Learn X in Y minutes

# Every file should have a "typed sigil" that tells Sorbet how strict to be
# during static type checking.
#
# Strictness levels (lax to strict):
#
# ignore: Sorbet won't even read the file. This means its contents are not
# visible during type checking. Avoid this.
#
# false: Sorbet will only report errors related to constant resolution. This is
# the default if no sigil is included.
#
# true: Sorbet will report all static type errors. This is the sweet spot of
# safety for effort.
#
# strict: Sorbet will require that all methods, constants, and instance
# variables have static types.
#
# strong: Sorbet will no longer allow anything to be T.untyped, even
# explicitly. Almost nothing satisfies this.

# typed: true

# Include the runtime type-checking library. This lets you write inline sigs
# and have them checked at runtime (instead of running Sorbet as RBI-only).
# These runtime checks happen even for files with `ignore` or `false` sigils.
require 'sorbet-runtime'

class BasicSigs
  # Bring in the type definition helpers. You'll almost always need this.
  extend T::Sig

  # Sigs are defined with `sig` and a block. Define the return value type with
  # `returns`.
  #
  # This method returns a value whose class is `String`. These are the most
  # common types, and Sorbet calls them "class types".
  sig { returns(String) }
  def greet
    'Hello, World!'
  end

  # Define parameter value types with `params`.
  sig { params(n: Integer).returns(String) }
  def greet_repeat(n)
    (1..n).map { greet }.join("\n")
  end

  # Define keyword parameters the same way.
  sig { params(n: Integer, sep: String).returns(String) }
  def greet_repeat_2(n, sep: "\n")
    (1..n).map { greet }.join(sep)
  end

  # Notice that positional/keyword and required/optional make no difference
  # here. They're all defined the same way in `params`.

  # For lots of parameters, it's nicer to use do..end and a multiline block
  # instead of curly braces.
  sig do
    params(
      str: String,
      num: Integer,
      sym: Symbol,
    ).returns(String)
  end
  def uhh(str:, num:, sym:)
    'What would you even do with these?'
  end

  # For a method whose return value is useless, use `void`.
  sig { params(name: String).void }
  def say_hello(name)
    puts "Hello, #{name}!"
  end

  # Splats! Also known as "rest parameters", "*args", "**kwargs", and others.
  #
  # Type the value that a _member_ of `args` or `kwargs` will have, not `args`
  # or `kwargs` itself.
  sig { params(args: Integer, kwargs: String).void }
  def no_op(*args, **kwargs)
    if kwargs[:op] == 'minus'
      args.each { |i| puts(i - 1) }
    else
      args.each { |i| puts(i + 1) }
    end
  end

  # Most initializers should be `void`.
  sig { params(name: String).void }
  def initialize(name:)
    # Instance variables must have annotated types to participate in static
    # type checking.

    # The value in `T.let` is checked statically and at runtime.
    @upname = T.let(name.upcase, String)

    # Sorbet can infer this one!
    @name = name
  end

  # Constants also need annotated types.
  SORBET = T.let('A delicious frozen treat', String)

  # Class variables too.
  @@the_answer = T.let(42, Integer)

  # Sorbet knows about the `attr_*` family.
  sig { returns(String) }
  attr_reader :upname

  sig { params(write_only: Integer).returns(Integer) }
  attr_writer :write_only

  # You say the reader part and Sorbet will say the writer part.
  sig { returns(String) }
  attr_accessor :name
end

module Debugging
  extend T::Sig

  # Sometimes it's helpful to know what type Sorbet has inferred for an
  # expression. Use `T.reveal_type` to make type-checking show a special error
  # with that information.
  #
  # This is most useful if you have Sorbet integrated into your editor so you
  # can see the result as soon as you save the file.

  sig { params(obj: Object).returns(String) }
  def debug(obj)
    T.reveal_type(obj) # Revealed type: Object
    repr = obj.inspect

    # Remember that Ruby methods can be called without arguments, so you can
    # save a couple characters!
    T.reveal_type repr # Revealed type: String

    "DEBUG: " + repr
  end
end

module StandardLibrary
  extend T::Sig
  # Sorbet provides some helpers for typing the Ruby standard library.

