Fix

A specification says what a subject MUST, SHOULD or MAY do, in the words of RFC 2119, and nothing about how it is made. Write it once, then test an Array against it today, your own class tomorrow, a library you depend on, or the next version of any of them, beside the RSpec or minitest suite you already have.

• Start the guide
• Read about Fix

Install

gem install fix

Version

1.0.1, for Ruby 3.3 or later

Source

github.com/fixrb/fix, MIT license

From RSpec

Coming from RSpec: what each idiom becomes, and what is left out

require "fix"

# A stack, whatever it is made of.
Fix :Stack do
  on :push, 1 do
    on :pop do
      it MUST eq 1
    end
  end

  on :pop do
    it SHOULD be_nil
  end

  on :peek do
    it MAY be_nil
  end
end

# A test picks the subject: an Array.
exit(Fix[:Stack].test { [] })

ruby stack.rb
stack.rb:7: Success: expected 1 to eq 1
stack.rb:12: Success: expected nil to be nil
stack.rb:16: Info: NoMethodError: undefined method 'peek' for an instance of Array

What makes Fix different

• A specification is a document

  It is read once, when the file is loaded: no setup, no mock, no logic, and anything wrong in it raises before a test runs. It names no class; the test picks the subject, and one specification serves as many implementations as you like.

• Three levels of requirement

  MUST, SHOULD and MAY tell what is required, what is recommended and what is optional, so that a contract between a library and its users, or between two teams, says what blocks a build and what does not. A SHOULD not met is a warning, a MAY on a method the subject lacks is an info, and both pass.

• One process per context

  Each context runs in a child process. What a subject does to a global, a constant or ENV stays in its context, and the order of the contexts cannot change a result. In a Rails application shows it with a database, which a process does not isolate.

Other gems

Fix is built from small libraries and extended by small gems. Each does one thing, in a few hundred lines of code at most, so that you can read it before you trust it.

From the blog

• Fix 1.0 is here

  October 31, 2026

  Fix 1.0 is out: a Ruby testing framework where a test suite is an RFC 2119 specification, tested against any subject, each context in its own process.

• The Three Levels of Requirements Inspired by RFC 2119

  December 30, 2024

  How Fix implements the MUST, SHOULD and MAY requirement levels for more precise specifications, and how this compares with RSpec’s historical evolution.

• Rethinking Test Architecture with Clear Specification Separation

  December 29, 2024

  How Fix’s approach to testing in Ruby keeps specifications apart from their implementations, for a test suite that is easier to maintain and to reuse.

All posts