tenum.md

id	title	sidebar_label
tenum	Typed Enums via T::Enum	T::Enum

Enumerations allow for type-safe declarations of a fixed set of values. "Type safe" means that the values in this set are the only values that belong to this type. Here's an example of how to define a typed enum with Sorbet:

# (1) New enumerations are defined by creating a subclass of T::Enum
class Suit < T::Enum
  # (2) Enum values are declared within an `enums do` block
  enums do
    Spades = new
    Hearts = new
    Clubs = new
    Diamonds = new
  end
end

Note how each enum value is created by calling new: each enum value is an instance of the enumeration class itself. This means that Suit::Spades.is_a?(Suit), and the same for all the other enum values. This guarantees that one enum value cannot be used where some other type is expected, and vice versa.

This also means that once an enum has been defined as a subclass of T::Enum, it behaves like any other Class Type and can be used in method signatures, type aliases, T.let annotations, and any other place a class type can be used:

sig {returns(Suit)}
def random_suit
  Suit.values.sample
end

Exhaustiveness

Sorbet knows about the values in an enumeration statically, and so it can use exhaustiveness checking to check whether all enum values have been considered. The easiest way is to use a case statement:

sig {params(suit: Suit).void}
def describe_suit_color(suit)
  case suit
  when Suit::Spades   then puts "Spades are black!"
  when Suit::Hearts   then puts "Hearts are red!"
  when Suit::Clubs    then puts "Clubs are black!"
  when Suit::Diamonds then puts "Diamonds are red!"
  else T.absurd(suit)
  end
end

Because of the call to T.absurd, if any of the individual suits had not been handled, Sorbet would report an error statically that one of the cases was missing. For more information on how exhaustiveness checking works, see Exhaustiveness Checking.

Converting enums to other types

Enumerations do not implicitly convert to any other type. Instead, all conversion must be done explicitly. One particularly convenient way to implement these conversion functions is to define instance methods on the enum class itself:

class Suit < T::Enum
  enums do
    # ...
  end

  sig {returns(Integer)}
  def rank
    # (1) Case on self (because this is an instance method)
    case self
    when Spades then 1
    when Hearts then 2
    when Clubs then 3
    when Diamonds then 4
    else
      # (2) Exhaustiveness still works when casing on `self`
      T.absurd(self)
    end
  end
end

A particularly common case to convert an enum to a String. Because this is so common, this conversion has been built in (it still must be explicitly called):

Suit::Spades.serialize # => 'spades'
Suit::Hearts.serialize # => 'hearts'
# ...

Again: this conversion must be done explicitly. When attempting to implicitly convert an enum value to a string, you'll get a non-human-friendly representation of the enum:

suit = Suit::Spades
puts "Got suit: #{suit}"
# =>  Got suit: #<Suit::Spades>

The default value used for serializing an enum is the name of the enum, all lowercase. To specify an alternative serialized value, pass an argument to new:

class Suit < T::Enum
  enums do
    Spades = new('SPADES')
    Hearts = new('HEARTS')
    Clubs = new('CLUBS')
    Diamonds = new('DIAMONDS')
  end
end

Suit::Diamonds.serialize # => 'DIAMONDS'

Each serialized value must be unique compared to all other serialized values for this enum.

Another common conversion is to take the serialized value and deserialize it back to the original enum value. This is also built into T::Enum:

serialized = Suit::Spades.serialize
suit = Suit.deserialize(serialized)

puts suit
# => #<Suit::Spades>

When the value being deserialized doesn't exist, a KeyError exception is raised:

Suit.deserialize('bad value')
# => KeyError: Enum Suit key not found: "bad value"

If this is not the behavior you want, you can use try_deserialize which returns nil when the value doesn't deserialize to anything:

Suit.try_deserialize('bad value')
# => nil

You can also ask whether a specific serialized value exists for an enum:

Suit.has_serialized?(Suit::Spades.serialize)
# => true

Suit.has_serialized?('bad value')
# => false

Listing the values of an enum

Sometimes it is useful to enumerate all the values of an enum:

Suit.values
# => [#<Suit::Spades>, #<Suit::Heart>, #<Suit::Clubs>, #<Suit::Diamonds>]

Attaching metadata to an enum

It can be tempting to "attach metadata" to each enum value by overriding the constructor for a T::Enum subclass such that it accepts more information and stores it on an instance variable.