  # Use T::Boolean to catch both `true` and `false`.
  #
  # For the curious, this is equivalent to
  #
  #     T.type_alias { T.any(TrueClass, FalseClass) }
  #
  sig { params(str: String).returns(T::Boolean) }
  def confirmed?(str)
    str == 'yes'
  end

  # Remember that the value `nil` is an instance of NilClass.
  sig { params(val: NilClass).void }
  def only_nil(val:); end

  # To avoid modifying standard library classes, Sorbet provides wrappers to
  # support common generics.
  #
  # Here's the full list:
  # * T::Array
  # * T::Enumerable
  # * T::Enumerator
  # * T::Hash
  # * T::Range
  # * T::Set
  sig { params(config: T::Hash[Symbol, String]).returns(T::Array[String]) }
  def merge_values(config)
    keyset = [:old_key, :new_key]
    config.each_pair.flat_map do |key, value|
      keyset.include?(key) ? value : 'sensible default'
    end
  end

  # Sometimes (usually dependency injection), a method will accept a reference
  # to a class rather than an instance of the class. Use `T.class_of(Dep)` to
  # accept the `Dep` class itself (or something that inherits from it).
  class Dep; end

  sig { params(dep: T.class_of(Dep)).returns(Dep) }
  def dependency_injection(dep:)
    dep.new
  end

  # Blocks, procs, and lambdas, oh my! All of these are typed with `T.proc`.
  #
  # Limitations:
  # 1. All parameters are assumed to be required positional parameters.
  # 2. The only runtime check is that the value is a `Proc`. The argument types
  #    are only checked statically.
  sig do
    params(
      data: T::Array[String],
      blk: T.proc.params(val: String).returns(Integer),
    ).returns(Integer)
  end
  def count(data, &blk)
    data.sum(&blk)
  end

  sig { returns(Integer) }
  def count_usage
    count(["one", "two", "three"]) { |word| word.length + 1 }
  end

  # If the method takes an implicit block, Sorbet will infer `T.untyped` for
  # it. Use the explicit block syntax if the types are important.
  sig { params(str: String).returns(T.untyped) }
  def implicit_block(str)
    yield(str)
  end

  # If you're writing a DSL and will execute the block in a different context,
  # use `bind`.
  sig { params(num: Integer, blk: T.proc.bind(Integer).void).void }
  def number_fun(num, &blk)
    num.instance_eval(&blk)
  end

  sig { params(num: Integer).void }
  def number_fun_usage(num)
    number_fun(10) { puts digits.join }
  end

  # If the block doesn't take any parameters, don't include `params`.
  sig { params(blk: T.proc.returns(Integer)).returns(Integer) }
  def doubled_block(&blk)
    2 * blk.call
  end
end

module Combinators
  extend T::Sig
  # These methods let you define new types from existing types.

  # Use `T.any` when you have a value that can be one of many types. These are
  # sometimes known as "union types" or "sum types".
  sig { params(num: T.any(Integer, Float)).returns(Rational) }
  def hundreds(num)
    num.rationalize
  end

  # `T.nilable(Type)` is a convenient alias for `T.any(Type, NilClass)`.
  sig { params(val: T.nilable(String)).returns(Integer) }
  def strlen(val)
    val.nil? ? -1 : val.length
  end

  # Use `T.all` when you have a value that must satisfy multiple types. These
  # are sometimes known as "intersection types". They're most useful for
  # interfaces (described later), but can also describe helper modules.

