Introducing ReActionView: A new ActionView-Compatible ERB Engine @ Kaigi on Rails 2025, Tokyo, Japan
by Marco Roth
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Marco Roth
Introducing ReActionView: A new
ActionView-Compatible ERB Engine
September 27, 2025 - Kaigi on Rails 2025, Toyko, Japan
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Marco Roth !
@marcoroth_
@[email protected]
🌐 marcoroth.dev
@marcoroth
Full-Stack Developer & Open Source Contributor
@marcoroth.dev
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I ❤ Open Source
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CableReady
Core Team
Maintainer
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I want to keep Ruby/Rails as
attractive and easy to get
started as possible
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Teaching Bootcamps
Mentoring Early-Career Engineers
Helping Businesses with Hotwire
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Lack of Feedback of Tooling
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Available Today
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HTML-only
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No ERB support
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For Rails apps it's quite common
to use ActionView ERB helpers
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Greet
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<%= tag.div data: { controller: "hello" } do %>
<%= content_tag :input, data: { hello_target: "name"} %>
<%= button_tag data: { action: "click->hello#greet" } do %>
Greet
<% end %>
<%= tag.span data: { hello_target: "output" } %>
<% end %>
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<%= tag.div(
data: {
controller: "filter",
filter_open_class: "border-white",
filter_close_class: "border-gray-300"
}
) %>
Missing full support for HTML+ERB and Rails-specific helpers
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No tooling support
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I had a few use-cases for a
smart ERB parser that could
handle use-cases like this
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I wanted to tackle
the bigger picture
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Empowering Developers with
HTML-Aware ERB Tooling
Marco Roth
April 16, RubyKaigi 2025
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HTML-aware ERB Parser
https://www.youtube.com/watch?v=EIoGfMGsiZI
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HTML-aware ERB Parser
HTML-aware ERB parser
Written in C99
Built on Prism
Designed for tooling
Support for Action View Helpers
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HTML-aware ERB Parser
An interleaved Syntax Tree of
HTML and Embedded Ruby
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HTML-aware ERB Parser
Structural Awareness of
HTML+ERB files
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<% if valid? %>
Title
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<% if valid? %>
Title
<% end %> <% if valid? %>Title
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<% if valid? %>
Title
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 39
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 40
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 41
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 42
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 43
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<% if valid? %>
Title
<% end %> @ ERBIfNode ├── tag_opening: "<%" ├── content: " if valid? " ├── tag_closing: "%>" ├── statements: │ └── @ HTMLElementNode │ ├── open_tag: │ │ └── @ HTMLOpenTagNode │ │ ├── tag_opening: "<" │ │ ├── tag_name: "h1" │ │ ├── tag_closing: ">" │ │ ├── children: [] │ │ └── is_void: false │ │ │ ├── tag_name: "h1" │ ├── body: │ │ └── @ HTMLTextNode │ │ └── content: "Title" │ │ │ └── close_tag: │ └── @ HTMLCloseTagNode │ ├── tag_opening: "" │ ├── subsequent: ∅ └── end_node: └── @ ERBEndNode ├── tag_opening: "<%" ├── content: " end " └── tag_closing: "%>"Slide 44
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HTML(+ERB)
Templating
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Vision on the future of
the view layer in Rails
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it doesn't come
without it's issues
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Encapsulation
View Boundaries
Reusability
Performance
Tooling
Testing
Frontend Logic/Complexity
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Developer Tooling
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Preview
Herb Formatter
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HTML+ERB Auto-Formatter
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<%=@title%>
<%if user_signed_in?%>Welcome,<%= current_user.name%>!
<%else%>Please<%= link_to"sign in",login_path%>.<%end%>
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<%=@title%>
<%if user_signed_in?%>Welcome,<%= current_user.name%>!
<%else%>Please<%= link_to"sign in",login_path%>.<%end%>
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<%=@title%>
<%if user_signed_in?%>Welcome,<%= current_user.name%>!
<%else%>Please<%= link_to"sign in",login_path%>.<%end%>
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<%= @title %>
<% if user_signed_in? %>Welcome, <%= current_user.name %>!
