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_endas it attempted to find dangling
endkeywords. 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:
And then execute:
$ bundle install
If your application is not calling
Bundler.require then you must manually add a require:
If you're using rspec add this to your
This is needed because people can execute a single test file via
bundle exec rspec path/to/file_spec.rband if that file has a syntax error, it won't load
spec_helper.rbto 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.
An extension is available for VSCode:
- Extension: https://marketplace.visualstudio.com/items?itemName=Zombocom.dead-end-vscode
- GitHub: https://github.com/zombocom/dead_end-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:
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
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 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
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.
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:
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
Stable internal interface(s):
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
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
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 (
There are some binstubs that already have this done for you. Instead of running
bundle exec rake you can run
bin/rake. Binstubs provided:
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
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>
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.