  module Reversible
    extend T::Sig
    sig { void }
    def reverse
      # Pretend this is actually implemented
    end
  end

  module Sortable
    extend T::Sig
    sig { void }
    def sort
      # Pretend this is actually implemented
    end
  end

  class List
    include Reversible
    include Sortable
  end

  sig { params(list: T.all(Reversible, Sortable)).void }
  def rev_sort(list)
    # reverse from Reversible
    list.reverse
    # sort from Sortable
    list.sort
  end

  def rev_sort_usage
    rev_sort(List.new)
  end

  # Sometimes, actually spelling out the type every time becomes more confusing
  # than helpful. Use type aliases to make them easier to work with.
  JSONLiteral = T.type_alias { T.any(Float, String, T::Boolean, NilClass) }

  sig { params(val: JSONLiteral).returns(String) }
  def stringify(val)
    val.to_s
  end
end

module DataClasses
  extend T::Sig
  # Use `T::Struct` to create a new class with type-checked fields. It combines
  # the best parts of the standard Struct and OpenStruct, and then adds static
  # typing on top.
  #
  # Types constructed this way are sometimes known as "product types".

  class Matcher < T::Struct
    # Use `prop` to define a field with both a reader and writer.
    prop :count, Integer
    # Use `const` to only define the reader and skip the writer.
    const :pattern, Regexp
    # You can still set a default value with `default`.
    const :message, String, default: 'Found one!'

    # This is otherwise a normal class, so you can still define methods.

    # You'll still need to bring `sig` in if you want to use it though.
    extend T::Sig

    sig { void }
    def reset
      self.count = 0
    end
  end

  sig { params(text: String, matchers: T::Array[Matcher]).void }
  def awk(text, matchers)
    matchers.each(&:reset)
    text.lines.each do |line|
      matchers.each do |matcher|
        if matcher.pattern =~ line
          Kernel.puts matcher.message
          matcher.count += 1
        end
      end
    end
  end

  # Gotchas and limitations

  # 1. `const` fields are not truly immutable. They don't have a writer method,
  #    but may be changed in other ways.
  class ChangeMe < T::Struct
    const :list, T::Array[Integer]
  end

  sig { params(change_me: ChangeMe).returns(T::Boolean) }
  def whoops!(change_me)
    change_me = ChangeMe.new(list: [1, 2, 3, 4])
    change_me.list.reverse!
    change_me.list == [4, 3, 2, 1]
  end

  # 2. `T::Struct` inherits its equality method from `BasicObject`, which uses
  #    identity equality (also known as "reference equality").
  class Coordinate < T::Struct
    const :row, Integer
    const :col, Integer
  end

  sig { returns(T::Boolean) }
  def never_equal!
    p1 = Coordinate.new(row: 1, col: 2)
    p2 = Coordinate.new(row: 1, col: 2)
    p1 != p2
  end

  # Define your own `#==` method to check the fields, if that's what you want.
  class Position < T::Struct
    extend T::Sig

    const :x, Integer
    const :y, Integer

    sig { params(other: Object).returns(T::Boolean) }
    def ==(other)
      # There's a real implementation here:
      # https://github.com/tricycle/sorbet-struct-comparable
      true
    end
  end

  # Use `T::Enum` to define a fixed set of values that are easy to reference.
  # This is especially useful when you don't care what the values _are_ as much
  # as you care that the set of possibilities is closed and static.
  class Crayon < T::Enum
    extend T::Sig

    # Initialize members with `enums`.
    enums do
      # Define each member with `new`. Each of these is an instance of the
      # `Crayon` class.
      Red = new
      Orange = new
      Yellow = new
      Green = new
      Blue = new
      Violet = new
      Brown = new
      Black = new
      # The default value of the enum is its name in all-lowercase. To change
      # that, pass a value to `new`.
      Gray90 = new('light-gray')
    end

    sig { returns(String) }
    def to_hex
      case self
      when Red   then '#ff0000'
      when Green then '#00ff00'
      # ...
      else            '#ffffff'
      end
    end
  end

  sig { params(crayon: Crayon, path: T::Array[Position]).void }
  def draw(crayon:, path:)
    path.each do |pos|
      Kernel.puts "(#{pos.x}, #{pos.y}) = " + crayon.to_hex
    end
  end

