mustache/mustache: logic-less Ruby templates and where the separation actually bites

The problem mustache/mustache solves, and who it is written for

The README frames the gem as a replacement for views built from ERB or Haml "with random helpers and arbitrary logic." That is the whole pitch: a view becomes two artifacts, a Ruby class the project calls the "view" and an HTML template, and the template "does nothing but reference methods in your view." The README states the goal plainly: "It emphasizes separating logic from presentation: it is impossible to embed application logic in this template language."

The audience is Ruby developers who already prefer writing Ruby to writing template dialects. The README's own list is blunt about it, saying the author does not like writing ERB, Haml, Liquid or Django Templates, or putting Ruby or JavaScript inside HTML. If you have ever debugged a view where a conditional, a query and a formatting rule all live in the same file, that is the situation this gem targets.

It is framework-agnostic by design. Nothing in the README ties rendering to Rails or Sinatra; the integration path it names is a separate gem, mustache-sinatra, plus an example application. That makes it usable in a plain Ruby script, a Rack app, or a static generation step, but it also means you supply the wiring yourself.

How rendering works: a Ruby view object, a template, and method lookup

The mechanism is small enough to describe in one example. You define a normal Ruby class inheriting from Mustache, define methods on it, and write a template that references those methods by name. The README's canonical view defines name, value, taxed_value and in_ca, where taxed_value calls value and multiplies by 0.6. Nothing special is required of the methods: some return strings, some return numbers, one returns a boolean.

The template then uses section tags for the boolean. In the README example, {{#in_ca}} ... {{/in_ca}} wraps the taxed line, so the wrapped content appears only when in_ca is truthy. Rendering is a single class-level call, Simple.render, and the README shows the result: "Hello Chris", the prize line with 10000, and the taxed line with 6000.0.

There is a second, procedural style for cases where a class per template is too much. The README calls it "Dict-Style Views" and shows a Winner instance where you assign view[:name] = 'George' and view[:value] = 100, then call view.render. The same object can be reused: reassigning view[:name] = 'Tony' and rendering again produces the new name with the old value. That mix-and-match is explicitly allowed, so a codebase can use classes in one place and hashes in another.

Escaping is part of the rendering contract. Values interpolated with the standard double mustache are escaped for &, backslash, double quote, < and >, plus the single quote on Ruby 2.0 and later. The README gives the concrete case: rendering a variable containing 5 > 2 through {{variable}} yields 5 &gt; 2, while {{{variable}}} yields 5 > 2. That default is the reason a template that emits pre-built HTML needs the triple form, and it is also the first thing to check when output looks wrong.

Installing mustache/mustache and rendering a first template

The README gives two install paths. For a one-off, install the gem directly. For an application, add it to the Gemfile with the version constraint the README shows.

gem install mustache

gem "mustache", "~> 1.0"

The fastest way to confirm it works is the one-liner from the README's usage section, which renders a string template against a hash and returns the interpolated result.

require 'mustache'
Mustache.render("Hello {{planet}}", planet: "World!")

That call returns "Hello World!". From there, the class-based form is the one you will live in. Define a view class and a template, then call render on the class.

class Simple < Mustache
  def name
    "Chris"
  end

  def value
    10_000
  end
end

With a simple.mustache file containing "Hello {{name}}" and a line for {{value}}, calling Simple.render produces the filled output. The repository also ships an examples directory, including examples/simple.rb and examples/simple.mustache, which is the shortest path to a working setup you can edit.

Template lookup, template_path and the cwd trap

By default a view looks for its template on disk in the current directory, following the Ruby naming convention: TemplatePartial maps to ./template_partial.mustache. The README is explicit that the default is cwd-relative, and that is the sharpest operational edge in the whole gem. A script that runs fine from its own directory can fail to find its template when invoked from elsewhere.

The fix the README documents is template_path, which can be set per class. Setting self.template_path = __dir__ inside the class makes Simple look for simple.mustache next to the class file "no matter the cwd." Multiple search paths are supported by joining them with the platform path separator, a colon on Unix. If you only want to point at a specific file, Mustache.template_file accepts a path directly.

Two more knobs matter in practice. template_extension changes the suffix the view searches for, so setting it to 'xml' makes the view look for ./blah.xml instead of a .mustache file. And the template can bypass disk entirely: Simple.template = 'Hi {{person}}!' sets it on the class, while Simple.new.template = 'Hi {{person}}!' sets it for one instance only. For test suites, the in-memory form is the one that removes filesystem state from the equation.

Helpers, initialize, and the boundary the gem refuses to cross

The README is candid that global helpers are just Ruby modules. Its gravatar example defines a module with gravatar, gravatar_for_id and gravatar_host methods, then includes that module into a Mustache subclass. Because the view is an ordinary class, you can also define initialize and set instance variables there. The README's example takes an ssl argument, stores it in @ssl, and renders with Simple.new(request.ssl?).render. The gravatar_host method then reads @ssl to choose between the https and http hosts.

