Type Checking With Sorbet

The majority of the code in Homebrew is written in Ruby which is a dynamic language. To avail the benefits of static type checking, we have set up Sorbet in our codebase which provides the benefits of static type checking to dynamic languages like Ruby.
Sorbet’s Documentation is a good place to get started if you want to dive deeper into Sorbet and it’s abilities.

Sorbet elements in the Homebrew Codebase

The sorbet/ directory in Library/Homebrew consists of:

  • The rbi/ directory. It contains all Ruby Interface files, which help Sorbet to learn about constants, ancestors, and methods defined in ways it doesn’t understand natively. RBI files for all gems are auto-generated using Tapioca. We can also create a RBI file to help Sorbet understand dynamic definitions. For example: Sorbet assumes that Kernel is not necessarily included in our modules and classes, hence we use RBI files to explicitly include the Kernel Module. Here is an example in our codebase.

  • The config file. It is actually a newline-separated list of arguments to pass to srb tc, the same as if they’d been passed at the command line. Arguments in the config file are always passed first (if it exists), followed by arguments provided on the command line. We use it ignore the Library/Homebrew/vendor directory, which contains gem definitions which we do not wish to type check.

  • Every Ruby file in the codebase is divided into three strictness levels: false, true and strict. The false files only report errors related to the syntax, constant resolution and correctness of the method signatures, and not type errors. We use this file to override strictness on a file-by-file basis. Our longtime goal is to move all false files to true and start reporting type errors on those files as well. If you are making changes that require adding a new ruby file, we would urge you to add it to true and work out the resulting type errors. Read more about Sorbet’s strictness levels here.

Using brew typecheck

When run without any arguments, brew typecheck, will run considering the strictness levels set in each of the individual Ruby files in the core Homebrew codebase. However, when typecheck is run on a specific file or directory, more errors may show up since Sorbet cannot resolve constants defined outside the scope of the specified file. These problems can be solved with RBI files. Currently brew typecheck provides --quiet, --file, --dir and --ignore options but you can explore more options with srb tc --help and passing them with srb tc.

Resolving Type Errors

Sorbet reports type errors along with an error reference code, which can be used to look up more information on how to debug the error, or what causes the error in the Sorbet documentation. Here is how we debug some common type errors:

  • Using T.reveal_type. In files which are true or higher, if we wrap a variable or method call in T.reveal_type, Sorbet will show us what type it thinks that variable has in the output of srb tc. This is particularly useful when writing method signatures and debugging. Make sure to remove this line from your code before committing your changes, since this is just a debugging tool.

  • One of the most frequent errors that we’ve encountered is: 7003: Method does not exist. Since Ruby is a very dynamic language, methods can be defined in ways Sorbet cannot see statically. In such cases, check if the method exists at runtime, if not, then Sorbet has caught a future bug! But, it is also possible that even though a method exists at runtime, Sorbet cannot see it. In such cases, we use *.rbi files. Read more about RBI files here.

  • Since Sorbet does not automatically assume that Kernel is to be included in Modules, we may encounter many errors while trying to use methods like puts, ohai, odebug et cetera. A simple workaround for this would be to add an extra include Kernel line in the respective RBI file.

  • The tips above are very generic and apply to lots of cases. For some common gotchas when using Sorbet, refer to the Sorbet Error Reference and FAQ.

Method Signatures

Detailed explanation about why we use Method Signatures and its syntax can be found here. The only extra thing to keep in mind is that we add method signatures to RBI files instead of the actual method definition in the code. This way we preserve the original code structure and everything related to Sorbet is kept within the Library/Homebrew/sorbet directory.

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