<% else %>Please <%= link_to "sign in", login_path %>
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Herb Linter
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18 new linter rules
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9 rules
Accessibility (A11y)
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Total of 32 linter rules
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Available Today
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Coming Soon
Stimulus LSP Update
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Planned
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<%= tag.div class: "px-4 py-2 text-white bg-sky-700" %>
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Tailwind Class Rewriter
Coming soon
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Vision
ERB Rendering Engine
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String Template Engine
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HTML Templating Engine
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An engine that cannot
produce invalid HTML
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.erb Embedded Ruby (ERB)
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Hello, <%= name %>!
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template = ActionView::Template.new(
"
Hello, <%= name %>!", "test.html.erb", ActionView::Template::Handlers::ERB, locals: [:name] )
Hello, <%= name %>!
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template = ActionView::Template.new(
"
Hello, <%= name %>!", "test.html.erb", ActionView::Template::Handlers::ERB, locals: [:name] ) compiled = template.handler.call( template, template.source )
Hello, <%= name %>!
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"@output_buffer.safe_append='
Hello, '.freeze; @output_buffer.append=( name ); @output_buffer.safe_append='!'.freeze; \n@output_buffer" compiled = template.handler.call( template, template.source )
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.herb HTML+ERB (Herb)
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Hello, <%= name %>!
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template = ActionView::Template.new(
"
Hello, <%= name %>!", "test.html.herb", ActionView::Template::Handlers::Herb, locals: [:name] ) compiled = template.handler.call( template, template.source )
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compiled = template.handler.call(
template,
template.source
)
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compiled = template.handler.call(
template,
template.source
)
#
Tag `
` opened at (1:1) was never closed before the end of document. UnclosedElementError in users/index.html.herb
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Marco Roth
Introducing ReActionView: A new
ActionView-Compatible ERB Engine
September 27, 2025 - Kaigi on Rails 2025, Toyko, Japan
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The Rails View Layer
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Action View
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July 2004 - Rails 0.5.0
https://rubytalk.org/t/ann-rails-0-5-0-the-end-of-vaporware/12744
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Rails is a open source web-application
framework for Ruby. It ships with an
answer for every letter in MVC:
Action Pack for the Controller and View,
Active Record for the Model.
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ERB
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Embedded Ruby
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"So basically the
templates are plain text files that can hold
anything (HTML, XML, LaTeX, emails)
sprinkled with Ruby embeddings to add
dynamic behavior."
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Today Action View Ships with
Erubi by default
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ERB is here to stay.
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Action View is here to stay.
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We want to level up
Action View while keeping it
Railsy.
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ActionView
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Reimagined
ActionView
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Reimagined
Reengineered
ActionView
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Reengineered
Reimagined
ActionView
Reactive
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An initiative to explore what’s
possible in the Rails view layer
in 2025 and beyond
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Backwards-Compatible
Improve DX
Embrace Web Standards
Integrate Frontend Frameworks
Explore what's possible
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Web Components
Progressive Web Apps (PWA)
Custom Elements
inert
modulepreload
Declarative Shadow DOM
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subgrid
Container Queries
:has()
@function
@property
CSS Nesting
@layer
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Speculation Rules API
Invoker Commands API
View Transition API
Popover API
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SSR
Rails App
+ HTML-over-the-wire
Hotwire/HTMX/Unpoly
Rails API
+ Single Page App
React/Vue/Svelte/etc.
CSR
server-side rendering client-side rendering
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High Fidelity UI
Existing Components/Libraries
A lot of React Developers
Split Backend/Frontend Teams
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Hotwire is great, simple, and gets
the job done in most of cases
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Transitioning from Hotwire to a
Frontend Framework is not
gradual, when you need it
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There is no onramp
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This is why some teams opt to
use SPAs from the beginning
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Which is giving away most of
what makes Rails so great and
productive
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Let's try to close the gap, so less
and less teams actually need
actually to abandon Action View
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I want to see what's possible
and how far we can go
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Release
Herb v0.7.0
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::Engine
Herb
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HTML-aware
ERB Rendering Engine
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Erubi::Engine
API-compatible
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Let's compare them
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Erubi::Engine
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Hello, <%= name %>!
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Hello, <%= name %>!
Regex ↓Slide 132
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Hello, <%= name %>!
"Hello, " name "!
" Regex ↓Slide 133
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Hello, <%= name %>!
add_text("Hello, ") add_expression("name") add_text("!