This is the part worth being clear-eyed about. The template language is logic-less, but the view class is not, and it should not be. Any branching, any query, any formatting decision belongs in Ruby. If your instinct is to put a comparison inside the template, the gem will not help you, and that is the intended outcome rather than a gap.

The constraint has a cost. A template that needs a slightly different shape for one caller requires a method on the view, or a subclass, or a different template. There is no inline expression to fall back on. Teams that like this trade accept the extra Ruby; teams that do not will find themselves writing thin methods whose only job is to precompute a boolean.

Where mustache/mustache is the wrong tool

The README does not document rollback, migration tooling, or a deprecation policy, and the release history in the repository is thin: the latest release listed is v1.0.2 from 2015-06-24, even though the repository's last push was on 2026-08-20. Those two dates describe different things, but together they mean you should not expect the gem's public surface to change under you, and you should not expect it to grow features either. If your selection criteria include a fast-moving dependency with frequent releases, this is not that.

The escaping default is the other place people get surprised. Double mustaches escape &, backslash, double quote, < and > (and the single quote on Ruby 2.0 and later). If your view returns markup that you intend to be emitted as-is, you need the triple-mustache form, and forgetting it produces visibly broken output rather than an error. There is no exception to catch.

Finally, the project is a template renderer and nothing more. The README points to mustache-sinatra for Sinatra integration and to view_namespace and view_path settings for people writing framework plugins, but it does not ship a Rails view resolver or an asset pipeline hook. If you need those, you are building them.

mustache/mustache against ERB and Liquid

The real alternative for most Ruby teams is ERB, which ships with the standard library and lets arbitrary Ruby run inside the template. The difference is not syntax, it is where decisions live. ERB permits a query or a conditional in the markup; mustache/mustache does not, and the README treats that as the feature: "it is impossible to embed application logic in this template language." An ERB view can be edited by someone who knows Ruby and nothing else. A mustache view requires a matching Ruby method to exist, or the tag renders empty.

Liquid is the closer comparison, since it is also logic-less by intent and is the template language behind Shopify themes. The README names Liquid among the dialects the author does not want to write. The practical difference is that Liquid ships filters and a tag vocabulary designed for user-editable templates, while mustache/mustache keeps the template vocabulary minimal and pushes everything into the Ruby class. If you want non-developers editing templates with a sanctioned set of filters, Liquid's design is aimed at that; if you want the template to be inert and the class to hold all authority, mustache/mustache is the stricter choice.

The other argument for mustache/mustache is portability of the template syntax itself. The README points to mustache.github.io for "a list of implementations (other than Ruby)", and the tag types are documented in a language-agnostic manpage shipped in the repository's man directory. A template written here is not Ruby-specific in form, which matters if the same markup must render in a JavaScript or Go service.

Licence and the cost of staying on this version

The repository is MIT licensed, with the LICENSE file at the top level. In practical terms that is a permissive licence that permits commercial use and modification, but the file itself is the authority and nothing here is legal advice; if your organisation has a licence review step, hand it the LICENSE file rather than a summary.

Upgrade cost is dominated by the release cadence. The latest release listed is v1.0.2 from 2015-06-24, so the version constraint in the README, gem "mustache", "~> 1.0", is not a moving target. The README does document one historical migration: projects upgrading to Sinatra 1.0 and Mustache 0.9.0 or later from 0.7.0 or lower had their settings renamed, and the README links a diff for that change. It says the settings "are named properly now" and are contained in a hash. Beyond that, the README does not describe an upgrade procedure, and the HISTORY.md file in the repository is where release-level changes would be recorded.

The maintenance reading is straightforward. The last push was on 2026-08-20, so the repository is not dormant, but the README does not make claims about a support window, and the gem's API as documented has been stable for a long time. Budget for reading HISTORY.md and the LICENSE file, not for tracking a changelog of breaking changes.

Editorial conclusion

Adopt mustache/mustache when a Ruby class should own every decision and the template should only name methods, and when you want the same template syntax to exist outside Ruby, since the project points to other implementations at mustache.github.io. Do not adopt it if your templates need conditionals the view class cannot express, or if you expect the gem itself to move quickly: the last push was on 2026-08-20, the latest release listed is v1.0.2 from 2015-06-24, and the README documents no upgrade path beyond the Sinatra 1.0 note. Before committing, verify two things against your own code: how your templates resolve when the current working directory differs from the file location, since the default lookup is relative to the cwd, and whether your output contains characters such as & or < that the double-mustache escaping will rewrite.

Frequently asked questions

Official sources

1. License: MIT
2. mustache/mustache on GitHub
3. Project website
4. README
5. Releases