If you’ve been working with Ruby on Rails for any length of time, you’ve probably encountered the age-old question: which templating engine should I use? With ERB as the default, Slim and Haml as popular alternatives, and Phlex as the new kid on the block, the choice can feel overwhelming.
In this comprehensive guide, I’ll break down each option, compare their strengths and weaknesses, and help you make an informed decision for your Rails projects.
Understanding the Landscape
Before diving into specifics, let’s understand what we’re comparing. Template engines are tools that help you generate HTML dynamically by embedding Ruby code within markup. Each engine has a different philosophy about how this should be done.
ERB (Embedded Ruby)
What is it? ERB is Rails’ default templating engine. It embeds Ruby code directly into HTML using special tags.
Syntax Example
<div class="user-profile">
<h1><%= @user.name %></h1>
<% if @user.admin? %>
<span class="badge">Admin</span>
<% end %>
<ul class="posts">
<% @user.posts.each do |post| %>
<li><%= link_to post.title, post_path(post) %></li>
<% end %>
</ul>
</div>
Code language: HTML, XML (xml)Pros
Zero Learning Curve: If you know HTML and Ruby, you already know ERB. There’s no new syntax to learn, making it perfect for beginners and mixed teams.
Universal Support: Every Rails developer knows ERB. Every gem, tutorial, and Stack Overflow answer uses ERB. This ubiquity is valuable.
No Setup Required: It works out of the box with every Rails installation. No gems to add, no configuration needed.
Familiar to Other Ecosystems: The concept of embedding code in angle brackets exists in PHP, ASP, JSP, and many other frameworks. Developers coming from other backgrounds will feel at home.
Cons
Verbose: Writing closing tags for everything gets tedious. Your files become longer than they need to be.
Easy to Create Messy Code: Because ERB doesn’t enforce structure, it’s easy to mix business logic with presentation logic, leading to hard-to-maintain views.
Repetitive: You’ll find yourself typing the same patterns over and over. The lack of shortcuts makes ERB feel inefficient once you’ve experienced alternatives.
When to Use ERB
ERB is ideal when you’re starting a new project with junior developers, working with a team that values convention over optimization, or building simple CRUD applications where template complexity is minimal. It’s also the safe choice for open-source projects where maximum accessibility matters.
Slim
What is it? Slim is a lightweight templating engine focused on reducing syntax to its bare essentials. Its motto is “what’s left when you take the fat off ERB.”
Syntax Example
.user-profile
h1 = @user.name
- if @user.admin?
span.badge Admin
ul.posts
- @user.posts.each do |post|
li = link_to post.title, post_path(post)
Code language: JavaScript (javascript)Pros
Dramatically Less Code: Slim templates are typically 30-40% shorter than their ERB equivalents. This means faster writing and easier scanning.
Clean and Readable: Once you learn the syntax, Slim templates are remarkably easy to read. The indentation-based structure naturally enforces good organization.
Fast Performance: Slim compiles to Ruby code that’s often faster than ERB, though the difference is negligible in most applications.
Enforces Good Structure: The indentation requirement prevents messy, unstructured code. You can’t create a Slim template that doesn’t follow proper nesting.
Cons
Learning Curve: Team members need to learn new syntax. The first week will involve frequent reference to documentation.
Indentation Sensitivity: Like Python, Slim uses significant whitespace. A misplaced space or tab can break your template, which can be frustrating when debugging.
Less Common: Fewer developers know Slim compared to ERB. Hiring and onboarding may take slightly longer.
Limited Ecosystem Examples: While most gems work fine with Slim, documentation and examples are usually in ERB, requiring mental translation.
When to Use Slim
Slim shines in applications with complex views where you want to maximize readability and minimize boilerplate. It’s perfect for teams that value developer experience and are willing to invest a small amount of time upfront to learn the syntax. If you find yourself frustrated by ERB’s verbosity, Slim is your answer.
Haml
What is it? Haml (HTML Abstraction Markup Language) was one of the first popular alternatives to ERB. It uses indentation to represent HTML structure and eliminates closing tags.
Syntax Example
.user-profile
%h1= @user.name
- if @user.admin?
%span.badge Admin
%ul.posts
- @user.posts.each do |post|
%li= link_to post.title, post_path(post)
Code language: JavaScript (javascript)Pros
Mature and Stable: Haml has been around since 2006. It’s battle-tested and reliable with excellent documentation.
Cleaner Than ERB: Like Slim, Haml eliminates closing tags and reduces boilerplate significantly.
Good Ecosystem Support: Many gems and libraries explicitly support Haml, and you’ll find plenty of examples and resources online.
Enforces Structure: The indentation requirement keeps your code organized and prevents deeply nested chaos.
Cons
Slower Than Slim: Haml is noticeably slower than Slim in benchmarks, though for most applications this won’t matter.
More Verbose Than Slim: The % prefix for tags makes Haml slightly more verbose than Slim’s minimalist approach.
Indentation Sensitivity: Like Slim, whitespace matters. Mixing tabs and spaces will cause problems.
Feeling Dated: While still widely used, Haml hasn’t evolved as quickly as Slim. It lacks some of the refinements that make Slim feel more modern.
When to Use Haml
Choose Haml if you want an alternative to ERB but prefer a more established option with extensive community support. It’s a safe middle ground between ERB’s verbosity and Slim’s minimalism. Haml is particularly good if you’re maintaining a legacy codebase that already uses it.
Phlex
What is it? Phlex represents a radical departure from traditional templating. Instead of mixing Ruby with HTML-like syntax, Phlex uses pure Ruby classes to build views. It’s component-oriented and type-safe.
