The project is in a healthy, maintained state
When you get an "unexpected end" in your syntax this gem helps you find it
 Project Readme


An error in your code forces you to stop. SyntaxSuggest 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

This project was previously named dead_end as it attempted to find dangling end keywords. The name was changed to be more descriptive and welcoming as part of the effort to merge it into Ruby 3.2.

Installation in your codebase

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

gem 'syntax_suggest'

And then execute:

$ bundle install

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

require "syntax_suggest"

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

--require syntax_suggest

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 syntax_suggest

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

Editor integration

An extension is available for VSCode:

What syntax errors does it handle?

Syntax suggest will fire against all syntax errors and can isolate any syntax error. In addition, syntax_suggest 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
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>|)
Unmatched `(', missing `)' ?

  1  class Dog
> 2    def speak(sound
> 4    end
  5  end
  • Any ambiguous or unknown errors will be annotated by the original parser error output:
Expected an expression after the operator

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

Stable internal interface(s):

  • SyntaxSuggest.handle_error(e)

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


Handling conflicts with the default gem

Because syntax_suggest is a default gem you can get conflicts when working on this project with Ruby 3.2+. To fix conflicts you can disable loading syntax_suggest as a default gem by using then environment variable RUBYOPT with the value --disable=syntax_suggest. The RUBYOPT environment variable works the same as if we had entered those flags directly in the ruby cli (i.e. ruby --disable=syntax_suggest is the same as RUBYOPT="--disable=syntax_suggest" ruby). It's needed because we don't always directly execute Ruby and RUBYOPT will be picked up when other commands load ruby (rspec, rake, or bundle etc.).

There are some binstubs that already have this done for you. Instead of running bundle exec rake you can run bin/rake. Binstubs provided:

  • bin/bundle
  • bin/rake
  • bin/rspec


After checking out the repo, run bin/setup to install dependencies. Then, run bin/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 bin/rake install. To release a new version, update the version number in version.rb, and then run bin/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/syntax_suggest_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


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

Environment variables

  • SYNTAX_SUGGEST_DEBUG - Enables debug output to STDOUT/STDERR and/or disk at ./tmp. The contents of debugging output are not stable and may change. If you would like stability, please open an issue to explain your use case.
  • SYNTAX_SUGGEST_TIMEOUT - Changes the default timeout value to the number set (in seconds).


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


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

Code of Conduct

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