  # To get all the values in the enum, use `.values`. For convenience there's
  # already a `#serialize` to get the enum string value.

  sig { returns(T::Array[String]) }
  def crayon_names
    Crayon.values.map(&:serialize)
  end

  # Use the "deserialize" family to go from string to enum value.

  sig { params(name: String).returns(T.nilable(Crayon)) }
  def crayon_from_name(name)
    if Crayon.has_serialized?(name)
      # If the value is not found, this will raise a `KeyError`.
      Crayon.deserialize(name)
    end

    # If the value is not found, this will return `nil`.
    Crayon.try_deserialize(name)
  end
end

module FlowSensitivity
  extend T::Sig
  # Sorbet understands Ruby's control flow constructs and uses that information
  # to get more accurate types when your code branches.

  # You'll see this most often when doing nil checks.
  sig { params(name: T.nilable(String)).returns(String) }
  def greet_loudly(name)
    if name.nil?
      'HELLO, YOU!'
    else
      # Sorbet knows that `name` must be a String here, so it's safe to call
      # `#upcase`.
      "HELLO, #{name.upcase}!"
    end
  end

  # The nils are a special case of refining `T.any`.
  sig { params(id: T.any(Integer, T::Array[Integer])).returns(T::Array[String]) }
  def database_lookup(id)
    if id.is_a?(Integer)
      # `ids` must be an Integer here.
      [id.to_s]
    else
      # `ids` must be a T::Array[Integer] here.
      id.map(&:to_s)
    end
  end

  # Sorbet recognizes these methods that narrow type definitions:
  # * is_a?
  # * kind_of?
  # * nil?
  # * Class#===
  # * Class#<
  # * block_given?
  #
  # Because they're so common, it also recognizes these Rails extensions:
  # * blank?
  # * present?
  #
  # Be careful to maintain Sorbet assumptions if you redefine these methods!

  # Have you ever written this line of code?
  #
  #     raise StandardError, "Can't happen"
  #
  # Sorbet can help you prove that statically (this is known as
  # "exhaustiveness") with `T.absurd`.  It's extra cool when combined with
  # `T::Enum`!

  class Size < T::Enum
    extend T::Sig

    enums do
      Byte = new('B')
      Kibibyte = new('KiB')
      Mebibyte = new('MiB')
      # "640K ought to be enough for anybody"
    end

    sig { returns(Integer) }
    def bytes
      case self
        when Byte     then 1 <<  0
        when Kibibyte then 1 << 10
        when Mebibyte then 1 << 20
        else
          # Sorbet knows you've checked all the cases, so there's no possible
          # value that `self` could have here.
          #
          # But if you _do_ get here somehow, this will raise at runtime.
          T.absurd(self)

          # If you're missing a case, Sorbet can even tell you which one it is!
      end
    end
  end

  # We're gonna need `puts` and `raise` for this next part.
  include Kernel

  # Sorbet knows that no code can execute after a `raise` statement because it
  # "never returns".
  sig { params(num: T.nilable(Integer)).returns(Integer) }
  def decrement(num)
    raise ArgumentError, '¯\_(ツ)_/¯' unless num

    num - 1
  end

  class CustomError < StandardError; end

  # You can annotate your own error-raising methods with `T.noreturn`.
  sig { params(message: String).returns(T.noreturn) }
  def oh_no(message = 'A bad thing happened')
    puts message
    raise CustomError, message
  end

  # Infinite loops also don't return.
  sig { returns(T.noreturn) }
  def loading
    loop do
      %q(-\|/).each_char do |c|
        print "\r#{c} reticulating splines..."
        sleep 1
      end
    end
  end

