Ruby code analysis gem that combines smarter verifying doubles, reflection and dynamic type sampling to combat NoMethodError
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TypeTracer collects a set of experimental approaches to checking Ruby "types" (particularly method signatures) while still allowing a decent level of metaprogramming and without needing to specify annotations in the production code.

For a detailed discussion of the three approaches below, see my talk "Auto-Analyzing Ruby for Fun and Quality" given at BostonRb in May 2016.

See tt_demos for an example of using approach 1., and quote-randomizer for a simple Rails app that incorporates support for approaches 2. and 3 below.

1. Enhancing RSpec instance_double to check stubbed argument values

Even if you have 100% unit test coverage, there still can be invalid method calls in the "seams" between your classes or groups of classes.

RSpec's Verifying Doubles, such as instance_double will check that stubbed method calls are actually defined on a instance of the class you specify and that the method arity (number of arguments) matches what you are stubbing.

This gem includes a file you can include which monkey-patches instance_double to parse the abstract syntax tree of the stubbed method to check whether the arguments to the stubbed call would result in an NoMethodError were they actually to be passed into the stubbed method.

To make your instance_double's behave like that, include type_tracer in your Gemfile and add require 'type_tracer/rspec/Instance_double_arg_checker.

It can only reliably assume that a method will be called on the actual value of an argument if that argument variable isn't reassigned and if the method doesn't contain any branches (because it could branch on the type of the argument). A lot of methods contain branches, so I may try to tighten that requirement (e.g. that the expression that it branches on must involve the argument).

2. Check instance method calls in app environment (for runtime defined methods)

It's very difficult / impossible to simply statically analyze Ruby code for undefined methods (though see the ruby-lint gem for something that does a decent job at it).

The approach here is instead to combine static and dynamic analysis of a Ruby program. That is to run the "undefined method" analysis in the context of a fully loaded app environment where many/most of the dynamically defined methods have been defined. In a typical Rails app for instance, the has_many association methods would be defined when a class is loaded.

However, the database attribute methods for ActiveRecord objects won't be defined until a request on that class is made e.g. from a call to new. My idea would be for TypeTracer to provide a config option for a block that could be called to induce most of the dynamically defined methods to get defined, e.g. by calling new on all of the ActiveRecord::Base subclasses and perhaps doing other application-specific method defining. Of course, not all dynamic methods are defined in easy-to-induce ways, but this would still catch a lot of them.

To use it, include the type_tracer gem in your Gemfile, then, assuming you're running a Rails app, you can run a new Rake task rake type_tracer:check_method_calls that will check for undefined top-level instance method calls (i.e. calls on self in methods).

It's possible that you will have methods that are defined not at the time that a class is first loaded but later in the app runtime. E.g. ActiveRecord attribute methods aren't defined until the first ActiveRecord operation for that model.

What type_tracer provides is a config option called attribute_methods_definer that specifies a proc that is called after the app environment is loaded but before the analysis starts. Here's an example that will call new on all classes that inherit from ActiveRecord::Base which will have the effect of making those classes define their attribute methods.

# config/initializers/type_tracer.rb
require 'type_tracer'

TypeTracer.config do |config|
  config.attribute_methods_definer = proc do
    # initialize all of the active record models so that they will define their
    # attribute methods.

This is similar to how you would define attribute methods for a dynamic class if you are using RSpec verifying doubles, see their doc for dynamic-classes

3A. Sampling implied types from production to use in further analysis

This is not a direct way to catch bad method calls, but it's an idea to use the running production application to gather the real implicit type signature for methods and then feed that back into the specs or analysis tools as more realistic type information (without needing to annotate production code or be super-specific about what the types actually are).

It's common, for instance, for a method to take either nil or a specific value. It's also decently comment for a method to take a "duck type" i.e. it could take instances of a range of classes but call a fixed set of methods on all of them. Sometimes too a method may have an explicit is_a? check and do different things for different types (e.g. a method that operates recursively on a nested structure that could be either an Array or Hash).

To try to represent as many of those cases as possible, the traced types are in effect union types of all the different classes that are passed in and what methods are called on instances of those different classes.

To use type tracing in your Rails app, include the type_tracer gem, and then add an initializer like this:

# config/initializers/type_tracer.rb
require 'type_tracer'

TypeTracer.config do |config|
  # This configures an Rack middleware for what requests to type sample on
  config.sample_types_for_requests do |_rack_env|
    # Only type sample 1% of requests
    rand() > 0.01

  # These configure the files to do type sampling on and the remote URL of the
  # sampled types endpoint in a deployed app.
  config.type_check_root_path = Rails.root
  config.type_check_path_regex = %r{\A(app|lib)/}
  config.sampled_types_url = ''

  # To make the sampled types useful for checking local changes, we need to be
  # able to know the git commit that produced the sampled types in case they are
  # no longer applicable given local changes (if you changed the callers of the
  # method in question).
  # This will give the git commit on Heroku, though requires this buildpack:
  config.git_commit = ENV['SOURCE_VERSION']

You'll also need to set up an endpoint in your app that will serve the sampled types (as referenced above with the sampled_types_url). Here's an example controller (that would need a corresponding route as well):

class SampledTypesController < ApplicationController
  def show
    render json: JSON.pretty_generate(TypeTracer::TypeSampler.sampled_type_info)

3B. Using sampled types to check local changes to a method for undefined method calls on arguments

To use the sampled method type signatures from a deployed app, run rake type_tracer:check_arg_sends. That will fetch the sampled types and Git commit hash from your specified sampled_types_url. It assumes you are using Git for your project and have the git executable installed.

A similar caveat to the expanded instance_double above holds, that the checking will assume the method is correct if it either contains branches or a re-assignment of the argument variable (because those cases are harder to analyze and a common idiom is e.g. return if x.nil? or x ||= default_value both of which could change the type of the x argument).


Add this line to your application's Gemfile:

gem 'type_tracer'

And then execute:

$ bundle

Or install it yourself as:

$ gem install type_tracer


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 tags, and push the .gem file to


Bug reports and pull requests are welcome on GitHub at[USERNAME]/type_tracer.


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