Purst

Purist is a tool designed to help detecting impure ruby code in runtime.

Ruby's stdlib and corelibs include a bunch of impure methods, including

1. randomization: Kernel.rand, Random.rand, SecureRandom.hex and so on

2. IO-related methods like Kernel.readline or IO.popen

3. specialized side-effects like Kernel.fork or Kernel.syscall

Purist hooks into ruby's tracepoint API to detect any invocation of these methods. You can see the full list of target methods in configuration.rb source file.

Installation

Install the gem and add to the application's Gemfile by executing:

$ bundle add purist

If bundler is not being used to manage dependencies, install the gem by executing:

$ gem install purist

Usage

To check if your code is pure, simply pass it into Purist.trace method:

Purist.trace { 3 * 3 } # 9

If provided block is impure, an exception will be raised:

irb(main):001> Purist.trace { p "I'm impure" }
gems/purist/lib/purist/handler.rb:23:in `call': {:path=>"(irb)", :lineno=>1, :module_name=>Kernel, :method_name=>:p} (Purist::Errors::PurityViolationError)

You can retrieve exception details like this:

exception = Purist.trace { p 1 } rescue $!

p exception.trace_point

{
  :path => ".../zeitwerk-2.6.13/lib/zeitwerk/kernel.rb",
  :lineno => 23,
  :module_name => Kernel,
  :method_name => :require,
  :backtrace => [...]
}

RSpec integration

Purist comes with built-in RSpec integration. To enable it, add require "purist/integrations/rspec" to your spec_helper.rb and manually include Purist::Integrations::RSpec::Matchers:

require "purist/integrations/rspec"

...

RSpec.configure do |config|
  ...
  config.include Purist::Integrations::RSpec::Matchers
  ...
end

And not be_pure and be_impure matchers are available:

expect { Module.new }.to be_pure
expect { User.where(name: :john) }.to be_impure

Caveats

1. Passing Purist.trace check does not mean your function is totally pure, for instance

def foo(n)
  if n > 0 # pure branch
    n.succ
  else # impure branch
    p n
  end
end

Purist.trace { foo(3) } # 4

2. Ruby stdlib/corelib is quite big, I'm pretty sure some impure functions are missing from the list.

3. Obviously, Purist is unable to detect anything within 3rd-party C extensions.

Development

After checking out the repo, run bin/setup to install dependencies. Then, run rake spec to run the tests. You can also run bin/console for an interactive prompt that will allow you to experiment.

To install this gem onto your local machine, run bundle exec rake install. To release a new version, update the version number in version.rb, and then run bundle exec rake release, which will create a git tag for the version, push git commits and the created tag, and push the .gem file to rubygems.org.

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/viralpraxis/purist. This project is intended to be a safe, welcoming space for collaboration, and contributors are expected to adhere to the code of conduct.

License

The gem is available as open source under the terms of the MIT License.

Code of Conduct

Everyone interacting in the Purist project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.