DeadEnd

An error in your code forces you to stop. DeadEnd helps you find those errors to get you back on your way faster.

Unmatched `end', missing keyword (`do', `def`, `if`, etc.) ?

  1  class Dog
❯ 2    defbark
❯ 4    end
  5  end

Installation in your codebase

To automatically annotate errors when they happen, add this to your Gemfile:

gem 'dead_end'

And then execute:

$ bundle install

If your application is not calling Bundler.require then you must manually add a require:

require "dead_end"

If you're using rspec add this to your .rspec file:

--require dead_end

This is needed because people can execute a single test file via bundle exec rspec path/to/file_spec.rb and if that file has a syntax error, it won't load spec_helper.rb to trigger any requires.

Install the CLI

To get the CLI and manually search for syntax errors (but not automatically annotate them), you can manually install the gem:

$ gem install dead_end

This gives you the CLI command $ dead_end for more info run $ dead_end --help.

Editor integration

An extension is available for VSCode:

What syntax errors does it handle?

Dead end will fire against all syntax errors and can isolate any syntax error. In addition, dead_end attempts to produce human readable descriptions of what needs to be done to resolve the issue. For example:

  • Missing end:
Unmatched keyword, missing `end' ?

❯ 1  class Dog
❯ 2    def bark
❯ 4  end
  • Missing keyword <!-- ruby class Dog def speak @sounds.each |sound| puts sound end end end -->
Unmatched `end', missing keyword (`do', `def`, `if`, etc.) ?

  1  class Dog
  2    def speak
❯ 3      @sounds.each |sound|
❯ 5      end
  6    end
  7  end
  • Missing pair characters (like {}, [], () , or |<var>|) <!--
class Dog
  def speak(sound
    puts sound
  end
end

-->

Unmatched `(', missing `)' ?

  1  class Dog
❯ 2    def speak(sound
❯ 4    end
  5  end
  • Any ambiguous or unknown errors will be annotated by the original ripper error output:
syntax error, unexpected end-of-input

  1  class Dog
  2    def meals_last_month
❯ 3      puts 3 *
  4    end
  5  end

How is it better than ruby -wc?

Ruby allows you to syntax check a file with warnings using ruby -wc. This emits a parser error instead of a human focused error. Ruby's parse errors attempt to narrow down the location and can tell you if there is a glaring indentation error involving end.

The dead_end algorithm doesn't just guess at the location of syntax errors, it re-parses the document to prove that it captured them.

This library focuses on the human side of syntax errors. It cares less about why the document could not be parsed (computer problem) and more on what the programmer needs (human problem) to fix the problem.

Sounds cool, but why isn't this baked into Ruby directly?

We are now talking about it https://bugs.ruby-lang.org/issues/18159#change-93682.

Artificial Inteligence?

This library uses a goal-seeking algorithm for syntax error detection similar to that of a path-finding search. For more information read the blog post about how it works under the hood.

How does it detect syntax error locations?

We know that source code that does not contain a syntax error can be parsed. We also know that code with a syntax error contains both valid code and invalid code. If you remove the invalid code, then we can programatically determine that the code we removed contained a syntax error. We can do this detection by generating small code blocks and searching for which blocks need to be removed to generate valid source code.

Since there can be multiple syntax errors in a document it's not good enough to check individual code blocks, we've got to check multiple at the same time. We will keep creating and adding new blocks to our search until we detect that our "frontier" (which contains all of our blocks) contains the syntax error. After this, we can stop our search and instead focus on filtering to find the smallest subset of blocks that contain the syntax error.

Here's an example:

Use internals

To use the dead_end gem without monkeypatching you can require 'dead_end/api'. This will allow you to load dead_end and use its internals without mutating require.

Stable internal interface(s):

  • DeadEnd.handle_error(e)

Any other entrypoints are subject to change without warning. If you want to use an internal interface from dead_end not on this list, open an issue to explain your use case.

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 tags, and push the .gem file to rubygems.org.

How to debug changes to output display

You can see changes to output against a variety of invalid code by running specs and using the DEBUG_DISPLAY=1 environment variable. For example:

$ DEBUG_DISPLAY=1 bundle exec rspec spec/ --format=failures

Run profiler

You can output profiler data to the tmp directory by running:

$ DEBUG_PERF=1 bundle exec rspec spec/integration/dead_end_spec.rb

Some outputs are in text format, some are html, the raw marshaled data is available in raw.rb.marshal. See https://ruby-prof.github.io/#reports for more info. One interesting one, is the "kcachegrind" interface. To view this on mac:

$ brew install qcachegrind

Open:

$ qcachegrind tmp/last/profile.callgrind.out.<numbers>

Contributing

Bug reports and pull requests are welcome on GitHub at https://github.com/zombocom/dead_end. 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 DeadEnd project's codebases, issue trackers, chat rooms and mailing lists is expected to follow the code of conduct.