Syntax Example
class UserProfile < Phlex::HTML
def initialize(user)
@user = user
end
def template
div(class: "user-profile") do
h1 { @user.name }
span(class: "badge") { "Admin" } if @user.admin?
ul(class: "posts") do
@user.posts.each do |post|
li { a(href: post_path(post)) { post.title } }
end
end
end
end
end
Code language: HTML, XML (xml)Pros
Pure Ruby: No context switching between Ruby and template syntax. Your entire view is just Ruby code, which means better IDE support, easier refactoring, and familiar debugging.
Component Architecture: Phlex encourages building reusable components, leading to better code organization and DRY principles.
Type Safety: Because it’s pure Ruby, you can use tools like Sorbet or RBS for type checking your views.
Excellent Performance: Phlex is extremely fast, often outperforming other template engines significantly.
Testable: Components are just Ruby classes, making them easy to unit test without rendering overhead.
No Markup Parsing: Since there’s no template syntax to parse, there’s one less layer of complexity in your stack.
Cons
Paradigm Shift: Phlex requires a completely different way of thinking about views. This isn’t just new syntax—it’s a new architecture.
Verbose for Simple Views: For basic templates, Phlex can feel like overkill. Writing div { h1 { "Hello" } } instead of <div><h1>Hello</h1></div> doesn’t feel like progress for simple cases.
Limited Ecosystem: Phlex is new. There are fewer examples, fewer ready-made components, and a smaller community.
No Designer-Friendly Workflow: Because Phlex is pure Ruby, front-end developers or designers who aren’t comfortable with Ruby will struggle to contribute to views.
Steep Learning Curve: Understanding how to structure Phlex components well takes time and experience.
When to Use Phlex
Phlex is ideal for component-heavy applications where you want maximum reusability and testability. It’s perfect for design systems, UI libraries, or applications with complex, interactive interfaces. Choose Phlex if your team is comfortable with Ruby and values type safety and performance. It’s also excellent for API-driven applications where you’re building JSON responses rather than full HTML pages.
The Comparison Matrix
Let me break down how these engines stack up across key criteria:
Performance
Winner: Phlex Phlex is the fastest, followed closely by Slim. Haml is slower, and ERB sits in the middle. However, for most applications, template rendering isn’t the bottleneck—database queries and business logic are.
Readability
Winner: Slim Once learned, Slim offers the best balance of conciseness and clarity. ERB is readable but verbose. Haml is good but slightly cluttered with % symbols. Phlex requires Ruby fluency to read comfortably.
Learning Curve
Winner: ERB ERB has virtually no learning curve. Slim and Haml require a day or two to feel comfortable. Phlex requires rethinking your entire approach to views.
Ecosystem Support
Winner: ERB ERB is universal. Everything supports it. Slim and Haml have good support but sometimes require translation. Phlex is still building its ecosystem.
Maintainability
Winner: Phlex/Slim Phlex’s component architecture and Slim’s enforced structure both lead to highly maintainable codebases. ERB’s flexibility can become a maintainability liability. Haml sits in the middle.
Team Onboarding
Winner: ERB Any Rails developer can contribute to ERB templates immediately. The alternatives require training time.
My Recommendations
After years of using all these engines in production, here’s what I recommend:
For New Projects with Small Teams
Use Slim. You’ll write less code, maintain cleaner views, and the learning investment pays off quickly. The performance gains are nice, but the real benefit is how much easier it is to scan and understand Slim templates.
For Large Teams or Open Source
Stick with ERB. The universal knowledge and zero onboarding friction outweigh the benefits of alternatives. Don’t underestimate the value of every Rails developer being able to contribute immediately.
For Component-Heavy Applications
Choose Phlex. If you’re building a complex UI with lots of reusable components, Phlex’s architecture will save you time in the long run. The learning curve is worth it for applications where component composition is central.
For Existing Projects
Don’t Rewrite. If your project already uses Haml or Slim, keep using it. If it uses ERB and you’re happy with it, don’t change. The cost of conversion rarely justifies the benefits.
For Learning
Start with ERB, then try Slim. Master Rails with its default templating engine first. Once you’re comfortable, experiment with Slim on a side project. After you understand the tradeoffs, you’ll be equipped to make informed decisions.
Mixing Engines
Here’s something many developers don’t realize: you can use multiple templating engines in the same Rails application. You might use ERB for most views but Phlex for a complex component or Slim for your admin interface.
This flexibility means you’re not locked into one choice forever. Start with ERB and migrate specific areas to alternatives as needs arise.
The Future
The Rails templating landscape is evolving. Phlex represents a new wave of thinking about views as components rather than templates. Meanwhile, tools like ViewComponent bridge the gap between traditional templates and component architecture.
My prediction? We’ll see more hybrid approaches where simple CRUD views use traditional templates while complex UIs leverage component-based systems like Phlex.
Conclusion
There’s no universally correct answer to “which templating engine should I use?” The right choice depends on your team, your project, and your priorities.
- ERB for maximum compatibility and zero friction
- Slim for optimal developer experience and clean code
- Haml for a mature alternative with good ecosystem support
- Phlex for component-driven architecture and maximum performance
My personal preference? I use Slim for most projects. The productivity boost is real, the syntax becomes second nature quickly, and I appreciate how it naturally encourages better code organization. But I’ve shipped successful applications with all four engines, and I wouldn’t hesitate to use any of them given the right context.
What matters most isn’t which engine you choose, but that you use it consistently and well. A well-structured ERB codebase beats a messy Slim project every time.
What’s your experience with Rails templating engines? Have you tried alternatives to ERB? I’d love to hear your thoughts in the comments below.
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