WidgetComponent.with_collection(widgets) %>