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Unsupported Ruby Features

Some features of Ruby Sorbet intentionally does not support. This doc aims to document the most commonly Ruby features which are not supported in Sorbet and for which Sorbet will not produce an error.

Note: This page is not exhaustive.

If something is missing from this page, it says nothing about whether Sorbet supports it, supports it but has a bug in the implementation, or does not ever intend to support it.

When in doubt, please open an issue.

Constant resolution through constant scopes via inheritance

class Parent
  X = 1
end

class Child < Parent
  p(X)  # ✅ okay in both
end

p(Child::X)  # in Ruby   => ✅ 1
             # in Sorbet => ❌ error

Sorbet does not support constant resolution through inheritance when given an explicit scope.

Alternative

Use Parent::X instead of Child::X

Why?

  • Performance

    Constant resolution is one of the most performance sensitive parts of Sorbet.

  • Understandability

    In this case, the code is easier to understand by simply replacing Child::X with Parent::X. This can always be done because Sorbet does not support dynamic constant references, so the scope is always known statically.

prepend

module WillBePrepended
  def foo
    puts 'WillBePrepended#foo'
  end
end

class Example
  prepend WillBePrepended
  def foo
    puts 'Example#foo'
  end
end

Example.new.foo # => WillBePrepended#foo

Sorbet does not model inheritance relationships introduced by prepend.

Alternative

  • Usually, static support for prepend is not required, even in codebases that make heavy use of prepend.
  • In the rare cases where prepend is required, we recommend using escape hatches to work around the problems.

Why?

  • Support for prepend would force Sorbet to use more memory throughout an entire codebase, even if the codebase makes no use of prepend. Usage of prepend is far more rare than the cost it would inflict in terms of memory.

  • Supporting prepend would add implementation complexity to Sorbet’s internals. For example: consider how to do Override Checking and generic bounds checking in the presence of prepended modules.

  • Historically, Sorbet was developed at Stripe, which lints against usage of prepend.

  • Historically and maybe still today: sorbet-runtime had (has?) poor support for runtime-checked type annotations on methods defined with prepended modules.

Refinements and refine do

class C
  def foo
    puts "C#foo"
  end
end

module M
  refine C do
    def foo
      puts "C#foo in M"
    end
  end
end

using M

c = C.new

c.foo # prints "C#foo in M"

Sorbet does not support refinements.

Alternatives

  • Use an RBI file to define the methods. Sorbet will assume that the methods are defined everywhere, not just where the using directive lives. This means that Sorbet will not reject code that would not have caused problems at runtime, at the expense of not catching situations that might have.

  • Use an Escape Hatch.

  • Use a monkey patch.

Why?

  • While refinements are better than monkey patching, they still amount to monkey patching. Sorbet’s role as a type checker is not only to catch errors, but to steer people towards simpler designs.

  • Historically, Sorbet was developed at Stripe, which does not use refinements.

  • Support for refinements would add implementation complexity to Sorbet.

  • Supporting refinements would require doing program-wide work to discover and use refinements even if a codebase does not use them at all, which comes with a performance cost.

Creating method aliases to methods in parent classes

class Parent
  def defined_in_parent; end
end

class Child < Parent
  alias_method :defined_in_child, :defined_in_parent
  # Sorbet thinks this method doesn't exist ^
end

Sorbet does not support aliasing to a method defined in a parent class from a child class.

Alternative

  1. Override the parent method in the child, and have the implementation just call super:

    class Parent
  def defined_in_parent; end
end

class Child < Parent
  def defined_in_parent
    super
  end

  alias_method :defined_in_child, :defined_in_parent
end

  2. Use RBI files to define the methods that would be defined this way:

    # -- foo.rb --
class Parent
  def defined_in_parent; end
end

class Child < Parent
  # Hide the alias_method call from Sorbet to silence the error
  T.unsafe(self).alias_method :defined_in_child, :defined_in_parent
end

# -- foo.rbi --
class Child < Parent
  # Define the method that will be defined with `alias_method` at runtime
  def defined_in_child; end
end

  3. Use an Escape Hatch to silence errors at call sites.

Why?

Due to original design decisions made in Sorbet’s architecture, all methods are defined before inheritance information is resolved.

There is nothing fundamental or performance sensitive which requires making this choice (i.e., resolving ancestor information does not require knowing the set of defined methods). But backing out this design decision at this point would require more work than we currently believe the payoff is.

Because this is not a fundamental nor ideological limitation, it’s possible this feature may gain support in the future.

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