Sorbet Cheatsheet Part 1

Published Nov 12, 2020

As I start using Sorbet more, I’ve realized that I need a quick reference for some common patterns. I’ve always been fond of Learn X in Y minutes, so I’m going to try to copy that style. It’ll take a few tries to get this right, so here’s the first.

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# 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(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)
end

class Helpers
  # TODO: T::Boolean
  # TODO: T.nilable
  # TODO: T.type_alias
  # TODO: T.reveal_type
end

class TheFunPart
end

class TrustMe
  # TODO: T.untyped
  # TODO: T.cast
  # TODO: T.unsafe
  # TODO: T.must
  # TODO: T.assert_type!
end

class Experimental
  # These are not listed in the documentation but are still available for use.
end

class Generics
  # TODO
end