  # You may run into a situation where Sorbet "loses" your type refinement.
  # Remember that almost everything you do in Ruby is a method call that could
  # return a different value next time you call it. Sorbet doesn't assume that
  # any methods are pure (even those from `attr_reader` and `attr_accessor`).
  sig { returns(T.nilable(Integer)) }
  def answer
    rand > 0.5 ? 42 : nil
  end

  sig { void }
  def bad_typecheck
    if answer.nil?
      0
    else
      # But answer might return `nil` if we call it again!
      answer + 1
      # ^ Method + does not exist on NilClass component of T.nilable(Integer)
    end
  end

  sig { void }
  def good_typecheck
    ans = answer
    if ans.nil?
      0
    else
      # This time, Sorbet knows that `ans` is non-nil.
      ans + 1
    end
  end
end

module InheritancePatterns
  extend T::Sig

  # If you have a method that always returns the type of its receiver, use
  # `T.self_type`. This is common in fluent interfaces and DSLs.
  #
  # Warning: This feature is still experimental!
  class Logging
    extend T::Sig

    sig { returns(T.self_type) }
    def log
      pp self
      self
    end
  end

  class Data < Logging
    extend T::Sig

    sig { params(x: Integer, y: String).void }
    def initialize(x: 0, y: '')
      @x = x
      @y = y
    end

    # You don't _have_ to use `T.self_type` if there's only one relevant class.
    sig { params(x: Integer).returns(Data) }
    def setX(x)
      @x = x
      self
    end

    sig { params(y: String).returns(Data) }
    def setY(y)
      @y = y
      self
    end
  end

  # Ta-da!
  sig { params(data: Data).void }
  def chaining(data)
    data.setX(1).log.setY('a')
  end

  # If it's a class method (a.k.a. singleton method), use `T.attached_class`.
  #
  # No warning here. This one is stable!
  class Box
    extend T::Sig

    sig { params(contents: String, weight: Integer).void }
    def initialize(contents, weight)
      @contents = contents
      @weight = weight
    end

    sig { params(contents: String).returns(T.attached_class) }
    def self.pack(contents)
      new(contents, contents.chars.uniq.length)
    end
  end

  class CompanionCube < Box
    extend T::Sig

    sig { returns(String) }
    def pick_up
      "♥#{@contents}🤍"
    end
  end

  sig { returns(String) }
  def befriend
    CompanionCube.pack('').pick_up
  end

  # Sorbet has support for abstract classes and interfaces. It can check that
  # all the concrete classes and implementations actually define the required
  # methods with compatible signatures.

  # Here's an abstract class:

  class WorkflowStep
    extend T::Sig

    # Bring in the inheritance helpers.
    extend T::Helpers

    # Mark this class as abstract. This means it cannot be instantiated with
    # `.new`, but it can still be subclassed.
    abstract!

    sig { params(args: T::Array[String]).void }
    def run(args)
      pre_hook
      execute(args)
      post_hook
    end

    # This is an abstract method, which means it _must_ be implemented by
    # subclasses. Add a signature with `abstract` to an empty method to tell
    # Sorbet about it.
    #
    # If this implementation of the method actually gets called at runtime, it
    # will raise `NotImplementedError`.
    sig { abstract.params(args: T::Array[String]).void }
    def execute(args); end

    # The following non-abstract methods _can_ be implemented by subclasses,
    # but they're optional.

    sig { void }
    def pre_hook; end

    sig { void }
    def post_hook; end
  end

  class Configure < WorkflowStep
    extend T::Sig

    sig { void }
    def pre_hook
      puts 'Configuring...'
    end

    # To implement an abstract method, mark the signature with `override`.
    sig { override.params(args: T::Array[String]).void }
    def execute(args)
      # ...
    end
  end

  # And here's an interface:

  module Queue
    extend T::Sig

    # Bring in the inheritance helpers.
    extend T::Helpers

    # Mark this module as an interface. This adds the following restrictions:
    # 1. All of its methods must be abstract.
    # 2. It cannot have any private or protected methods.
    interface!