") Regex ↓Slide 134
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_buf = ::String.new
_buf << '
Hello, ' _buf << ::Erubi.h(name) _buf << '!
' _buf.to_s (simplified+formatted)Slide 135
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Herb::Engine
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Hello, <%= name %>!
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Herb.parse("source")
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@ DocumentNode
└── children: (1 item)
└── @ HTMLElementNode
├── open_tag:
│ └── @ HTMLOpenTagNode
│ ├── tag_opening: "<"
│ ├── tag_name: "h1"
│ └── tag_closing: ">"
│
├── tag_name: "h1"
├── body: (3 items)
│ ├── @ HTMLTextNode
│ │ └── content: "Hello, "
│ │
│ ├── @ ERBContentNode
│ │ ├── tag_opening: "<%="
│ │ ├── content: " name "
│ │ └── tag_closing: "%>"
│ │
│ └── @ HTMLTextNode
│ └── content: "!"
│
└── close_tag:
└── @ HTMLCloseTagNode
├── tag_opening: ""
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@ DocumentNode
└── children: (1 item)
└── @ HTMLElementNode
├── open_tag:
│ └── @ HTMLOpenTagNode (
) │ ├── body: (3 items) │ ├── @ HTMLTextNode ("Hello, ") │ ├── @ ERBContentNode (<%= name %>) │ └── @ HTMLTextNode ("!") │ └── close_tag: └── @ HTMLCloseTagNode (
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class Compiler < Herb::Visitor
def visit_document_node(node)
def visit_html_element_node(node)
def visit_html_open_tag_node(node)
def visit_html_close_tag_node(node)
def visit_html_text_node(node)
def visit_erb_content_node(node)
end
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def visit_html_element_node(node)
visit(node.open_tag)
visit_all(node.body)
visit(node.close_tag)
end
@ HTMLElementNode
├── open_tag:
│ └── @ HTMLOpenTagNode
│
├── body: (3 items)
│ ├── @ HTMLTextNode
│ ├── @ ERBContentNode
│ └── @ HTMLTextNode
│
└── close_tag:
└── @ HTMLCloseTagNode
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def visit_html_open_tag_node(node)
add_text(node.tag_opening)
add_text(node.tag_name)
add_text(node.tag_closing)
end
@ HTMLOpenTagNode
├── tag_opening: "<"
├── tag_name: "h1"
└── tag_closing: ">"
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def visit_html_text_node(node)
add_text(node.content)
end
@ HTMLTextNode
└── content: "Hello, "
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@ ERBContentNode
├── tag_opening: "<%="
├── content: " name "
└── tag_closing: "%>"
def visit_erb_content_node(node)
if node.tag_opening == "<%="
add_expression(node.content)
else
add_code(node.content)
end
end
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add_text("<")
add_text("h1")
add_text(">")
add_text("Hello, ")
add_expression("name")
add_text("!")
add_text("")
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add_text("<")
add_text("h1")
add_text(">")
add_text("Hello, ")
add_expression("name")
add_text("!")
add_text("")
add_text("
Hello, ") add_expression("name") add_text("!
") Erubi::Engine Herb::EngineSlide 147
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def add_text(text)
@tokens << [:text, text]
end
def add_expression(code)
@tokens << [:expr, code]
end
def add_code(code)
@tokens << [:code, code]
end
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@tokens = [
["<", :text],
["h1", :text],
[">", :text],
["Hello, ", :text],
["name", :expr],
["!", :text],
["", :text],
]
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@tokens = [
["<", :text],
["h1", :text],
[">", :text],
["Hello, ", :text],
["name", :expr],
["!", :text],
["", :text],
]
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@tokens = [
["
Hello, ", :text], ["name", :expr], ["!
", :text], ]Slide 151
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@tokens = [
["
Hello, ", :text], ["name", :expr], ["!
", :text], ]Slide 152
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add_text("
Hello, ") add_expression("name") add_text("!
") Erubi::Engine Herb::Engine add_text("Hello, ") add_expression("name") add_text("!
")Slide 153
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_buf = ::String.new
_buf << '
Hello, ' _buf << ::Erubi.h(name) _buf << '!
' _buf.to_s (simplified+formatted) _buf = ::String.new _buf << 'Hello, ' _buf << ::Herb.h(name) _buf << '!