This is strongly discouraged. It's likely that Sorbet will enforce this discouragement with a future change.

Concretely, consider some code like this that is discouraged:

→ View on sorbet.run

This code is discouraged because it...

• overrides the T::Enum constructor, making it brittle to potential future changes in the T::Enum API.
• stores state on each enum value. Enum values are singleton instances, meaning that if someone accidentally mutates this state, it's observed globally throughout an entire program.

Rather than thinking of enums as data containers, instead think of them as dumb immutable values. A more idiomatic way to express the code above looks similar to the example given in the section [Converting enums to other types](#converting-enums-to-other types) above:

# typed: strict
class Suit < T::Enum
  extend T::Sig

  enums do
    Spades = new
    Hearts = new
    Clubs = new
    Diamonds = new
  end

  sig {returns(Integer)}
  def rank
    case self
    when Spades then 1
    when Hearts then 2
    when Clubs then 3
    when Diamonds then 4
    else T.absurd(self)
    end
  end
end

→ View on sorbet.run

This example uses exhaustiveness on the enum to associate a rank with each suit. It does this without needing to override anything built into T::Enum, and without mutating state.

If you need exhaustiveness over a set of cases which do carry data, see Approximating algebraic data types.

Defining one enum as a subset of another enum

One thing that comes up from time to time is having one large enum, but knowing that in certain places only a subset of those enums are valid. With T::Enum, there are a number of ways to encode this:

1. By using a sealed module
2. By explicitly converting between multiple enums

Let's elaborate on those two one at a time.

All the examples below will be for days of the week. There are 7 days total, but there are two clear groups: weekdays and weekends, and sometimes it makes sense to have the type system enforce that a value can only be a weekday enum value or only a weekend enum value.

By using a sealed module

Sealed modules are a way to limit where a module is allowed to be included. See the docs if you'd like to learn more, but here's how they can be used together with T::Enum:

# (1) Define an interface / module
module DayOfWeek
  extend T::Helpers
  sealed!
end

class Weekday < T::Enum
  # (2) include DayOfWeek when defining the Weekday enum
  include DayOfWeek

  enums do
    Monday = new
    Tuesday = new
    Wednesday = new
    Thursday = new
    Friday = new
  end
end

class Weekend < T::Enum
  # (3) ditto
  include DayOfWeek

  enums do
    Saturday = new
    Sunday = new
  end
end

→ view full example on sorbet.run

Now we can use the type DayOfWeek for "any day of the week" or the types Weekday & Weekend in places where only one specific enum is allowed.

There are a couple limitations with this approach:

1. Sorbet doesn't allow calling methods on T::Enum when we have a value of type DayOfWeek. Since it's an interface, only the methods defined that interface can be called (so for example day_of_week.serialize doesn't type check).

   One way to get around this is to declare abstract methods for all of the T::Enum methods that we'd like to be able to call (serialize, for example).

2. It's not the case that T.class_of(DayOfWeek) is a valid T.class_of(T::Enum). This means that we can't pass DayOfWeek (the class object) to a method that calls enum_class.values on whatever enum class it was given to list the valid values of an enum.

The second approach addresses these two issues, at the cost of some verbosity.

By explicitly converting between multiple enums

The second approach is to define multiple enums, each of which overlap values with the other enums, and to define explicit conversion functions between the enums:

→ View full example on sorbet.run

As you can see, this example is significantly more verbose, but it is an alternative when the type safety is worth the tradeoff.

What's next?

• Union types

  Enums are great for defining simple sets of related constants. When the values are not simple constants (for example, "any instance of these two classes"), union types provide a more powerful mechanism for organizing code.

• Sealed Classes and Modules

  While union types provide an ad hoc mechanism to group related types, sealed classes and modules provide a way to establish this grouping at these types' definitions.

• Exhaustiveness Checking

  For union types, sealed classes, and enums, Sorbet has powerful exhaustiveness checking that can statically catch when certain cases have not been handled.