non-forcing-constants.md

id	title	sidebar_label
non-forcing-constants	Non-Forcing Constants	T::NonForcingConstants

Note: This is an advanced feature of Sorbet. It's more likely that you're looking about how Sorbet handles is_a? using Flow-Sensitive Typing. Please read carefully before using this feature.

This is a variant of is_a? that doesn't force its class argument to be loaded.

class Outer
  autoload :Nested, './expensive_file_to_load.rb'
end

# ...

def foo(x)
  if T::NonForcingConstants.non_forcing_is_a?(x, '::Outer::Nested')
    # ...
  end

  # The above can be better than this, which will cause
  # `./expensive_file_to_load.rb` to load:
  if x.is_a?(::Outer::Nested)
    # ...
  end
end

The idea is basically: if a constant like ::Outer::Nested hasn't loaded (e.g., because it hasn't been referenced yet), then certainly no instances of ::Outer::Nested can exist yet, so the instance of check will short circuit and return false. If ::Outer::Nested has loaded, the is_a? check will be carried out like normal.

This method should only be used when there's a measurable performance benefit from using it. This method is rarely used, which means that people will not be familiar with it when they see it. This makes code harder to read and reason about. Only use this method sparingly and when there's proof that it speeds a certain code path up.

Usage

The klass given as a string must be an absolute constant reference (starting with ::). Even though the argument is a string, the constant that the string refers to must still exist from Sorbet's perspective of the whole codebase. This means it will be an error if there's a typo in the string or the constant is renamed in the future.

class MyClass; end

T::NonForcingConstants.non_forcing_is_a?(
  nil,
  '::MyClas' # error: Unable to resolve constant `::MyClas`
)

→ View on sorbet.run

While Sorbet can see whether the constant exists or not statically, it cannot do so at runtime (since it does not force constants). This means that is_a? and non_forcing_is_a? can have different behaviors at runtime when the constant doesn't exist, which can cause subtle bugs if misused:

val.is_a?(::DoesntExist) # NameError (uninitialized constant DoesntExist)

T::NonForcingConstants.non_forcing_is_a?(val, '::DoesntExist') # => false

In this case, only Sorbet's static check that the constant in the string literal must resolve prevents this situation.