' _buf.to_s Erubi::Engine Herb::EngineSlide 154
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if we now handle all
nodes the Herb Syntax
Tree has
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We end up with the (almost)
same compiled template output
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Let's change Rails to use
Herb::Engine instead of
Erubi::Engine
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# actionview/lib/action_view/template/handlers/erb/erubi.rb
require "erubi"
module ActionView
class Template
module Handlers
class ERB
class Erubi < ::Erubi::Engine
def initialize(input, properties = {})
# ...
super
end
end
end
end
end
end
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# actionview/lib/action_view/template/handlers/erb/herb.rb
require "herb"
module ActionView
class Template
module Handlers
class ERB
class Herb < ::Herb::Engine
def initialize(input, properties = {})
# ...
super
end
end
end
end
end
end
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# actionview/lib/action_view/template/handlers/erb.rb
module ActionView
class Template
module Handlers
class ERB
autoload :Erubi, "action_view/template/handlers/erb/erubi"
class_attribute :erb_implementation, default: Erubi
end
end
end
end
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# actionview/lib/action_view/template/handlers/erb.rb
module ActionView
class Template
module Handlers
class ERB
autoload :Erubi, "action_view/template/handlers/erb/erubi"
autoload :Herb, "action_view/template/handlers/erb/herb"
class_attribute :erb_implementation, default: Erubi
end
end
end
end
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# actionview/lib/action_view/template/handlers/erb.rb
module ActionView
class Template
module Handlers
class ERB
autoload :Erubi, "action_view/template/handlers/erb/erubi"
autoload :Herb, "action_view/template/handlers/erb/herb"
class_attribute :erb_implementation, default: Erubi
end
end
end
end
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# actionview/lib/action_view/template/handlers/erb.rb
module ActionView
class Template
module Handlers
class ERB
autoload :Erubi, "action_view/template/handlers/erb/erubi"
autoload :Herb, "action_view/template/handlers/erb/herb"
class_attribute :erb_implementation, default: Herb
end
end
end
end
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bin/dev
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$
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Gives us all the guarantees
the Herb Parser gives us
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Ensures valid
HTML Templates and Syntax
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String Template Engine
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HTML Templating Engine
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But it also gives us
valuable feedback
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The same parser that's now
compiling/rendering your views
is also running in your editor
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Makes the tooling a lot more
accurate and instant
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Since our engine now has full
introspection and understanding
of HTML+ERB...
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... we can do a lot of cool things!
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Let's improve error messages
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The Herb Parser already
exactly knows where the error is
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So it can pinpoint it exactly
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But since we deal with
HTML+ERB we can also have
some smart validations built-in
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config.annotate_rendered_view_with_filenames
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You can't really style HTML
comment
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But, our Compiler already knows
how to render HTML elements
and attributes
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So, what if we...
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... inject additional HTML
attributes in development?
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...
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[data-herb-debug-file-name] {
outline: #3B82F6 dotted 2px;
outline-offset: 2px;
}
// ...
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Views
Partials
Components
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...
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...
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...
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CSS outline + HTML
attributes don't interrupt
the natural flow
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Provides immensely
helpful visual debugging
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Fast jump-to-source
clicking on the
view outline label
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Can we also do this for
ERB Output tags?
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Yes.
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Show ERB content on hover
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Jump-to-source
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Vision
What is next
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6 Adoption Levels
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Level 1 - Better Errors Messages
Level 2 - HTML-aware Engine
Level 3 - Action View Optimizations
Level 4 - Reactive ERB Views
Level 5 - Client-side Templates
Level 6 - External Components
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bundle add reactionview
rails generate reactionview:install
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# config/initializers/reactionview.rb
ReActionView.configure do |config|
# Intercept .html.erb templates
# config.intercept_erb = true
# Enable debug mode in development
config.debug_mode = Rails.env.development?
end
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# config/initializers/reactionview.rb
ReActionView.configure do |config|
# Intercept .html.erb templates
config.intercept_erb = true
# Enable debug mode in development
config.debug_mode = Rails.env.development?