    sig { abstract.params(num: Integer).void }
    def push(num); end

    sig { abstract.returns(T.nilable(Integer)) }
    def pop; end
  end

  class PriorityQueue
    extend T::Sig

    # Include the interface to tell Sorbet that this class implements it.
    # Sorbet doesn't support implicitly implemented interfaces (also known as
    # "duck typing").
    include Queue

    sig { void }
    def initialize
      @items = T.let([], T::Array[Integer])
    end

    # Implement the Queue interface's abstract methods. Remember to use
    # `override`!

    sig { override.params(num: Integer).void }
    def push(num)
      @items << num
      @items.sort!
    end

    sig { override.returns(T.nilable(Integer)) }
    def pop
      @items.shift
    end
  end

  # If you use the `included` hook to get class methods from your modules,
  # you'll have to use `mixes_in_class_methods` to get them to type-check.

  module Mixin
    extend T::Helpers
    interface!

    module ClassMethods
      extend T::Sig

      sig { void }
      def whisk
        'fskfskfsk'
      end
    end

    mixes_in_class_methods(ClassMethods)
  end

  class EggBeater
    include Mixin
  end

  EggBeater.whisk # Meringue!
end

module EscapeHatches
  extend T::Sig

  # Ruby is a very dynamic language, and sometimes Sorbet can't infer the
  # properties you already know to be true. Although there are ways to rewrite
  # your code so Sorbet can prove safety, you can also choose to "break out" of
  # Sorbet using these "escape hatches".

  # Once you start using `T.nilable`, Sorbet will start telling you _all_ the
  # places you're not handling nils. Sometimes, you know a value can't be nil,
  # but it's not practical to fix the sigs so Sorbet can prove it. In that
  # case, you can use `T.must`.
  sig { params(maybe_str: T.nilable(String)).returns(String) }
  def no_nils_here(maybe_str)
    # If maybe_str _is_ actually nil, this will error at runtime.
    str = T.must(maybe_str)
    str.downcase
  end

  # More generally, if you know that a value must be a specific type, you can
  # use `T.cast`.
  sig do
    params(
      str_or_ary: T.any(String, T::Array[String]),
      idx_or_range: T.any(Integer, T::Range[Integer]),
    ).returns(T::Array[String])
  end
  def slice2(str_or_ary, idx_or_range)
    # Let's say that, for some reason, we want individual characters from
    # strings or sub-arrays from arrays. The other options are not allowed.
    if str_or_ary.is_a?(String)
      # Here, we know that `idx_or_range` must be a single index. If it's not,
      # this will error at runtime.
      idx = T.cast(idx_or_range, Integer)
      [str_or_ary.chars.fetch(idx)]
    else
      # Here, we know that `idx_or_range` must be a range. If it's not, this
      # will error at runtime.
      range = T.cast(idx_or_range, T::Range[Integer])
      str_or_ary.slice(range) || []
    end
  end

  # If you know that a method exists, but Sorbet doesn't, you can use
  # `T.unsafe` so Sorbet will let you call it. Although we tend to think of
  # this as being an "unsafe method call", `T.unsafe` is called on the receiver
  # rather than the whole expression.
  sig { params(count: Integer).returns(Date) }
  def the_future(count)
    # Let's say you've defined some extra date helpers that Sorbet can't find.
    # So `2.decades` is effectively `(2*10).years` from ActiveSupport.
    Date.today + T.unsafe(count).decades
  end

  # If this is a method on the implicit `self`, you'll have to make that
  # explicit to use `T.unsafe`.
  sig { params(count: Integer).returns(Date) }
  def the_past(count)
    # Let's say that metaprogramming defines a `now` helper method for
    # `Time.new`. Using it would normally look like this:
    #
    #     now - 1234
    #
    T.unsafe(self).now - 1234
  end

  # There's a special type in Sorbet called `T.untyped`. For any value of this
  # type, Sorbet will allow it to be used for any method argument and receive
  # any method call.

  sig { params(num: Integer, anything: T.untyped).returns(T.untyped) }
  def nothing_to_see_here(num, anything)
    anything.digits # Is it an Integer...
    anything.upcase # ... or a String?

