brew/docs/Typechecking.md

# 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.

The [Sorbet Documentation] is a good place
to get started if you want to dive deeper into Sorbet and it's abilities.

## Sorbet in the Homebrew Codebase

### Inline Type Annotations

To add type annotations to a class or module, we need to first extend it with
the `T::Sig` module (read this as `Type::Signature`). This adds the `sig`
method which is used to annotate method signatures. Here's a simple example:

```ruby
class MyClass
  extend T::Sig

  sig { params(name: String).returns(String) }
  def my_method(name)
    "Hello, #{name}!"
  end
end
```

With `params`, we specify that we have a parameter `name` which must be a
`String` and with `returns`, we specify that this method always returns
a `String`.

For more information on how to express more complex types, refer to the
official documentation:

  - [Method Signatures](https://sorbet.org/docs/sigs)
  - [Class Types](https://sorbet.org/docs/class-types)
  - [Nilable Types](https://sorbet.org/docs/nilable-types)
  - [Union Types](https://sorbet.org/docs/union-types)

### Ruby Interface Files (`.rbi`)

RBI files help Sorbet learn about constants, ancestors and methods
defined in ways it doesn’t understand natively. We can also create a
RBI file to help Sorbet understand dynamic definitions.

Sometimes it is necessary to explicitly include the `Kernel` module in
order for Sorbet to know that methods such as `puts` are available in
a given context. This is mostly necessary for modules since they can
be used in both `BasicObject`s (which don't include `Kernel`) and
`Object`s (which include `Kernel` by default). In this case, it is
necessary to create an `.rbi` file ([example]) since re-including the
`Kernel` module in actual code can break things.

Read more about RBI files [here](https://sorbet.org/docs/rbi).

[example]: https://github.com/Homebrew/brew/blob/61b79318ed089b5010501e2cbf163fd8e48e2dfc/Library/Homebrew/global.rbi

### The [`Library/Homebrew/sorbet`] Directory

[`Library/Homebrew/sorbet`]: https://github.com/Homebrew/brew/tree/master/Library/Homebrew/sorbet

- The `rbi` directory contains all Ruby Interface (`.rbi`) files
  auto-generated by running `brew typecheck --update`:

  - RBI files for all gems are generated using
    [Tapioca](https://github.com/Shopify/tapioca#tapioca).
  - Definitions for dynamic code (i.e. meta-programming) are generated using
    `srb rbi hidden-definitions`.
  - Definitions for missing constants are generated using `srb rbi todo`.

- The `config` file is 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, followed by arguments provided
  on the command-line. We use it to ignore Gem directories which we do not
  wish to type check.

- Every Ruby file in the codebase has a magic `# typed: <level>` comment at the
  top, where `<level>` is one of [Sorbet's strictness levels], usually `false`,
  `true` or `strict`. The `false` files only report errors related to the
  syntax, constant resolution and correctness of the method signatures, but no
  type errors. Our long-term goal is to move all `false` files to `true` and
  start reporting type errors on those files as well. Therefore, when adding
  new files, you should ideally mark it with `# typed: true` and work out any
  resulting type errors.

  [Sorbet's strictness levels]: https://sorbet.org/docs/static#file-level-granularity-strictness-levels

## 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
it 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 to 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](https://sorbet.org/docs/sigs) 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](#ruby-interface-files-rbi).

- 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](https://sorbet.org/docs/error-reference)
  and [FAQ](https://sorbet.org/docs/faq).

[Sorbet Documentation]: https://sorbet.org/docs/overview