end
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bin/dev
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This is just the beginning
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Please give it a shot, even if it's
just locally in your development
environment
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Please report any weird
behavior your might encounter
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Level 1 - Better Errors
Level 2 - HTML-aware Engine
Level 3 - Action View Optimizations
Level 4 - Reactive ERB Views
Level 5 - Client-side Templates
Level 6 - External Components
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Level 3 - Action View
Optimizations
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Inlining Partial render calls
Inlining Action View Tag Helpers
Minifying insignificant whitespace
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Probably more
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Level 4 - Reactive ERB Views
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Phoenix LiveView HEEx-like
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@items = [1, 2, 3, 4]
- 1
- 2
- 3
- 4
-
<% @items.each do |item| %>
- <%= item %> <% end %>
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Level 5 - Client-side Templates
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Limited Set of ERB
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RSC - React Server
Components
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"ERB Client Components"
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Web Components
React Components
...
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Optimistic UI updates
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Improved offline functionality
(+ service workers)
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Level 6 - External Components
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When you need more and
don't want to abandon
HTML+ERB alltogether
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Only on certain pages
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pin "@rails/actioncable", to: "actioncable.esm.js"
pin "@rails/activestorage", to: "activestorage.esm.js"
pin "@rails/actiontext", to: "actiontext.esm.js"
pin "trix"
pin_all_from "controllers"
importmap-rails
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Register external components
import [:Alert, :AlertTitle], from: "shadcn"
import_all_from "app/javascript/components"
config/initializers/reactionview.rb
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<%= @post.title %>
Heads up! app/views/posts/show.html.herbSlide 248
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The engine knows that these
components are client-side
and knows how to mount/
hydrate them
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Tightly integrated
Built-in Tooling
No setup
Works out of the box
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Ideas
More ideas to explore
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One off
Stimulus Controllers
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https://alpinejs.dev/start-here
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"Inline Stimulus"
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View File
Scoped CSS
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.example {
color: red;
}
Hello World
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.example {
color: red;
}
Hello World
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color: red;
}
Hello World
.example {
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color: red;
}
Hello World
.example[data-v-f3f3eg9]
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Selective Rendering
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Turbo Frames
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<%= @product.name %>
Customer Reviews
<%= render @reviews %> <%= link_to "Load More Reviews", reviews_path %>Slide 265
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<%= @product.name %>
Customer Reviews
<%= render @reviews %> <%= link_to "Load More Reviews", reviews_path %>Slide 266
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<%= render "card", selector: "#reviews" %>
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<%= @product.name %>
Customer Reviews
<%= render @reviews %> <%= link_to "Load More Reviews", reviews_path %>Slide 268
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The idea is to bring more
of these modern frontend
features to Rails
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So that there is less of a
need to abandon the
Rails way
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Action View has
been really stable
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Most features/changes
were additive
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Makes it easy to
rely and build on
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Rails shouldn't
discourage the use of
(modern) JavaScript
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Rails should embrace it
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embrace modern
web standards
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Provide good defaults and
build-in options/migration
paths when you need more
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So that the view layer scales
the same way everything
else does in the framework
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This is a vision.
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If we, as a framework and
community, want to stay
relevant we need to explore
what's possible.
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New innovations require
exploration.
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Even if none of this is viable in
production it's still worth
exploring.
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Conclusion
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Prism had a big effect on Ruby
internals and the tooling landscape
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Herb already has a similar effect for
HTML Templating and Tooling
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Herb started as a Parser
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and is now an ecosystem
of valuable tools
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ReActionView started as a
vision to address some of the
shortcomings in the Rails
View Layer
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Today it might be just an
alternative ERB Rendering
engine
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But I believe that this already
shows what could be possible
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We can push more boundaries
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and bring the whole Rails
Framework forward
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so that it can hold up with the
current JavaScript stacks
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and that teams don't have to
abandon the beauty of
Action View
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The community has built
incredible tooling for Ruby
itself.
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Now it's time to bring that
same care to the view layer.
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With Herb and ReActionView
I believe we have a path to
level up the view layer
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Herb::Engine
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Rails has a unique position as
a full stack framework
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I want to keep Rails as
attractive and competitive as
possible
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github.com/sponsors/marcoroth
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herb-tools.dev
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reactionview.dev
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Please try the tools, report
issues, and give feedback .
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I believe there is a bright future
for the Rails Frontend Story
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Thank you %
@marcoroth_
@[email protected]
🌐 marcoroth.dev
@marcoroth
@marcoroth.dev
/in/marco-roth