    # Sorbet will not be able to infer anything about this return value because
    # it's untyped.
    BasicObject.new
  end

  def see_here
    # It's actually nil!  This will crash at runtime, but Sorbet allows it.
    nothing_to_see_here(1, nil)
  end

  # For a method without a sig, Sorbet infers the type of each argument and the
  # return value to be `T.untyped`.
end

# The following types are not officially documented but are still useful. They
# may be experimental, deprecated, or not supported.

module ValueSet
  extend T::Sig

  # A common pattern in Ruby is to have a method accept one value from a set of
  # options. Especially when starting out with Sorbet, it may not be practical
  # to refactor the code to use `T::Enum`. In this case, you can use `T.enum`.
  #
  # Note: Sorbet can't check this statically becuase it doesn't track the
  # values themselves.
  sig do
    params(
      data: T::Array[Numeric],
      shape: T.enum([:circle, :square, :triangle])
    ).void
  end
  def plot_points(data, shape: :circle)
    data.each_with_index do |y, x|
      Kernel.puts "#{x}: #{y}"
    end
  end
end

module Generics
  extend T::Sig

  # Generics are useful when you have a class whose method types change based
  # on the data it contains or a method whose method type changes based on what
  # its arguments are.

  # A generic method uses `type_parameters` to declare type variables and
  # `T.type_parameter` to refer back to them.
  sig do
    type_parameters(:element)
      .params(
        element: T.type_parameter(:element),
        count: Integer,
      ).returns(T::Array[T.type_parameter(:element)])
  end
  def repeat_value(element, count)
    count.times.each_with_object([]) do |elt, ary|
      ary << elt
    end
  end

  sig do
    type_parameters(:element)
      .params(
        count: Integer,
        block: T.proc.returns(T.type_parameter(:element)),
      ).returns(T::Array[T.type_parameter(:element)])
  end
  def repeat_cached(count, &block)
    elt = block.call
    ary = []
    count.times do
      ary << elt
    end
    ary
  end

  # A generic class uses `T::Generic.type_member` to define type variables that
  # can be like regular type names.
  class BidirectionalHash
    extend T::Sig
    extend T::Generic

    Left = type_member
    Right = type_member

    sig { void }
    def initialize
      @left_hash = T.let({}, T::Hash[Left, Right])
      @right_hash = T.let({}, T::Hash[Right, Left])
    end

    # Implement just enough to make the methods below work.

    sig { params(lkey: Left).returns(T::Boolean) }
    def lhas?(lkey)
      @left_hash.has_key?(lkey)
    end

    sig { params(rkey: Right).returns(T.nilable(Left)) }
    def rget(rkey)
      @right_hash[rkey]
    end
  end

  # To specialize a generic type, use brackets.
  sig do
    params(
      options: BidirectionalHash[Symbol, Integer],
      choice: T.any(Symbol, Integer),
    ).returns(T.nilable(String))
  end
  def lookup(options, choice)
    case choice
    when Symbol
      options.lhas?(choice) ? choice.to_s : nil
    when Integer
      options.rget(choice).to_s
    else
      T.absurd(choice)
    end
  end

  # To specialize through inheritance, re-declare the `type_member` with
  # `fixed`.
  class Options < BidirectionalHash
    Left = type_member(fixed: Symbol)
    Right = type_member(fixed: Integer)
  end

  sig do
    params(
      options: Options,
      choice: T.any(Symbol, Integer),
    ).returns(T.nilable(String))
  end
  def lookup2(options, choice)
    lookup(options, choice)
  end

  # There are other variance annotations you can add to `type_member`, but
  # they're rarely used.
end